Red Hat Enterprise Linux 6

Resource Management Guide

Managing system resources on Red Hat Enterprise Linux 6

Edition 6

Milan Navrátil

Red Hat Customer Content Services

Marie Doleželová

Red Hat Customer Content Services

Peter Ondrejka

Red Hat Customer Content Services

Eva Majoršinová

Red Hat Customer Content Services

Martin Prpič

Red Hat Product Security

Rüdiger Landmann

Red Hat Customer Content Services

Douglas Silas

Red Hat Customer Content Services

Legal Notice

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Abstract

Managing system resources on Red Hat Enterprise Linux 6.
Preface
1. Document Conventions
1.1. Typographic Conventions
1.2. Pull-quote Conventions
1.3. Notes and Warnings
2. Getting Help and Giving Feedback
2.1. Do You Need Help?
2.2. We Need Feedback
1. Introduction to Control Groups (Cgroups)
1.1. How Control Groups Are Organized
1.2. Relationships Between Subsystems, Hierarchies, Control Groups and Tasks
1.3. Implications for Resource Management
2. Using Control Groups
2.1. The cgconfig Service
2.1.1. The /etc/cgconfig.conf File
2.1.2. The /etc/cgconfig.d/ Directory
2.2. Creating a Hierarchy and Attaching Subsystems
2.3. Attaching Subsystems to, and Detaching Them from, an Existing Hierarchy
2.4. Unmounting a Hierarchy
2.5. Creating Control Groups
2.6. Removing Control Groups
2.7. Setting Parameters
2.8. Moving a Process to a Control Group
2.8.1. The cgred Service
2.9. Starting a Process in a Control Group
2.9.1. Starting a Service in a Control Group
2.9.2. Process Behavior in the Root Control Group
2.10. Generating the /etc/cgconfig.conf File
2.10.1. Blacklisting Parameters
2.10.2. Whitelisting Parameters
2.11. Obtaining Information About Control Groups
2.11.1. Finding a Process
2.11.2. Finding a Subsystem
2.11.3. Finding Hierarchies
2.11.4. Finding Control Groups
2.11.5. Displaying Parameters of Control Groups
2.12. Unloading Control Groups
2.13. Using the Notification API
2.14. Additional Resources
3. Subsystems and Tunable Parameters
3.1. blkio
3.1.1. Proportional Weight Division Tunable Parameters
3.1.2. I/O Throttling Tunable Parameters
3.1.3. blkio Common Tunable Parameters
3.1.4. Example Usage
3.2. cpu
3.2.1. CFS Tunable Parameters
3.2.2. RT Tunable Parameters
3.2.3. Example Usage
3.3. cpuacct
3.4. cpuset
3.5. devices
3.6. freezer
3.7. memory
3.7.1. Example Usage
3.8. net_cls
3.9. net_prio
3.10. ns
3.11. perf_event
3.12. Common Tunable Parameters
3.13. Additional Resources
4. Control Group Application Examples
4.1. Prioritizing Database I/O
4.2. Prioritizing Network Traffic
4.3. Per-group Division of CPU and Memory Resources
A. Revision History

Preface

1. Document Conventions

This manual uses several conventions to highlight certain words and phrases and draw attention to specific pieces of information.

1.1. Typographic Conventions

Four typographic conventions are used to call attention to specific words and phrases. These conventions, and the circumstances they apply to, are as follows.
Mono-spaced Bold
Used to highlight system input, including shell commands, file names and paths. Also used to highlight keys and key combinations. For example:
To see the contents of the file my_next_bestselling_novel in your current working directory, enter the cat my_next_bestselling_novel command at the shell prompt and press Enter to execute the command.
The above includes a file name, a shell command and a key, all presented in mono-spaced bold and all distinguishable thanks to context.
Key combinations can be distinguished from an individual key by the plus sign that connects each part of a key combination. For example:
Press Enter to execute the command.
Press Ctrl+Alt+F2 to switch to a virtual terminal.
The first example highlights a particular key to press. The second example highlights a key combination: a set of three keys pressed simultaneously.
If source code is discussed, class names, methods, functions, variable names and returned values mentioned within a paragraph will be presented as above, in mono-spaced bold. For example:
File-related classes include filesystem for file systems, file for files, and dir for directories. Each class has its own associated set of permissions.
Proportional Bold
This denotes words or phrases encountered on a system, including application names; dialog-box text; labeled buttons; check-box and radio-button labels; menu titles and submenu titles. For example:
Choose SystemPreferencesMouse from the main menu bar to launch Mouse Preferences. In the Buttons tab, select the Left-handed mouse check box and click Close to switch the primary mouse button from the left to the right (making the mouse suitable for use in the left hand).
To insert a special character into a gedit file, choose ApplicationsAccessoriesCharacter Map from the main menu bar. Next, choose SearchFind… from the Character Map menu bar, type the name of the character in the Search field and click Next. The character you sought will be highlighted in the Character Table. Double-click this highlighted character to place it in the Text to copy field and then click the Copy button. Now switch back to your document and choose EditPaste from the gedit menu bar.
The above text includes application names; system-wide menu names and items; application-specific menu names; and buttons and text found within a GUI interface, all presented in proportional bold and all distinguishable by context.
Mono-spaced Bold Italic or Proportional Bold Italic
Whether mono-spaced bold or proportional bold, the addition of italics indicates replaceable or variable text. Italics denotes text you do not input literally or displayed text that changes depending on circumstance. For example:
To connect to a remote machine using ssh, type ssh username@domain.name at a shell prompt. If the remote machine is example.com and your username on that machine is john, type ssh john@example.com.
The mount -o remount file-system command remounts the named file system. For example, to remount the /home file system, the command is mount -o remount /home.
To see the version of a currently installed package, use the rpm -q package command. It will return a result as follows: package-version-release.
Note the words in bold italics above: username, domain.name, file-system, package, version and release. Each word is a placeholder, either for text you enter when issuing a command or for text displayed by the system.
Aside from standard usage for presenting the title of a work, italics denotes the first use of a new and important term. For example:
Publican is a DocBook publishing system.

1.2. Pull-quote Conventions

Terminal output and source code listings are set off visually from the surrounding text.
Output sent to a terminal is set in mono-spaced roman and presented thus:
books        Desktop   documentation  drafts  mss    photos   stuff  svn
books_tests  Desktop1  downloads      images  notes  scripts  svgs
Source-code listings are also set in mono-spaced roman but add syntax highlighting as follows:
static int kvm_vm_ioctl_deassign_device(struct kvm *kvm,
                 struct kvm_assigned_pci_dev *assigned_dev)
{
         int r = 0;
         struct kvm_assigned_dev_kernel *match;

         mutex_lock(&kvm->lock);

         match = kvm_find_assigned_dev(&kvm->arch.assigned_dev_head,
                                       assigned_dev->assigned_dev_id);
         if (!match) {
                 printk(KERN_INFO "%s: device hasn't been assigned before, "
                   "so cannot be deassigned\n", __func__);
                 r = -EINVAL;
                 goto out;
         }

         kvm_deassign_device(kvm, match);

         kvm_free_assigned_device(kvm, match);

out:
         mutex_unlock(&kvm->lock);
         return r;
}

1.3. Notes and Warnings

Finally, we use three visual styles to draw attention to information that might otherwise be overlooked.

Note

Notes are tips, shortcuts or alternative approaches to the task at hand. Ignoring a note should have no negative consequences, but you might miss out on a trick that makes your life easier.

Important

Important boxes detail things that are easily missed: configuration changes that only apply to the current session, or services that need restarting before an update will apply. Ignoring a box labeled “Important” will not cause data loss but may cause irritation and frustration.

Warning

Warnings should not be ignored. Ignoring warnings will most likely cause data loss.

2. Getting Help and Giving Feedback

2.1. Do You Need Help?

If you experience difficulty with a procedure described in this documentation, visit the Red Hat Customer Portal at http://access.redhat.com. From the Customer Portal, you can:
  • Search or browse through a knowledge base of technical support articles about Red Hat products.
  • Submit a support case to Red Hat Global Support Services (GSS).
  • Access other product documentation.
Red Hat also hosts a large number of electronic mailing lists for discussion of Red Hat software and technology. You can find a list of publicly available mailing lists at https://www.redhat.com/mailman/listinfo. Click the name of any mailing list to subscribe to that list or to access the list archives.

2.2. We Need Feedback

If you find a typographical error in this manual, or if you have thought of a way to make this manual better, we would love to hear from you. Please submit a report in Bugzilla: http://bugzilla.redhat.com/ against the product Red Hat Enterprise Linux 6.
When submitting a bug report, be sure to mention the manual's identifier: doc-Resource_Management_Guide
If you have a suggestion for improving the documentation, try to be as specific as possible when describing it. If you have found an error, please include the section number and some of the surrounding text so we can find it easily.

Chapter 1. Introduction to Control Groups (Cgroups)

Red Hat Enterprise Linux 6 provides a new kernel feature: control groups, which are called by their shorter name cgroups in this guide. Cgroups allow you to allocate resources — such as CPU time, system memory, network bandwidth, or combinations of these resources — among user-defined groups of tasks (processes) running on a system. You can monitor the cgroups you configure, deny cgroups access to certain resources, and even reconfigure your cgroups dynamically on a running system. The cgconfig (control group config) service can be configured to start up at boot time and reestablish your predefined cgroups, thus making them persistent across reboots.
By using cgroups, system administrators gain fine-grained control over allocating, prioritizing, denying, managing, and monitoring system resources. Hardware resources can be appropriately divided up among tasks and users, increasing overall efficiency.

1.1. How Control Groups Are Organized

Cgroups are organized hierarchically, like processes, and child cgroups inherit some of the attributes of their parents. However, there are differences between the two models.

The Linux Process Model

All processes on a Linux system are child processes of a common parent: the init process, which is executed by the kernel at boot time and starts other processes (which may in turn start child processes of their own). Because all processes descend from a single parent, the Linux process model is a single hierarchy, or tree.
Additionally, every Linux process except init inherits the environment (such as the PATH variable)[1] and certain other attributes (such as open file descriptors) of its parent process.

The Cgroup Model

Cgroups are similar to processes in that:
  • they are hierarchical, and
  • child cgroups inherit certain attributes from their parent cgroup.
The fundamental difference is that many different hierarchies of cgroups can exist simultaneously on a system. If the Linux process model is a single tree of processes, then the cgroup model is one or more separate, unconnected trees of tasks (i.e. processes).
Multiple separate hierarchies of cgroups are necessary because each hierarchy is attached to one or more subsystems. A subsystem[2] represents a single resource, such as CPU time or memory. Red Hat Enterprise Linux 6 provides ten cgroup subsystems, listed below by name and function.

Available Subsystems in Red Hat Enterprise Linux

  • blkio — this subsystem sets limits on input/output access to and from block devices such as physical drives (disk, solid state, or USB).
  • cpu — this subsystem uses the scheduler to provide cgroup tasks access to the CPU.
  • cpuacct — this subsystem generates automatic reports on CPU resources used by tasks in a cgroup.
  • cpuset — this subsystem assigns individual CPUs (on a multicore system) and memory nodes to tasks in a cgroup.
  • devices — this subsystem allows or denies access to devices by tasks in a cgroup.
  • freezer — this subsystem suspends or resumes tasks in a cgroup.
  • memory — this subsystem sets limits on memory use by tasks in a cgroup and generates automatic reports on memory resources used by those tasks.
  • net_cls — this subsystem tags network packets with a class identifier (classid) that allows the Linux traffic controller (tc) to identify packets originating from a particular cgroup task.
  • net_prio — this subsystem provides a way to dynamically set the priority of network traffic per network interface.
  • ns — the namespace subsystem.
  • perf_event — this subsystem identifies cgroup membership of tasks and can be used for performance analysis.

Note

You may come across the term resource controller or simply controller in cgroup literature such as the man pages or kernel documentation. Both of these terms are synonymous with subsystem and arise from the fact that a subsystem typically schedules a resource or applies a limit to the cgroups in the hierarchy it is attached to.
The definition of a subsystem (resource controller) is quite general: it is something that acts upon a group of tasks, i.e. processes.

1.2. Relationships Between Subsystems, Hierarchies, Control Groups and Tasks

Remember that system processes are called tasks in cgroup terminology.
Here are a few simple rules governing the relationships between subsystems, hierarchies of cgroups, and tasks, along with explanations of the consequences of those rules.

Rule 1

A single hierarchy can have one or more subsystems attached to it.
As a consequence, the cpu and memory subsystems (or any number of subsystems) can be attached to a single hierarchy, as long as each one is not attached to any other hierarchy which has any other subsystems attached to it already (see Rule 2).
Rule 1

Figure 1.1. Rule 1

Rule 2

Any single subsystem (such as cpu) cannot be attached to more than one hierarchy if one of those hierarchies has a different subsystem attached to it already.
As a consequence, the cpu subsystem can never be attached to two different hierarchies if one of those hierarchies already has the memory subsystem attached to it. However, a single subsystem can be attached to two hierarchies if both of those hierarchies have only that subsystem attached.
Rule 2—The numbered bullets represent a time sequence in which the subsystems are attached.

Figure 1.2. Rule 2—The numbered bullets represent a time sequence in which the subsystems are attached.

Rule 3

Each time a new hierarchy is created on the systems, all tasks on the system are initially members of the default cgroup of that hierarchy, which is known as the root cgroup. For any single hierarchy you create, each task on the system can be a member of exactly one cgroup in that hierarchy. A single task may be in multiple cgroups, as long as each of those cgroups is in a different hierarchy. As soon as a task becomes a member of a second cgroup in the same hierarchy, it is removed from the first cgroup in that hierarchy. At no time is a task ever in two different cgroups in the same hierarchy.
As a consequence, if the cpu and memory subsystems are attached to a hierarchy named cpu_mem_cg, and the net_cls subsystem is attached to a hierarchy named net, then a running httpd process could be a member of any one cgroup in cpu_mem_cg, and any one cgroup in net.
The cgroup in cpu_mem_cg that the httpd process is a member of might restrict its CPU time to half of that allotted to other processes, and limit its memory usage to a maximum of 1024 MB. Additionally, the cgroup in net that the httpd process is a member of might limit its transmission rate to 30 MB/s (megabytes per second).
When the first hierarchy is created, every task on the system is a member of at least one cgroup: the root cgroup. When using cgroups, therefore, every system task is always in at least one cgroup.
Rule 3

Figure 1.3. Rule 3

Rule 4

Any process (task) on the system which forks itself creates a child task. A child task automatically inherits the cgroup membership of its parent but can be moved to different cgroups as needed. Once forked, the parent and child processes are completely independent.
As a consequence, consider the httpd task that is a member of the cgroup named half_cpu_1gb_max in the cpu_and_mem hierarchy, and a member of the cgroup trans_rate_30 in the net hierarchy. When that httpd process forks itself, its child process automatically becomes a member of the half_cpu_1gb_max cgroup, and the trans_rate_30 cgroup. It inherits the exact same cgroups its parent task belongs to.
From that point forward, the parent and child tasks are completely independent of each other: changing the cgroups that one task belongs to does not affect the other. Neither will changing cgroups of a parent task affect any of its grandchildren in any way. To summarize: any child task always initially inherits memberships to the exact same cgroups as their parent task, but those memberships can be changed or removed later.
Rule 4—The numbered bullets represent a time sequence in which the task forks.

Figure 1.4. Rule 4—The numbered bullets represent a time sequence in which the task forks.

1.3. Implications for Resource Management

  • Because a task can belong to only a single cgroup in any one hierarchy, there is only one way that a task can be limited or affected by any single subsystem. This is logical: a feature, not a limitation.
  • You can group several subsystems together so that they affect all tasks in a single hierarchy. Because cgroups in that hierarchy have different parameters set, those tasks will be affected differently.
  • It may sometimes be necessary to refactor a hierarchy. An example would be removing a subsystem from a hierarchy that has several subsystems attached, and attaching it to a new, separate hierarchy.
  • Conversely, if the need for splitting subsystems among separate hierarchies is reduced, you can remove a hierarchy and attach its subsystems to an existing one.
  • The design allows for simple cgroup usage, such as setting a few parameters for specific tasks in a single hierarchy, such as one with just the cpu and memory subsystems attached.
  • The design also allows for highly specific configuration: each task (process) on a system could be a member of each hierarchy, each of which has a single attached subsystem. Such a configuration would give the system administrator absolute control over all parameters for every single task.


[1] The parent process is able to alter the environment before passing it to a child process.
[2] You should be aware that subsystems are also called resource controllers, or simply controllers, in the libcgroup man pages and other documentation.

Chapter 2. Using Control Groups

As explained in Chapter 3, Subsystems and Tunable Parameters, control groups and the subsystems to which they relate can be manipulated using shell commands and utilities. However, the easiest way to work with cgroups is to install the libcgroup package, which contains a number of cgroup-related command line utilities and their associated man pages. It is possible to mount hierarchies and set cgroup parameters (non-persistently) using shell commands and utilities available on any system. However, using the libcgroup-provided utilities simplifies the process and extends your capabilities. Therefore, this guide focuses on libcgroup commands throughout. In most cases, we have included the equivalent shell commands to help describe the underlying mechanism. However, we recommend that you use the libcgroup commands wherever practical.

Note

In order to use cgroups, first ensure the libcgroup package is installed on your system by running, as root:
~]# yum install libcgroup

2.1. The cgconfig Service

The cgconfig service installed with the libcgroup package provides a convenient way to create hierarchies, attach subsystems to hierarchies, and manage cgroups within those hierarchies. It is recommended that you use cgconfig to manage hierarchies and cgroups on your system.
The cgconfig service is not started by default on Red Hat Enterprise Linux 6. When you start the service with chkconfig, it reads the cgroup configuration file — /etc/cgconfig.conf. Cgroups are therefore recreated from session to session and remain persistent. Depending on the contents of the configuration file, cgconfig can create hierarchies, mount necessary file systems, create cgroups, and set subsystem parameters for each group.
The default /etc/cgconfig.conf file installed with the libcgroup package creates and mounts an individual hierarchy for each subsystem, and attaches the subsystems to these hierarchies. The cgconfig service also allows to create configuration files in the /etc/cgconfig.d/ directory and to invoke them from /etc/cgconfig.conf.
If you stop the cgconfig service (with the service cgconfig stop command), it unmounts all the hierarchies that it mounted.

2.1.1. The /etc/cgconfig.conf File

The /etc/cgconfig.conf file contains two major types of entries — mount and group. Mount entries create and mount hierarchies as virtual file systems, and attach subsystems to those hierarchies. Mount entries are defined using the following syntax:
mount {
    subsystem = /cgroup/hierarchy;
    …
}
The libcgroup package automatically creates a default /etc/cgconfig.conf file when it is installed. The default configuration file looks as follows:
mount {
	cpuset	= /cgroup/cpuset;
	cpu	= /cgroup/cpu;
	cpuacct	= /cgroup/cpuacct;
	memory	= /cgroup/memory;
	devices	= /cgroup/devices;
	freezer	= /cgroup/freezer;
	net_cls	= /cgroup/net_cls;
	blkio	= /cgroup/blkio;
}
The subsystems listed in the above configuration are automatically mounted to their respective hierarchies under the /cgroup/ directory. It is recommended to use these default hierarchies for specifying control groups. However, in certain cases you may need to create hierarchies manually, for example when they were deleted before, or it is beneficial to have a single hierarchy for multiple subsystems (as in Section 4.3, “Per-group Division of CPU and Memory Resources”). Note that multiple subsystems can be mounted to a single hierarchy, but each subsystem can be mounted only once. See Example 2.1, “Creating a mount entry” for an example of creating a hierarchy.

Example 2.1. Creating a mount entry

The following example creates a hierarchy for the cpuset subsystem:
mount {
    cpuset = /cgroup/red;
}
the equivalent of the shell commands:
~]# mkdir /cgroup/red
~]# mount -t cgroup -o cpuset red /cgroup/red
Since each subsystem can be mounted only once, the above commands would fail if cpuset is already mounted.
Group entries create cgroups and set subsystem parameters. Group entries are defined using the following syntax:
group <name> {
    [<permissions>]
    <controller> {
        <param name> = <param value>;
        …
    }
    …
}
Note that the permissions section is optional. To define permissions for a group entry, use the following syntax:
perm {
    task {
        uid = <task user>;
        gid = <task group>;
    }
    admin {
       uid = <admin name>;
       gid = <admin group>;
    }
}

Example 2.2. Creating a group entry

The following example creates a cgroup for SQL daemons, with permissions for users in the sqladmin group to add tasks to the cgroup and the root user to modify subsystem parameters:
group daemons {
     cpuset {
         cpuset.mems = 0;
         cpuset.cpus = 0;
     }
}
group daemons/sql {
     perm {
         task {
             uid = root;
             gid = sqladmin;
         } admin {
             uid = root;
             gid = root;
         }
     }
     cpuset {
         cpuset.mems = 0;
         cpuset.cpus = 0;
     }
}
When combined with the example of the mount entry in Example 2.1, “Creating a mount entry”, the equivalent shell commands are:
~]# mkdir -p /cgroup/red/daemons/sql
~]# chown root:root /cgroup/red/daemons/sql/*
~]# chown root:sqladmin /cgroup/red/daemons/sql/tasks
~]# echo $(cgget -n -v -r cpuset.mems /) > /cgroup/red/daemons/cpuset.mems
~]# echo $(cgget -n -v -r cpuset.cpus /) > /cgroup/red/daemons/cpuset.cpus
~]# echo 0 > /cgroup/red/daemons/sql/cpuset.mems
~]# echo 0 > /cgroup/red/daemons/sql/cpuset.cpus

Note

You must restart the cgconfig service for the changes in the /etc/cgconfig.conf file to take effect. However, note that restarting this service causes the entire cgroup hierarchy to be rebuilt, which removes any previously existing cgroups (for example, any existing cgroups used by libvirtd). To restart the cgconfig service, use the following command:
~]# service cgconfig restart
When you install the libcgroup package, a sample configuration file is written to /etc/cgconfig.conf. The hash symbols ('#') at the start of each line comment that line out and make it invisible to the cgconfig service.

2.1.2. The /etc/cgconfig.d/ Directory

The /etc/cgconfig.d/ directory is reserved for storing configuration files for specific applications and use cases. These files should be created with the .conf suffix and adhere to the same syntax rules as /etc/cgconfig.conf.
The cgconfig service first parses the /etc/cgconfig.conf file and then continues with files in the /etc/cgconfig.d/ directory. Note that the order of file parsing is not defined, because it does not make a difference provided that each configuration file is unique. Therefore, do not define the same group or template in multiple configuration files, otherwise they would interfere with each other.
Storing specific configuration files in a separate directory makes them easily reusable. If an application is shipped with a dedicated configuration file, you can easily set up cgroups for the application just by copying its configuration file to /etc/cgconfig.d/.

2.2. Creating a Hierarchy and Attaching Subsystems

Warning

The following instructions, which cover creating a new hierarchy and attaching subsystems to it, assume that cgroups are not already configured on your system. In this case, these instructions will not affect the operation of the system. Changing the tunable parameters in a cgroup with tasks, however, can immediately affect those tasks. This guide alerts you the first time it illustrates changing a tunable cgroup parameter that can affect one or more tasks.
On a system on which cgroups are already configured (either manually, or by the cgconfig service) these commands fail unless you first unmount existing hierarchies, which affects the operation of the system. Do not experiment with these instructions on production systems.
To create a hierarchy and attach subsystems to it, edit the mount section of the /etc/cgconfig.conf file as root. Entries in the mount section have the following format:
subsystem = /cgroup/hierarchy;
When cgconfig next starts, it will create the hierarchy and attach the subsystems to it.
The following example creates a hierarchy called cpu_and_mem and attaches the cpu, cpuset, cpuacct, and memory subsystems to it.
mount {
    cpuset  = /cgroup/cpu_and_mem;
    cpu     = /cgroup/cpu_and_mem;
    cpuacct = /cgroup/cpu_and_mem;
    memory  = /cgroup/cpu_and_mem;
}

Alternative method

You can also use shell commands and utilities to create hierarchies and attach subsystems to them.
Create a mount point for the hierarchy as root. Include the name of the cgroup in the mount point:
~]# mkdir /cgroup/name
For example:
~]# mkdir /cgroup/cpu_and_mem
Next, use the mount command to mount the hierarchy and simultaneously attach one or more subsystems. For example:
~]# mount -t cgroup -o subsystems name /cgroup/name
Where subsystems is a comma‑separated list of subsystems and name is the name of the hierarchy. Brief descriptions of all available subsystems are listed in Available Subsystems in Red Hat Enterprise Linux, and Chapter 3, Subsystems and Tunable Parameters provides a detailed reference.

Example 2.3. Using the mount command to attach subsystems

In this example, a directory named /cgroup/cpu_and_mem already exists and will serve as the mount point for the hierarchy that you create. Attach the cpu, cpuset, and memory subsystems to a hierarchy named cpu_and_mem, and mount the cpu_and_mem hierarchy on /cgroup/cpu_and_mem:
~]# mount -t cgroup -o cpu,cpuset,memory cpu_and_mem /cgroup/cpu_and_mem
You can list all available subsystems along with their current mount points (i.e. where the hierarchy they are attached to is mounted) with the lssubsys [3] command:
~]# lssubsys -am
cpu,cpuset,memory /cgroup/cpu_and_mem
net_cls
ns
cpuacct
devices
freezer
blkio
This output indicates that:
  • the cpu, cpuset, and memory subsystems are attached to a hierarchy mounted on /cgroup/cpu_and_mem, and
  • the net_cls, ns, cpuacct, devices, freezer, and blkio subsystems are as yet unattached to any hierarchy, as illustrated by the lack of a corresponding mount point.

2.3. Attaching Subsystems to, and Detaching Them from, an Existing Hierarchy

To add a subsystem to an existing hierarchy, detach it from an existing hierarchy, or move it to a different hierarchy, edit the mount section of the /etc/cgconfig.conf file as root, using the same syntax described in Section 2.2, “Creating a Hierarchy and Attaching Subsystems”. When cgconfig next starts, it will reorganize the subsystems according to the hierarchies that you specify.

Alternative method

To add an unattached subsystem to an existing hierarchy, remount the hierarchy. Include the extra subsystem in the mount command, together with the remount option.

Example 2.4. Remounting a hierarchy to add a subsystem

The lssubsys command shows cpu, cpuset, and memory subsystems attached to the cpu_and_mem hierarchy:
~]# lssubsys -am
cpu,cpuset,memory /cgroup/cpu_and_mem
net_cls
ns
cpuacct
devices
freezer
blkio
Remount the cpu_and_mem hierarchy, using the remount option, and include cpuacct in the list of subsystems:
~]# mount -t cgroup -o remount,cpu,cpuset,cpuacct,memory cpu_and_mem /cgroup/cpu_and_mem
The lssubsys command now shows cpuacct attached to the cpu_and_mem hierarchy:
~]# lssubsys -am
cpu,cpuacct,cpuset,memory /cgroup/cpu_and_mem
net_cls
ns
devices
freezer
blkio
Analogously, you can detach a subsystem from an existing hierarchy by remounting the hierarchy and omitting the subsystem name from the -o options. For example, to then detach the cpuacct subsystem, simply remount and omit it:
~]# mount -t cgroup -o remount,cpu,cpuset,memory cpu_and_mem /cgroup/cpu_and_mem

2.4. Unmounting a Hierarchy

You can unmount a hierarchy of cgroups with the umount command:
~]# umount /cgroup/name
For example:
~]# umount /cgroup/cpu_and_mem
If the hierarchy is currently empty (that is, it contains only the root cgroup) the hierarchy is deactivated when it is unmounted. If the hierarchy contains any other cgroups, the hierarchy remains active in the kernel even though it is no longer mounted.
To remove a hierarchy, ensure that all child cgroups are removed before you unmount the hierarchy, or use the cgclear command which can deactivate a hierarchy even when it is not empty — refer to Section 2.12, “Unloading Control Groups”.

2.5. Creating Control Groups

Use the cgcreate command to create cgroups. The syntax for cgcreate is:
cgcreate -t uid:gid -a uid:gid -g subsystems:path
where:
  • -t (optional) — specifies a user (by user ID, uid) and a group (by group ID, gid) to own the tasks pseudofile for this cgroup. This user can add tasks to the cgroup.

    Note

    Note that the only way to remove a task from a cgroup is to move it to a different cgroup. To move a task, the user has to have write access to the destination cgroup; write access to the source cgroup is not necessary.
  • -a (optional) — specifies a user (by user ID, uid) and a group (by group ID, gid) to own all pseudofiles other than tasks for this cgroup. This user can modify access of the tasks in this cgroup to system resources.
  • -g — specifies the hierarchy in which the cgroup should be created, as a comma‑separated list of subsystems associated with hierarchies. If the subsystems in this list are in different hierarchies, the group is created in each of these hierarchies. The list of hierarchies is followed by a colon and the path to the child group relative to the hierarchy. Do not include the hierarchy mount point in the path.
    For example, the cgroup located in the directory /cgroup/cpu_and_mem/lab1/ is called just lab1 — its path is already uniquely determined because there is at most one hierarchy for a given subsystem. Note also that the group is controlled by all the subsystems that exist in the hierarchies in which the cgroup is created, even though these subsystems have not been specified in the cgcreate command — refer to Example 2.5, “cgcreate usage”.
Because all cgroups in the same hierarchy have the same controllers, the child group has the same controllers as its parent.

Example 2.5. cgcreate usage

Consider a system where the cpu and memory subsystems are mounted together in the cpu_and_mem hierarchy, and the net_cls controller is mounted in a separate hierarchy called net. Run the following command:
~]# cgcreate -g cpu,net_cls:/test-subgroup
The cgcreate command creates two groups named test-subgroup, one in the cpu_and_mem hierarchy and one in the net hierarchy. The test-subgroup group in the cpu_and_mem hierarchy is controlled by the memory subsystem, even though it was not specified in the cgcreate command.

Alternative method

To create a child of the cgroup directly, use the mkdir command:
~]# mkdir /cgroup/hierarchy/name/child_name
For example:
~]# mkdir /cgroup/cpu_and_mem/group1

2.6. Removing Control Groups

Use cgdelete to remove cgroups. It has similar syntax as cgcreate. Run the following command:
cgdelete subsystems:path
where:
  • subsystems is a comma‑separated list of subsystems.
  • path is the path to the cgroup relative to the root of the hierarchy.
For example:
~]# cgdelete cpu,net_cls:/test-subgroup
cgdelete can also recursively remove all subgroups with the option -r.
When you delete a cgroup, all its tasks move to its parent group.

2.7. Setting Parameters

Set subsystem parameters by running the cgset command from a user account with permission to modify the relevant cgroup. For example, if cpuset is mounted to /cgroup/cpu_and_mem/ and the /cgroup/cpu_and_mem/group1 subdirectory exists, specify the CPUs to which this group has access with the following command:
cpu_and_mem]# cgset -r cpuset.cpus=0-1 group1
The syntax for cgset is:
cgset -r parameter=value path_to_cgroup
where:
  • parameter is the parameter to be set, which corresponds to the file in the directory of the given cgroup.
  • value is the value for the parameter.
  • path_to_cgroup is the path to the cgroup relative to the root of the hierarchy. For example, to set the parameter of the root group (if the cpuacct subsystem is mounted to /cgroup/cpu_and_mem/), change to the /cgroup/cpu_and_mem/ directory, and run:
    cpu_and_mem]# cgset -r cpuacct.usage=0 /
    Alternatively, because . is relative to the root group (that is, the root group itself) you could also run:
    cpu_and_mem]# cgset -r cpuacct.usage=0 .
    Note, however, that / is the preferred syntax.

    Note

    Only a small number of parameters can be set for the root group (such as the cpuacct.usage parameter shown in the examples above). This is because a root group owns all of the existing resources, therefore, it would make no sense to limit all existing processes by defining certain parameters, for example the cpuset.cpu parameter.
    To set the parameter of group1, which is a subgroup of the root group, run:
    cpu_and_mem]# cgset -r cpuacct.usage=0 group1
    A trailing slash after the name of the group (for example, cpuacct.usage=0 group1/) is optional.
The values that you can set with cgset might depend on values set higher in a particular hierarchy. For example, if group1 is limited to use only CPU 0 on a system, you cannot set group1/subgroup1 to use CPUs 0 and 1, or to use only CPU 1.
You can also use cgset to copy the parameters of one cgroup into another existing cgroup. For example:
cpu_and_mem]# cgset --copy-from group1/ group2/
The syntax to copy parameters with cgset is:
cgset --copy-from path_to_source_cgroup path_to_target_cgroup
where:
  • path_to_source_cgroup is the path to the cgroup whose parameters are to be copied, relative to the root group of the hierarchy.
  • path_to_target_cgroup is the path to the destination cgroup, relative to the root group of the hierarchy.
Ensure that any mandatory parameters for the various subsystems are set before you copy parameters from one group to another, or the command will fail. For more information on mandatory parameters, refer to Important.

Alternative method

To set parameters in a cgroup directly, insert values into the relevant subsystem pseudofile using the echo command. In the following example, the echo command inserts the value of 0-1 into the cpuset.cpus pseudofile of the cgroup group1:
~]# echo 0-1 > /cgroup/cpu_and_mem/group1/cpuset.cpus
With this value in place, the tasks in this cgroup are restricted to CPUs 0 and 1 on the system.

2.8. Moving a Process to a Control Group

Move a process into a cgroup by running the cgclassify command, for example:
~]# cgclassify -g cpu,memory:group1 1701
The syntax for cgclassify is:
cgclassify -g subsystems:path_to_cgroup pidlist
where:
  • subsystems is a comma‑separated list of subsystems, or * to launch the process in the hierarchies associated with all available subsystems. Note that if cgroups of the same name exist in multiple hierarchies, the -g option moves the processes in each of those groups. Ensure that the cgroup exists within each of the hierarchies whose subsystems you specify here.
  • path_to_cgroup is the path to the cgroup within its hierarchies.
  • pidlist is a space-separated list of process identifier (PIDs).
If the -g option is not specified, cgclassify automatically searches the /etc/cgrules.conf file (see Section 2.8.1, “The cgred Service”) and uses the first applicable configuration line. According to this line, cgclassify determines the hierarchies and cgroups to move the process under. Note that for the move to be successful, the destination hierarchies must exist. The subsystems specified in /etc/cgrules.conf also have to be properly configured for the corresponding hierarchy in /etc/cgconfig.conf.
You can also add the --sticky option before the pid to keep any child processes in the same cgroup. If you do not set this option and the cgred service is running, child processes are allocated to cgroups based on the settings found in /etc/cgrules.conf. However, the parent process remains in the cgroup in which it was first started.
Using cgclassify, you can move several processes simultaneously. For example, this command moves the processes with PIDs 1701 and 1138 into cgroup group1/:
~]# cgclassify -g cpu,memory:group1 1701 1138
Note that the PIDs to be moved are separated by spaces and that the specified groups should be in different hierarchies.

Alternative method

To move a process into a cgroup directly, write its PID to the tasks file of the cgroup. For example, to move a process with the PID 1701 into a cgroup at /cgroup/cpu_and_mem/group1/:
~]# echo 1701 > /cgroup/cpu_and_mem/group1/tasks

2.8.1. The cgred Service

Cgred is a service (which starts the cgrulesengd service) that moves tasks into cgroups according to parameters set in the /etc/cgrules.conf file. Entries in the /etc/cgrules.conf file can take one of these two forms:
user subsystems control_group
user:command subsystems control_group
Replace user with a user name or a group name prefixed with the "@" character. Replace subsystems with a comma‑separated list of subsystem names, control_group represents a path to the cgroup, and command stands for a process name or a full command path of a process.
For example:
maria			devices		/usergroup/staff
This entry specifies that any processes that belong to the user named maria access the devices subsystem according to the parameters specified in the /usergroup/staff cgroup. To associate particular commands with particular cgroups, add the command parameter, as follows:
maria:ftp		devices		/usergroup/staff/ftp
The entry now specifies that when the user named maria uses the ftp command, the process is automatically moved to the /usergroup/staff/ftp cgroup in the hierarchy that contains the devices subsystem. Note, however, that the daemon moves the process to the cgroup only after the appropriate condition is fulfilled. Therefore, the ftp process might run for a short time in the wrong group. Furthermore, if the process quickly spawns children while in the wrong group, these children might not be moved.
Entries in the /etc/cgrules.conf file can include the following extra notation:
  • @ — indicates a group instead of an individual user. For example, @admins are all users in the admins group.
  • * — represents "all". For example, * in the subsystem field represents all mounted subsystems.
  • % — represents an item that is the same as the item on the line above.
For example, the entries specified in the /etc/cgrules.conf file can have the following form:
@adminstaff		devices		/admingroup
@labstaff		%		%
The above configuration ensures that processes owned by the adminstaff and labstaff access the devices subsystem according to the limits set in the admingroup cgroup.
Rules specified in /etc/cgrules.conf can be linked to templates configured either in the /etc/cgconfig.conf file or in configuration files stored in the /etc/cgconfig.d/ directory, allowing for flexible cgroup assignment and creation.
For example, specify the following template in /etc/cgconfig.conf:
template users/%g/%u {
                     cpuacct{
                     }
                     cpu {
                        cpu.shares = "1000";
                     }
          }
Then use the users/%g/%u template in the third row of a /etc/cgrules.conf entry, which can look as follows:
peter:ftp		cpu		users/%g/%u
The %g and %u variables used above are automatically replaced with group and user name depending on the owner of the ftp process. If the process belongs to peter from the adminstaff group, the above path is translated to users/adminstaff/peter. The cgred service then searches for this directory, and if it does not exist, cgred creates it and assigns the process to users/adminstaff/peter/tasks. Note that template rules apply only to definitions of templates in configuration files, so even if "group users/adminstaff/peter" was defined in /etc/cgconfig.conf, it would be ignored in favor of "template users/%g/%u".
There are several other variables that can be used for specifying cgroup paths in templates:
  • %u — is replaced with the name of the user who owns the current process. If name resolution fails, UID is used instead.
  • %U — is replaced with the UID of the specified user who owns the current process.
  • %g — is replaced with the name of the user group that owns the current process, or with the GID if name resolution fails.
  • %G — is replaced with the GID of the cgroup that owns the current process.
  • %p — is replaced with the name of the current process. PID is used in case of name resolution failure.
  • %P — is replaced with the PID of the current processes.

2.9. Starting a Process in a Control Group

Important

Some subsystems have mandatory parameters that must be set before you can move a task into a cgroup which uses any of those subsystems. For example, before you move a task into a cgroup which uses the cpuset subsystem, the cpuset.cpus and cpuset.mems parameters must be defined for that cgroup.
The examples in this section illustrate the correct syntax for the command, but only work on systems on which the relevant mandatory parameters have been set for any controllers used in the examples. If you have not already configured the relevant controllers, you cannot copy example commands directly from this section and expect them to work on your system.
Refer to Chapter 3, Subsystems and Tunable Parameters for a description of which parameters are mandatory for given subsystems.
Launch processes in a cgroup by running the cgexec command. For example, this command launches the firefox web browser within the group1 cgroup, subject to the limitations imposed on that group by the cpu subsystem:
~]# cgexec -g cpu:group1 firefox http://www.redhat.com
The syntax for cgexec is:
cgexec -g subsystems:path_to_cgroup command arguments
where:
  • subsystems is a comma‑separated list of subsystems, or * to launch the process in the hierarchies associated with all available subsystems. Note that, as with cgset described in Section 2.7, “Setting Parameters”, if cgroups of the same name exist in multiple hierarchies, the -g option creates processes in each of those groups. Ensure that the cgroup exists within each of the hierarchies whose subsystems you specify here.
  • path_to_cgroup is the path to the cgroup relative to the hierarchy.
  • command is the command to run.
  • arguments are any arguments for the command.
You can also add the --sticky option before the command to keep any child processes in the same cgroup. If you do not set this option and the cgred service is running, child processes will be allocated to cgroups based on the settings found in /etc/cgrules.conf. The process itself, however, will remain in the cgroup in which you started it.

Alternative method

When you start a new process, it inherits the group of its parent process. Therefore, an alternative method for starting a process in a particular cgroup is to move your shell process to that group (refer to Section 2.8, “Moving a Process to a Control Group”), and then launch the process from that shell. For example:
~]# echo $$ > /cgroup/cpu_and_mem/group1/tasks
~]# firefox
Note that after exiting firefox, your existing shell is still in the group1 cgroup. Therefore, an even better way would be:
~]# sh -c "echo \$$ > /cgroup/cpu_and_mem/group1/tasks && firefox"

2.9.1. Starting a Service in a Control Group

You can start certain services in a cgroup. Services that can be started in cgroups must:
  • use a /etc/sysconfig/servicename file
  • use the daemon() function from /etc/init.d/functions to start the service
To make an eligible service start in a cgroup, edit its file in the /etc/sysconfig directory to include an entry in the form CGROUP_DAEMON="subsystem:control_group" where subsystem is a subsystem associated with a particular hierarchy, and control_group is a cgroup in that hierarchy. For example:
CGROUP_DAEMON="cpuset:group1"
If cpuset is mounted to /cgroup/cpu_and_mem/, the above configuration translates to /cgroup/cpu_and_mem/group1.

2.9.2. Process Behavior in the Root Control Group

Certain blkio and cpu configuration options affect processes (tasks) running in the root cgroup in a different way than those in a subgroup. Consider the following example:
  1. Create two subgroups under one root group: /rootgroup/red/ and /rootgroup/blue/
  2. In each subgroup and in the root group, define the cpu.shares configuration option and set it to 1.
In the scenario configured above, one process placed in each group (that is, one task in /rootgroup/tasks, /rootgroup/red/tasks and /rootgroup/blue/tasks) consumes 33.33% of the CPU:
/rootgroup/ process:      33.33%
/rootgroup/blue/ process: 33.33%
/rootgroup/red/ process:  33.33%
Any other processes placed in subgroups blue and red result in the 33.33% percent of the CPU assigned to that specific subgroup to be split among the multiple processes in that subgroup.
However, multiple processes placed in the root group cause the CPU resource to be split per process, rather than per group. For example, if /rootgroup/ contains three processes, /rootgroup/red/ contains one process and /rootgroup/blue/ contains one process, and the cpu.shares option is set to 1 in all groups, the CPU resource is divided as follows:
/rootgroup/ processes:    20% + 20% + 20%
/rootgroup/blue/ process: 20%
/rootgroup/red/ process:  20%
Therefore, it is recommended to move all processes from the root group to a specific subgroup when using the blkio and cpu configuration options which divide an available resource based on a weight or a share (for example, cpu.shares or blkio.weight). To move all tasks from the root group into a specific subgroup, you can use the following commands:
rootgroup]# cat tasks >> red/tasks
rootgroup]# echo > tasks

2.10. Generating the /etc/cgconfig.conf File

Configuration for the /etc/cgconfig.conf file can be generated from the current cgroup configuration using the cgsnapshot utility. This utility takes a snapshot of the current state of all subsystems and their cgroups and returns their configuration as it would appear in the /etc/cgconfig.conf file. Example 2.6, “Using the cgsnapshot utility” shows an example usage of the cgsnapshot utility.

Example 2.6. Using the cgsnapshot utility

Configure cgroups on the system using the following commands:
~]# mkdir /cgroup/cpu
~]# mount -t cgroup -o cpu cpu /cgroup/cpu
~]# mkdir /cgroup/cpu/lab1
~]# mkdir /cgroup/cpu/lab2
~]# echo 2 > /cgroup/cpu/lab1/cpu.shares
~]# echo 3 > /cgroup/cpu/lab2/cpu.shares
~]# echo 5000000 > /cgroup/cpu/lab1/cpu.rt_period_us
~]# echo 4000000 > /cgroup/cpu/lab1/cpu.rt_runtime_us
~]# mkdir /cgroup/cpuacct
~]# mount -t cgroup -o cpuacct cpuacct /cgroup/cpuacct
The above commands mounted two subsystems and created two cgroups, for the cpu subsystem, with specific values for some of their parameters. Executing the cgsnapshot command (with the -s option and an empty /etc/cgsnapshot_blacklist.conf file[4]) then produces the following output:
~]$ cgsnapshot -s
# Configuration file generated by cgsnapshot
mount {
        cpu = /cgroup/cpu;
        cpuacct = /cgroup/cpuacct;
}

group lab2 {
        cpu {
                cpu.rt_period_us="1000000";
                cpu.rt_runtime_us="0";
                cpu.shares="3";
        }
}

group lab1 {
        cpu {
                cpu.rt_period_us="5000000";
                cpu.rt_runtime_us="4000000";
                cpu.shares="2";
        }
}
The -s option used in the example above tells cgsnapshot to ignore all warnings in the output file caused by parameters not being defined in the blacklist or whitelist of the cgsnapshot utility. For more information on parameter blacklisting, refer to Section 2.10.1, “Blacklisting Parameters”. For more information on parameter whitelisting, refer to Section 2.10.2, “Whitelisting Parameters”.
By default, the output generated by cgsnapshot is returned on the standard output. Use the -f option to specify a file to which the output should be redirected. For example:
~]$ cgsnapshot -f ~/test/cgconfig_test.conf

Warning

When using the -f option, note that it overwrites any content in the file you specify. Therefore, it is recommended not to direct the output straight to the /etc/cgconfig.conf file.
The cgsnapshot utility can also create configuration files per subsystem. By specifying the name of a subsystem, the output will consist of the corresponding configuration for that subsystem:
~]$ cgsnapshot cpuacct
# Configuration file generated by cgsnapshot
mount {
        cpuacct = /cgroup/cpuacct;
}

2.10.1. Blacklisting Parameters

The cgsnapshot utility allows parameter blacklisting. If a parameter is blacklisted, it does not appear in the output generated by cgsnapshot. By default, the /etc/cgsnapshot_blacklist.conf file is checked for blacklisted parameters. If a parameter is not present in the blacklist, the whitelist is checked. To specify a different blacklist, use the -b option. For example:
~]$ cgsnapshot -b ~/test/my_blacklist.conf

2.10.2. Whitelisting Parameters

The cgsnapshot utility also allows parameter whitelisting. If a parameter is whitelisted, it appears in the output generated by cgsnapshot. If a parameter is neither blacklisted or whitelisted, a warning appears informing of this:
~]$ cgsnapshot -f ~/test/cgconfig_test.conf
WARNING: variable cpu.rt_period_us is neither blacklisted nor whitelisted
WARNING: variable cpu.rt_runtime_us is neither blacklisted nor whitelisted
By default, there is no whitelist configuration file. To specify which file to use as a whitelist, use the -w option. For example:
~]$ cgsnapshot -w ~/test/my_whitelist.conf
Specifying the -t option tells cgsnapshot to generate a configuration with parameters from the whitelist only.

2.11. Obtaining Information About Control Groups

There are several ways to find and monitor control groups, subsystems, and hierarchies configured on your system.

2.11.1. Finding a Process

To find the cgroup to which a process belongs, run:
~]$ ps -O cgroup
Or, if you know the PID for the process, run:
~]$ cat /proc/PID/cgroup
where PID stands for a PID of the inspected process.

2.11.2. Finding a Subsystem

To find out what subsystems are available in your kernel and how they are mounted together into hierarchies, run:
~]$ cat /proc/cgroups
#subsys_name    hierarchy       num_cgroups     enabled
cpuset  2       1       1
ns      0       1       1
cpu     3       1       1
cpuacct 4       1       1
memory  5       1       1
devices 6       1       1
freezer 7       1       1
net_cls 8       1       1
blkio   9       3       1
perf_event      0       1       1
net_prio        0       1       1
In the example output above, the hierarchy column lists IDs of the existing hierarchies on the system. Subsystems with the same hierarchy ID are attached to the same hierarchy. The num_cgroup column lists the number of existing cgroups in the hierarchy that uses a particular subsystem. The enabled column reports the value of 1 if a particular subsystem is enabled, or 0 if it is not.
Or, to find the mount points of particular subsystems, run:
~]$ lssubsys -m subsystems
where subsystems is a list of the subsystems in which you are interested. Note that the lssubsys -m command returns only the top-level mount point per each hierarchy.

2.11.3. Finding Hierarchies

It is recommended that you mount hierarchies under the /cgroup/ directory. Assuming this is the case on your system, list or browse the contents of that directory to obtain a list of hierarchies. If the tree utility is installed on your system, run it to obtain an overview of all hierarchies and the cgroups within them:
~]$ tree /cgroup

2.11.4. Finding Control Groups

To list the cgroups on a system, run:
~]$ lscgroup
You can restrict the output to a specific hierarchy by specifying a controller and path in the format controller:path. For example:
~]$ lscgroup cpuset:group1
lists only subgroups of the group1 cgroup in the hierarchy to which the cpuset subsystem is attached.

2.11.5. Displaying Parameters of Control Groups

To display the parameters of specific cgroups, run:
~]$ cgget -r parameter list_of_cgroups
where parameter is a pseudofile that contains values for a subsystem, and list_of_cgroups is a list of cgroups separated with spaces. For example:
~]$ cgget -r cpuset.cpus -r memory.limit_in_bytes group1 group2
displays the values of cpuset.cpus and memory.limit_in_bytes for cgroups group1 and group2.
If you do not know the names of the parameters themselves, use a command like:
~]$ cgget -g cpuset /

2.12. Unloading Control Groups

Warning

The cgclear command destroys all cgroups in all hierarchies. If you do not have these hierarchies stored in a configuration file, you will not be able to readily reconstruct them.
To clear an entire cgroup file system, use the cgclear command.
All tasks in the cgroup are reallocated to the root node of the hierarchies, all cgroups are removed, and the file system itself is unmounted from the system, destroying all previously mounted hierarchies. Finally, the directory where the cgroup file system was mounted is removed.

Note

Using the mount command to create cgroups (as opposed to creating them using the cgconfig service) results in the creation of an entry in the /etc/mtab file (the mounted file systems table). This change is also reflected in the /proc/mounts file. However, the unloading of cgroups with the cgclear command, along with other cgconfig commands, uses a direct kernel interface which does not reflect its changes into the /etc/mtab file and only writes the new information into the /proc/mounts file. After unloading cgroups with the cgclear command, the unmounted cgroups can still be visible in the /etc/mtab file, and, consequently, displayed when the mount command is executed. Refer to the /proc/mounts file for an accurate listing of all mounted cgroups.

2.13. Using the Notification API

The cgroups notification API allows user space applications to receive notifications about the changing status of a cgroup. Currently, the notification API only supports monitoring of the Out of Memory (OOM) control file: memory.oom_control. To create a notification handler, write a C program using the following instructions:
  1. Using the eventfd() function, create a file descriptor for event notifications. For more information, refer to the eventfd(2) man page.
  2. To monitor the memory.oom_control file, open it using the open() function. For more information, refer to the open(2) man page.
  3. Use the write() function to write the following arguments to the cgroup.event_control file of the cgroup whose memory.oom_control file you are monitoring:
    <event_file_descriptor> <OOM_control_file_descriptor>
    where:
    • event_file_descriptor is used to open the cgroup.event_control file,
    • and OOM_control_file_descriptor is used to open the respective memory.oom_control file.
    For more information on writing to a file, refer to the write(1) man page.
When the above program is started, it will be notified of any OOM situation in the cgroup it is monitoring. Note that OOM notifications only work in non-root cgroups.
For more information on the memory.oom_control tunable parameter, refer to Section 3.7, “memory”. For more information on configuring notifications for OOM control, refer to Example 3.3, “OOM Control and Notifications”.

2.14. Additional Resources

The ultimate documentation for cgroup commands is available on the manual pages provided with the libcgroup package. The section numbers are specified in the list of man pages below.

The libcgroup Man Pages

  • man 1 cgclassify — the cgclassify command is used to move running tasks to one or more cgroups.
    man 1 cgclear — the cgclear command is used to delete all cgroups in a hierarchy.
    man 5 cgconfig.conf — cgroups are defined in the cgconfig.conf file.
    man 8 cgconfigparser — the cgconfigparser command parses the cgconfig.conf file and mounts hierarchies.
    man 1 cgcreate — the cgcreate command creates new cgroups in hierarchies.
    man 1 cgdelete — the cgdelete command removes specified cgroups.
    man 1 cgexec — the cgexec command runs tasks in specified cgroups.
    man 1 cgget — the cgget command displays cgroup parameters.
    man 1 cgsnapshot — the cgsnapshot command generates a configuration file from existing subsystems.
    man 5 cgred.confcgred.conf is the configuration file for the cgred service.
    man 5 cgrules.confcgrules.conf contains the rules used for determining when tasks belong to certain cgroups.
    man 8 cgrulesengd — the cgrulesengd service distributes tasks to cgroups.
    man 1 cgset — the cgset command sets parameters for a cgroup.
    man 1 lscgroup — the lscgroup command lists the cgroups in a hierarchy.
    man 1 lssubsys — the lssubsys command lists the hierarchies containing the specified subsystems.


[3] The lssubsys command is one of the utilities provided by the libcgroup package. You have to install libcgroup to use it: refer to Chapter 2, Using Control Groups if you are unable to run lssubsys.
[4] The cpu.shares parameter is specified in the /etc/cgsnapshot_blacklist.conf file by default, which would cause it to be omitted in the generated output in Example 2.6, “Using the cgsnapshot utility”. Thus, for the purposes of the example, an empty /etc/cgsnapshot_blacklist.conf file is used.

Chapter 3. Subsystems and Tunable Parameters

Subsystems are kernel modules that are aware of cgroups. Typically, they are resource controllers that allocate varying levels of system resources to different cgroups. However, subsystems could be programmed for any other interaction with the kernel where the need exists to treat different groups of processes differently. The application programming interface (API) to develop new subsystems is documented in cgroups.txt in the kernel documentation, installed on your system at /usr/share/doc/kernel-doc-kernel-version/Documentation/cgroups/ (provided by the kernel-doc package). The latest version of the cgroups documentation is also available online at http://www.kernel.org/doc/Documentation/cgroup-v1/cgroups.txt. Note, however, that the features in the latest documentation might not match those available in the kernel installed on your system.
State objects that contain the subsystem parameters for a cgroup are represented as pseudofiles within the cgroup virtual file system. These pseudofiles can be manipulated by shell commands or their equivalent system calls. For example, cpuset.cpus is a pseudofile that specifies which CPUs a cgroup is permitted to access. If /cgroup/cpuset/webserver is a cgroup for the web server that runs on a system, and the following command is executed,
~]# echo 0,2 > /cgroup/cpuset/webserver/cpuset.cpus
the value 0,2 is written to the cpuset.cpus pseudofile and therefore limits any tasks whose PIDs are listed in /cgroup/cpuset/webserver/tasks to use only CPU 0 and CPU 2 on the system.

3.1. blkio

The Block I/O (blkio) subsystem controls and monitors access to I/O on block devices by tasks in cgroups. Writing values to some of these pseudofiles limits access or bandwidth, and reading values from some of these pseudofiles provides information on I/O operations.
The blkio subsystem offers two policies for controlling access to I/O:
  • Proportional weight division — implemented in the Completely Fair Queuing (CFQ) I/O scheduler, this policy allows you to set weights to specific cgroups. This means that each cgroup has a set percentage (depending on the weight of the cgroup) of all I/O operations reserved. For more information, refer to Section 3.1.1, “Proportional Weight Division Tunable Parameters”
  • I/O throttling (Upper limit) — this policy is used to set an upper limit for the number of I/O operations performed by a specific device. This means that a device can have a limited rate of read or write operations. For more information, refer to Section 3.1.2, “I/O Throttling Tunable Parameters”

Important

Currently, the Block I/O subsystem does not work for buffered write operations. It is primarily targeted at direct I/O, although it works for buffered read operations.

3.1.1. Proportional Weight Division Tunable Parameters

blkio.weight
specifies the relative proportion (weight) of block I/O access available by default to a cgroup, in the range from 100 to 1000. This value is overridden for specific devices by the blkio.weight_device parameter. For example, to assign a default weight of 500 to a cgroup for access to block devices, run:
~]# echo 500 > blkio.weight
blkio.weight_device
specifies the relative proportion (weight) of I/O access on specific devices available to a cgroup, in the range from 100 to 1000. The value of this parameter overrides the value of the blkio.weight parameter for the devices specified. Values take the format of major:minor weight, where major and minor are device types and node numbers specified in Linux Allocated Devices, otherwise known as the Linux Devices List and available from http://www.kernel.org/doc/Documentation/devices.txt. For example, to assign a weight of 500 to a cgroup for access to /dev/sda, run:
~]# echo 8:0 500 > blkio.weight_device
In the Linux Allocated Devices notation, 8:0 represents /dev/sda.

3.1.2. I/O Throttling Tunable Parameters

blkio.throttle.read_bps_device
specifies the upper limit on the number of read operations a device can perform. The rate of the read operations is specified in bytes per second. Entries have three fields: major, minor, and bytes_per_second. Major and minor are device types and node numbers specified in Linux Allocated Devices, and bytes_per_second is the upper limit rate at which read operations can be performed. For example, to allow the /dev/sda device to perform read operations at a maximum of 10 MBps, run:
~]# echo "8:0 10485760" > /cgroup/blkio/test/blkio.throttle.read_bps_device
blkio.throttle.read_iops_device
specifies the upper limit on the number of read operations a device can perform. The rate of the read operations is specified in operations per second. Entries have three fields: major, minor, and operations_per_second. Major and minor are device types and node numbers specified in Linux Allocated Devices, and operations_per_second is the upper limit rate at which read operations can be performed. For example, to allow the /dev/sda device to perform a maximum of 10 read operations per second, run:
~]# echo "8:0 10" > /cgroup/blkio/test/blkio.throttle.read_iops_device
blkio.throttle.write_bps_device
specifies the upper limit on the number of write operations a device can perform. The rate of the write operations is specified in bytes per second. Entries have three fields: major, minor, and bytes_per_second. Major and minor are device types and node numbers specified in Linux Allocated Devices, and bytes_per_second is the upper limit rate at which write operations can be performed. For example, to allow the /dev/sda device to perform write operations at a maximum of 10 MBps, run:
~]# echo "8:0 10485760" > /cgroup/blkio/test/blkio.throttle.write_bps_device
blkio.throttle.write_iops_device
specifies the upper limit on the number of write operations a device can perform. The rate of the write operations is specified in operations per second. Entries have three fields: major, minor, and operations_per_second. Major and minor are device types and node numbers specified in Linux Allocated Devices, and operations_per_second is the upper limit rate at which write operations can be performed. For example, to allow the /dev/sda device to perform a maximum of 10 write operations per second, run:
~]# echo "8:0 10" > /cgroup/blkio/test/blkio.throttle.write_iops_device
blkio.throttle.io_serviced
reports the number of I/O operations performed on specific devices by a cgroup as seen by the throttling policy. Entries have four fields: major, minor, operation, and number. Major and minor are device types and node numbers specified in Linux Allocated Devices, operation represents the type of operation (read, write, sync, or async) and number represents the number of operations.
blkio.throttle.io_service_bytes
reports the number of bytes transferred to or from specific devices by a cgroup. The only difference between blkio.io_service_bytes and blkio.throttle.io_service_bytes is that the former is not updated when the CFQ scheduler is operating on a request queue. Entries have four fields: major, minor, operation, and bytes. Major and minor are device types and node numbers specified in Linux Allocated Devices, operation represents the type of operation (read, write, sync, or async) and bytes is the number of transferred bytes.

3.1.3. blkio Common Tunable Parameters

The following parameters may be used for either of the policies listed in Section 3.1, “blkio”.
blkio.reset_stats
resets the statistics recorded in the other pseudofiles. Write an integer to this file to reset the statistics for this cgroup.
blkio.time
reports the time that a cgroup had I/O access to specific devices. Entries have three fields: major, minor, and time. Major and minor are device types and node numbers specified in Linux Allocated Devices, and time is the length of time in milliseconds (ms).
blkio.sectors
reports the number of sectors transferred to or from specific devices by a cgroup. Entries have three fields: major, minor, and sectors. Major and minor are device types and node numbers specified in Linux Allocated Devices, and sectors is the number of disk sectors.
blkio.avg_queue_size
reports the average queue size for I/O operations by a cgroup, over the entire length of time of the group's existence. The queue size is sampled every time a queue for this cgroup receives a timeslice. Note that this report is available only if CONFIG_DEBUG_BLK_CGROUP=y is set on the system.
blkio.group_wait_time
reports the total time (in nanoseconds — ns) a cgroup spent waiting for a timeslice for one of its queues. The report is updated every time a queue for this cgroup gets a timeslice, so if you read this pseudofile while the cgroup is waiting for a timeslice, the report will not contain time spent waiting for the operation currently queued. Note that this report is available only if CONFIG_DEBUG_BLK_CGROUP=y is set on the system.
blkio.empty_time
reports the total time (in nanoseconds — ns) a cgroup spent without any pending requests. The report is updated every time a queue for this cgroup has a pending request, so if you read this pseudofile while the cgroup has no pending requests, the report will not contain time spent in the current empty state. Note that this report is available only if CONFIG_DEBUG_BLK_CGROUP=y is set on the system.
blkio.idle_time
reports the total time (in nanoseconds — ns) the scheduler spent idling for a cgroup in anticipation of a better request than the requests already in other queues or from other groups. The report is updated every time the group is no longer idling, so if you read this pseudofile while the cgroup is idling, the report will not contain time spent in the current idling state. Note that this report is available only if CONFIG_DEBUG_BLK_CGROUP=y is set on the system.
blkio.dequeue
reports the number of times requests for I/O operations by a cgroup were dequeued by specific devices. Entries have three fields: major, minor, and number. Major and minor are device types and node numbers specified in Linux Allocated Devices, and number is the number of times requests by the group were dequeued. Note that this report is available only if CONFIG_DEBUG_BLK_CGROUP=y is set on the system.
blkio.io_serviced
reports the number of I/O operations performed on specific devices by a cgroup as seen by the CFQ scheduler. Entries have four fields: major, minor, operation, and number. Major and minor are device types and node numbers specified in Linux Allocated Devices, operation represents the type of operation (read, write, sync, or async) and number represents the number of operations.
blkio.io_service_bytes
reports the number of bytes transferred to or from specific devices by a cgroup as seen by the CFQ scheduler. Entries have four fields: major, minor, operation, and bytes. Major and minor are device types and node numbers specified in Linux Allocated Devices, operation represents the type of operation (read, write, sync, or async) and bytes is the number of transferred bytes.
blkio.io_service_time
reports the total time between request dispatch and request completion for I/O operations on specific devices by a cgroup as seen by the CFQ scheduler. Entries have four fields: major, minor, operation, and time. Major and minor are device types and node numbers specified in Linux Allocated Devices, operation represents the type of operation (read, write, sync, or async) and time is the length of time in nanoseconds (ns). The time is reported in nanoseconds rather than a larger unit so that this report is meaningful even for solid-state devices.
blkio.io_wait_time
reports the total time I/O operations on specific devices by a cgroup spent waiting for service in the scheduler queues. When you interpret this report, note:
  • the time reported can be greater than the total time elapsed, because the time reported is the cumulative total of all I/O operations for the cgroup rather than the time that the cgroup itself spent waiting for I/O operations. To find the time that the group as a whole has spent waiting, use the blkio.group_wait_time parameter.
  • if the device has a queue_depth > 1, the time reported only includes the time until the request is dispatched to the device, not any time spent waiting for service while the device reorders requests.
Entries have four fields: major, minor, operation, and time. Major and minor are device types and node numbers specified in Linux Allocated Devices, operation represents the type of operation (read, write, sync, or async) and time is the length of time in nanoseconds (ns). The time is reported in nanoseconds rather than a larger unit so that this report is meaningful even for solid-state devices.
blkio.io_merged
reports the number of BIOS requests merged into requests for I/O operations by a cgroup. Entries have two fields: number and operation. Number is the number of requests, and operation represents the type of operation (read, write, sync, or async).
blkio.io_queued
reports the number of requests queued for I/O operations by a cgroup. Entries have two fields: number and operation. Number is the number of requests, and operation represents the type of operation (read, write, sync, or async).

3.1.4. Example Usage

Refer to Example 3.1, “blkio proportional weight division” for a simple test of running two dd threads in two different cgroups with various blkio.weight values.

Example 3.1. blkio proportional weight division

  1. Mount the blkio subsystem:
    ~]# mount -t cgroup -o blkio blkio /cgroup/blkio/
  2. Create two cgroups for the blkio subsystem:
    ~]# mkdir /cgroup/blkio/test1/
    ~]# mkdir /cgroup/blkio/test2/
  3. Set blkio weights in the previously created cgroups:
    ~]# echo 1000 > /cgroup/blkio/test1/blkio.weight
    ~]# echo 500 > /cgroup/blkio/test2/blkio.weight
  4. Create two large files:
    ~]# dd if=/dev/zero of=file_1 bs=1M count=4000
    ~]# dd if=/dev/zero of=file_2 bs=1M count=4000
    The above commands create two files (file_1 and file_2) of size 4 GB.
  5. For each of the test cgroups, execute a dd command (which reads the contents of a file and outputs it to the null device) on one of the large files:
    ~]# cgexec -g blkio:test1 time dd if=file_1 of=/dev/null
    ~]# cgexec -g blkio:test2 time dd if=file_2 of=/dev/null
    Both commands will output their completion time once they have finished.
  6. Simultaneously with the two running dd threads, you can monitor the performance in real time by using the iotop utility. To install the iotop utility, execute, as root, the yum install iotop command. The following is an example of the output as seen in the iotop utility while running the previously started dd threads:
    Total DISK READ: 83.16 M/s | Total DISK WRITE: 0.00 B/s
        TIME  TID  PRIO  USER     DISK READ  DISK WRITE  SWAPIN      IO    COMMAND
    15:18:04 15071 be/4 root       27.64 M/s    0.00 B/s  0.00 % 92.30 % dd if=file_2 of=/dev/null
    15:18:04 15069 be/4 root       55.52 M/s    0.00 B/s  0.00 % 88.48 % dd if=file_1 of=/dev/null
    
In order to get the most accurate result in Example 3.1, “blkio proportional weight division”, prior to the execution of the dd commands, flush all file system buffers and free pagecache, dentries and inodes using the following commands:
~]# sync
~]# echo 3 > /proc/sys/vm/drop_caches
Additionally, you can enable group isolation which provides stronger isolation between groups at the expense of throughput. When group isolation is disabled, fairness can be expected only for a sequential workload. By default, group isolation is enabled and fairness can be expected for random I/O workloads as well. To enable group isolation, use the following command:
~]# echo 1 > /sys/block/<disk_device>/queue/iosched/group_isolation
where <disk_device> stands for the name of the desired device, for example sda.

3.2. cpu

The cpu subsystem schedules CPU access to cgroups. Access to CPU resources can be scheduled using two schedulers:
  • Completely Fair Scheduler (CFS) — a proportional share scheduler which divides the CPU time (CPU bandwidth) proportionately between groups of tasks (cgroups) depending on the priority/weight of the task or shares assigned to cgroups. For more information about resource limiting using CFS, refer to Section 3.2.1, “CFS Tunable Parameters”.
  • Real-Time scheduler (RT) — a task scheduler that provides a way to specify the amount of CPU time that real-time tasks can use. For more information about resource limiting of real-time tasks, refer to Section 3.2.2, “RT Tunable Parameters”.

3.2.1. CFS Tunable Parameters

In CFS, a cgroup can get more than its share of CPU if there are enough idle CPU cycles available in the system, due to the work conserving nature of the scheduler. This is usually the case for cgroups that consume CPU time based on relative shares. Ceiling enforcement can be used for cases when a hard limit on the amount of CPU that a cgroup can utilize is required (that is, tasks cannot use more than a set amount of CPU time).
The following options can be used to configure ceiling enforcement or relative sharing of CPU:

Ceiling Enforcement Tunable Parameters

cpu.cfs_period_us
specifies a period of time in microseconds (µs, represented here as "us") for how regularly a cgroup's access to CPU resources should be reallocated. If tasks in a cgroup should be able to access a single CPU for 0.2 seconds out of every 1 second, set cpu.cfs_quota_us to 200000 and cpu.cfs_period_us to 1000000. The upper limit of the cpu.cfs_quota_us parameter is 1 second and the lower limit is 1000 microseconds.
cpu.cfs_quota_us
specifies the total amount of time in microseconds (µs, represented here as "us") for which all tasks in a cgroup can run during one period (as defined by cpu.cfs_period_us). As soon as tasks in a cgroup use up all the time specified by the quota, they are throttled for the remainder of the time specified by the period and not allowed to run until the next period. If tasks in a cgroup should be able to access a single CPU for 0.2 seconds out of every 1 second, set cpu.cfs_quota_us to 200000 and cpu.cfs_period_us to 1000000. Note that the quota and period parameters operate on a CPU basis. To allow a process to fully utilize two CPUs, for example, set cpu.cfs_quota_us to 200000 and cpu.cfs_period_us to 100000.
Setting the value in cpu.cfs_quota_us to -1 indicates that the cgroup does not adhere to any CPU time restrictions. This is also the default value for every cgroup (except the root cgroup).
cpu.stat
reports CPU time statistics using the following values:
  • nr_periods — number of period intervals (as specified in cpu.cfs_period_us) that have elapsed.
  • nr_throttled — number of times tasks in a cgroup have been throttled (that is, not allowed to run because they have exhausted all of the available time as specified by their quota).
  • throttled_time — the total time duration (in nanoseconds) for which tasks in a cgroup have been throttled.

Relative Shares Tunable Parameters

cpu.shares
contains an integer value that specifies a relative share of CPU time available to the tasks in a cgroup. For example, tasks in two cgroups that have cpu.shares set to 100 will receive equal CPU time, but tasks in a cgroup that has cpu.shares set to 200 receive twice the CPU time of tasks in a cgroup where cpu.shares is set to 100. The value specified in the cpu.shares file must be 2 or higher.
Note that shares of CPU time are distributed per all CPU cores on multi-core systems. Even if a cgroup is limited to less than 100% of CPU on a multi-core system, it may use 100% of each individual CPU core. Consider the following example: if cgroup A is configured to use 25% and cgroup B 75% of the CPU, starting four CPU-intensive processes (one in A and three in B) on a system with four cores results in the following division of CPU shares:

Table 3.1. CPU share division

PIDcgroupCPUCPU share
100A0100% of CPU0
101B1100% of CPU1
102B2100% of CPU2
103B3100% of CPU3
Using relative shares to specify CPU access has two implications on resource management that should be considered:
  • Because the CFS does not demand equal usage of CPU, it is hard to predict how much CPU time a cgroup will be allowed to utilize. When tasks in one cgroup are idle and are not using any CPU time, the leftover time is collected in a global pool of unused CPU cycles. Other cgroups are allowed to borrow CPU cycles from this pool.
  • The actual amount of CPU time that is available to a cgroup can vary depending on the number of cgroups that exist on the system. If a cgroup has a relative share of 1000 and two other cgroups have a relative share of 500, the first cgroup receives 50% of all CPU time in cases when processes in all cgroups attempt to use 100% of the CPU. However, if another cgroup is added with a relative share of 1000, the first cgroup is only allowed 33% of the CPU (the rest of the cgroups receive 16.5%, 16.5%, and 33% of CPU).

3.2.2. RT Tunable Parameters

The RT scheduler works in a similar way to the ceiling enforcement control of the CFS (for more information, refer to Section 3.2.1, “CFS Tunable Parameters”) but limits CPU access to real-time tasks only. The amount of time for which real-time task can access the CPU is decided by allocating a run time and a period for each cgroup. All tasks in a cgroup are then allowed to access the CPU for the defined period of time for one run time (for example, tasks in a cgroup can be allowed to run 0.1 seconds in every 1 second). Please note that the following parameters take into account all available logical CPUs, therefore the access times specified with these parameters are in fact multiplied by the number of CPUs.
cpu.rt_period_us
applicable to real-time scheduling tasks only, this parameter specifies a period of time in microseconds (µs, represented here as "us") for how regularly a cgroup's access to CPU resources is reallocated.
cpu.rt_runtime_us
applicable to real-time scheduling tasks only, this parameter specifies a period of time in microseconds (µs, represented here as "us") for the longest continuous period in which the tasks in a cgroup have access to CPU resources. Establishing this limit prevents tasks in one cgroup from monopolizing CPU time. As mentioned above, the access times are multiplied by the number of logical CPUs. For example, setting cpu.rt_runtime_us to 200000 and cpu.rt_period_us to 1000000 translates to the task being able to access a single CPU for 0.4 seconds out of every 1 second on systems with two CPUs (0.2 x 2), or 0.8 seconds on systems with four CPUs (0.2 x 4).

3.2.3. Example Usage

Example 3.2. Limiting CPU access

The following examples assume you have an existing hierarchy of cgroups configured and the cpu subsystem mounted on your system:
  • To allow one cgroup to use 25% of a single CPU and a different cgroup to use 75% of that same CPU, use the following commands:
    ~]# echo 250 > /cgroup/cpu/blue/cpu.shares
    ~]# echo 750 > /cgroup/cpu/red/cpu.shares
  • To limit a cgroup to fully utilize a single CPU, use the following commands:
    ~]# echo 10000 > /cgroup/cpu/red/cpu.cfs_quota_us
    ~]# echo 10000 > /cgroup/cpu/red/cpu.cfs_period_us
  • To limit a cgroup to utilize 10% of a single CPU, use the following commands:
    ~]# echo 10000 > /cgroup/cpu/red/cpu.cfs_quota_us
    ~]# echo 100000 > /cgroup/cpu/red/cpu.cfs_period_us
  • On a multi-core system, to allow a cgroup to fully utilize two CPU cores, use the following commands:
    ~]# echo 200000 > /cgroup/cpu/red/cpu.cfs_quota_us
    ~]# echo 100000 > /cgroup/cpu/red/cpu.cfs_period_us

3.3. cpuacct

The CPU Accounting (cpuacct) subsystem generates automatic reports on CPU resources used by the tasks in a cgroup, including tasks in child groups. Three reports are available:
cpuacct.usage
reports the total CPU time (in nanoseconds) consumed by all tasks in this cgroup (including tasks lower in the hierarchy).

Note

To reset the value in cpuacct.usage, execute the following command:
~]# echo 0 > /cgroup/cpuacct/cpuacct.usage
The above command also resets values in cpuacct.usage_percpu.
cpuacct.stat
reports the user and system CPU time consumed by all tasks in this cgroup (including tasks lower in the hierarchy) in the following way:
  • user — CPU time consumed by tasks in user mode.
  • system — CPU time consumed by tasks in system (kernel) mode.
CPU time is reported in the units defined by the USER_HZ variable.
cpuacct.usage_percpu
reports the CPU time (in nanoseconds) consumed on each CPU by all tasks in this cgroup (including tasks lower in the hierarchy).

3.4. cpuset

The cpuset subsystem assigns individual CPUs and memory nodes to cgroups. Each cpuset can be specified according to the following parameters, each one in a separate pseudofile within the cgroup virtual file system:

Important

Some subsystems have mandatory parameters that must be set before you can move a task into a cgroup which uses any of those subsystems. For example, before you move a task into a cgroup which uses the cpuset subsystem, the cpuset.cpus and cpuset.mems parameters must be defined for that cgroup.
cpuset.cpus (mandatory)
specifies the CPUs that tasks in this cgroup are permitted to access. This is a comma-separated list, with dashes ("-") to represent ranges. For example,
0-2,16
represents CPUs 0, 1, 2, and 16.
cpuset.mems (mandatory)
specifies the memory nodes that tasks in this cgroup are permitted to access. This is a comma-separated list in the ASCII format, with dashes ("-") to represent ranges. For example,
0-2,16
represents memory nodes 0, 1, 2, and 16.
cpuset.memory_migrate
contains a flag (0 or 1) that specifies whether a page in memory should migrate to a new node if the values in cpuset.mems change. By default, memory migration is disabled (0) and pages stay on the node to which they were originally allocated, even if the node is no longer among the nodes specified in cpuset.mems. If enabled (1), the system migrates pages to memory nodes within the new parameters specified by cpuset.mems, maintaining their relative placement if possible — for example, pages on the second node on the list originally specified by cpuset.mems are allocated to the second node on the new list specified by cpuset.mems, if the place is available.
cpuset.cpu_exclusive
contains a flag (0 or 1) that specifies whether cpusets other than this one and its parents and children can share the CPUs specified for this cpuset. By default (0), CPUs are not allocated exclusively to one cpuset.
cpuset.mem_exclusive
contains a flag (0 or 1) that specifies whether other cpusets can share the memory nodes specified for the cpuset. By default (0), memory nodes are not allocated exclusively to one cpuset. Reserving memory nodes for the exclusive use of a cpuset (1) is functionally the same as enabling a memory hardwall with the cpuset.mem_hardwall parameter.
cpuset.mem_hardwall
contains a flag (0 or 1) that specifies whether kernel allocations of memory page and buffer data should be restricted to the memory nodes specified for the cpuset. By default (0), page and buffer data is shared across processes belonging to multiple users. With a hardwall enabled (1), each tasks' user allocation can be kept separate.
cpuset.memory_pressure
a read-only file that contains a running average of the memory pressure created by the processes in the cpuset. The value in this pseudofile is automatically updated when cpuset.memory_pressure_enabled is enabled, otherwise, the pseudofile contains the value 0.
cpuset.memory_pressure_enabled
contains a flag (0 or 1) that specifies whether the system should compute the memory pressure created by the processes in the cgroup. Computed values are output to cpuset.memory_pressure and represent the rate at which processes attempt to free in-use memory, reported as an integer value of attempts to reclaim memory per second, multiplied by 1000.
cpuset.memory_spread_page
contains a flag (0 or 1) that specifies whether file system buffers should be spread evenly across the memory nodes allocated to the cpuset. By default (0), no attempt is made to spread memory pages for these buffers evenly, and buffers are placed on the same node on which the process that created them is running.
cpuset.memory_spread_slab
contains a flag (0 or 1) that specifies whether kernel slab caches for file input/output operations should be spread evenly across the cpuset. By default (0), no attempt is made to spread kernel slab caches evenly, and slab caches are placed on the same node on which the process that created them is running.
cpuset.sched_load_balance
contains a flag (0 or 1) that specifies whether the kernel will balance loads across the CPUs in the cpuset. By default (1), the kernel balances loads by moving processes from overloaded CPUs to less heavily used CPUs.
Note, however, that setting this flag in a cgroup has no effect if load balancing is enabled in any parent cgroup, as load balancing is already being carried out at a higher level. Therefore, to disable load balancing in a cgroup, disable load balancing also in each of its parents in the hierarchy. In this case, you should also consider whether load balancing should be enabled for any siblings of the cgroup in question.
cpuset.sched_relax_domain_level
contains an integer between -1 and a small positive value, which represents the width of the range of CPUs across which the kernel should attempt to balance loads. This value is meaningless if cpuset.sched_load_balance is disabled.
The precise effect of this value varies according to system architecture, but the following values are typical:
Values of cpuset.sched_relax_domain_level
ValueEffect
-1 Use the system default value for load balancing
0 Do not perform immediate load balancing; balance loads only periodically
1 Immediately balance loads across threads on the same core
2 Immediately balance loads across cores in the same package or book (in case of s390x architectures)
3 Immediately balance loads across books in the same package (available only for s390x architectures)
4 Immediately balance loads across CPUs on the same node or blade
5 Immediately balance loads across several CPUs on architectures with non-uniform memory access (NUMA)
6 Immediately balance loads across all CPUs on architectures with NUMA

Note

With the release of Red Hat Enterprise Linux 6.1 the BOOK scheduling domain has been added to the list of supported domain levels. This change affected the meaning of cpuset.sched_relax_domain_level values. Please note that the effect of values from 3 to 5 changed. For example, to get the old effect of value 3, which was "Immediately balance loads across CPUs on the same node or blade" the value 4 needs to be selected. Similarly, the old 4 is now 5, and the old 5 is now 6.

3.5. devices

The devices subsystem allows or denies access to devices by tasks in a cgroup.

Important

The Device Whitelist (devices) subsystem is considered to be a Technology Preview in Red Hat Enterprise Linux 6.
Technology Preview features are currently not supported under Red Hat Enterprise Linux 6 subscription services, might not be functionally complete and are generally not suitable for production use. However, Red Hat includes these features in the operating system as a customer convenience and to increase the features' exposure. You might find these features useful in a non-production environment and are also welcome to provide feedback and functionality suggestions for a Technology Preview feature before it becomes fully supported.
devices.allow
specifies devices to which tasks in a cgroup have access. Each entry has four fields: type, major, minor, and access. The values used in the type, major, and minor fields correspond to device types and node numbers specified in Linux Allocated Devices, otherwise known as the Linux Devices List and available from http://www.kernel.org/doc/Documentation/devices.txt.
type
type can have one of the following three values:
  • a — applies to all devices, both character devices and block devices
  • b — specifies a block device
  • c — specifies a character device
major, minor
major and minor are device node numbers specified by Linux Allocated Devices. The major and minor numbers are separated by a colon. For example, 8 is the major number that specifies Small Computer System Interface (SCSI) disk drives, and the minor number 1 specifies the first partition on the first SCSI disk drive; therefore 8:1 fully specifies this partition, corresponding to a file system location of /dev/sda1.
* can stand for all major or all minor device nodes, for example 9:* (all RAID devices) or *:* (all devices).
access
access is a sequence of one or more of the following letters:
  • r — allows tasks to read from the specified device
  • w — allows tasks to write to the specified device
  • m — allows tasks to create device files that do not yet exist
For example, when access is specified as r, tasks can only read from the specified device, but when access is specified as rw, tasks can read from and write to the device.
devices.deny
specifies devices that tasks in a cgroup cannot access. The syntax of entries is identical with devices.allow.
devices.list
reports the devices for which access controls have been set for tasks in this cgroup.

3.6. freezer

The freezer subsystem suspends or resumes tasks in a cgroup.
freezer.state
freezer.state is only available in non-root cgroups and has three possible values:
  • FROZEN — tasks in the cgroup are suspended.
  • FREEZING — the system is in the process of suspending tasks in the cgroup.
  • THAWED — tasks in the cgroup have resumed.
To suspend a specific process:
  1. Move that process to a cgroup in a hierarchy which has the freezer subsystem attached to it.
  2. Freeze that particular cgroup to suspend the process contained in it.
It is not possible to move a process into a suspended (frozen) cgroup.
Note that while the FROZEN and THAWED values can be written to freezer.state, FREEZING cannot be written, only read.

3.7. memory

The memory subsystem generates automatic reports on memory resources used by the tasks in a cgroup, and sets limits on memory use of those tasks:

Note

By default, the memory subsystem uses 40 bytes of memory per physical page on x86_64 systems. These resources are consumed even if memory is not used in any hierarchy. If you do not plan to use the memory subsystem, you can disable it to reduce the resource consumption of the kernel.
To permanently disable the memory subsystem, open the /boot/grub/grub.conf configuration file as root and append the following text to the line that starts with the kernel keyword:
cgroup_disable=memory
For more information on working with /boot/grub/grub.conf, see the Configuring the GRUB Boot Loader chapter in the Red Hat Enterprise Linux 6 Deployment Guide.
To temporarily disable the memory subsystem for a single session, perform the following steps when starting the system:
  1. At the GRUB boot screen, press any key to enter the GRUB interactive menu.
  2. Select Red Hat Enterprise Linux with the version of the kernel that you want to boot and press the a key to modify the kernel parameters.
  3. Type cgroup_disable=memory at the end of the line and press Enter to exit GRUB edit mode.
With cgroup_disable=memory enabled, memory is not visible as an individually mountable subsystem and it is not automatically mounted when mounting all cgroups in a single hierarchy. Please note that memory is currently the only subsystem that can be effectively disabled with cgroup_disable to save resources. Using this option with other subsystems only disables their usage, but does not cut their resource consumption. However, other subsystems do not consume as much resources as the memory subsystem.
The following tunable parameters are available for the memory subsystem:
memory.stat
reports a wide range of memory statistics, as described in the following table:

Table 3.2. Values reported by memory.stat

StatisticDescription
cache page cache, including tmpfs (shmem), in bytes
rss anonymous and swap cache, not including tmpfs (shmem), in bytes
mapped_file size of memory-mapped mapped files, including tmpfs (shmem), in bytes
pgpgin number of pages paged into memory
pgpgout number of pages paged out of memory
swap swap usage, in bytes
active_anon anonymous and swap cache on active least-recently-used (LRU) list, including tmpfs (shmem), in bytes
inactive_anon anonymous and swap cache on inactive LRU list, including tmpfs (shmem), in bytes
active_file file-backed memory on active LRU list, in bytes
inactive_file file-backed memory on inactive LRU list, in bytes
unevictable memory that cannot be reclaimed, in bytes
hierarchical_memory_limit memory limit for the hierarchy that contains the memory cgroup, in bytes
hierarchical_memsw_limit memory plus swap limit for the hierarchy that contains the memory cgroup, in bytes
Additionally, each of these files other than hierarchical_memory_limit and hierarchical_memsw_limit has a counterpart prefixed total_ that reports not only on the cgroup, but on all its children as well. For example, swap reports the swap usage by a cgroup and total_swap reports the total swap usage by the cgroup and all its child groups.
When you interpret the values reported by memory.stat, note how the various statistics inter-relate:
  • active_anon + inactive_anon = anonymous memory + file cache for tmpfs + swap cache
    Therefore, active_anon + inactive_anonrss, because rss does not include tmpfs.
  • active_file + inactive_file = cache - size of tmpfs
memory.usage_in_bytes
reports the total current memory usage by processes in the cgroup (in bytes).
memory.memsw.usage_in_bytes
reports the sum of current memory usage plus swap space used by processes in the cgroup (in bytes).
memory.max_usage_in_bytes
reports the maximum memory used by processes in the cgroup (in bytes).
memory.memsw.max_usage_in_bytes
reports the maximum amount of memory and swap space used by processes in the cgroup (in bytes).
memory.limit_in_bytes
sets the maximum amount of user memory (including file cache). If no units are specified, the value is interpreted as bytes. However, it is possible to use suffixes to represent larger units — k or K for kilobytes, m or M for megabytes, and g or G for gigabytes. For example, to set the limit to 1 gigabyte, execute:
~]# echo 1G > /cgroup/memory/lab1/memory.limit_in_bytes
You cannot use memory.limit_in_bytes to limit the root cgroup; you can only apply values to groups lower in the hierarchy.
Write -1 to memory.limit_in_bytes to remove any existing limits.
memory.memsw.limit_in_bytes
sets the maximum amount for the sum of memory and swap usage. If no units are specified, the value is interpreted as bytes. However, it is possible to use suffixes to represent larger units — k or K for kilobytes, m or M for megabytes, and g or G for gigabytes.
You cannot use memory.memsw.limit_in_bytes to limit the root cgroup; you can only apply values to groups lower in the hierarchy.
Write -1 to memory.memsw.limit_in_bytes to remove any existing limits.

Important

It is important to set the memory.limit_in_bytes parameter before setting the memory.memsw.limit_in_bytes parameter: attempting to do so in the reverse order results in an error. This is because memory.memsw.limit_in_bytes becomes available only after all memory limitations (previously set in memory.limit_in_bytes) are exhausted.
Consider the following example: setting memory.limit_in_bytes = 2G and memory.memsw.limit_in_bytes = 4G for a certain cgroup will allow processes in that cgroup to allocate 2 GB of memory and, once exhausted, allocate another 2 GB of swap only. The memory.memsw.limit_in_bytes parameter represents the sum of memory and swap. Processes in a cgroup that does not have the memory.memsw.limit_in_bytes parameter set can potentially use up all the available swap (after exhausting the set memory limitation) and trigger an Out Of Memory situation caused by the lack of available swap.
The order in which the memory.limit_in_bytes and memory.memsw.limit_in_bytes parameters are set in the /etc/cgconfig.conf file is important as well. The following is a correct example of such a configuration:
memory {
    memory.limit_in_bytes = 1G;
    memory.memsw.limit_in_bytes = 1G;
}
memory.failcnt
reports the number of times that the memory limit has reached the value set in memory.limit_in_bytes.
memory.memsw.failcnt
reports the number of times that the memory plus swap space limit has reached the value set in memory.memsw.limit_in_bytes.
memory.soft_limit_in_bytes
enables flexible sharing of memory. Under normal circumstances, control groups are allowed to use as much of the memory as needed, constrained only by their hard limits set with the memory.limit_in_bytes parameter. However, when the system detects memory contention or low memory, control groups are forced to restrict their consumption to their soft limits. To set the soft limit for example to 256 MB, execute:
~]# echo 256M > /cgroup/memory/lab1/memory.soft_limit_in_bytes
This parameter accepts the same suffixes as memory.limit_in_bytes to represent units. To have any effect, the soft limit must be set below the hard limit. If lowering the memory usage to the soft limit does not solve the contention, cgroups are pushed back as much as possible to make sure that one control group does not starve the others of memory. Note that soft limits take effect over a long period of time, since they involve reclaiming memory for balancing between memory cgroups.
memory.force_empty
when set to 0, empties memory of all pages used by tasks in the cgroup. This interface can only be used when the cgroup has no tasks. If memory cannot be freed, it is moved to a parent cgroup if possible. Use the memory.force_empty parameter before removing a cgroup to avoid moving out-of-use page caches to its parent cgroup.
memory.swappiness
sets the tendency of the kernel to swap out process memory used by tasks in this cgroup instead of reclaiming pages from the page cache. This is the same tendency, calculated the same way, as set in /proc/sys/vm/swappiness for the system as a whole. The default value is 60. Values lower than 60 decrease the kernel's tendency to swap out process memory, values greater than 60 increase the kernel's tendency to swap out process memory, and values greater than 100 permit the kernel to swap out pages that are part of the address space of the processes in this cgroup.
Note that a value of 0 does not prevent process memory being swapped out; swap out might still happen when there is a shortage of system memory because the global virtual memory management logic does not read the cgroup value. To lock pages completely, use mlock() instead of cgroups.
You cannot change the swappiness of the following groups:
  • the root cgroup, which uses the swappiness set in /proc/sys/vm/swappiness.
  • a cgroup that has child groups below it.
memory.move_charge_at_immigrate
allows moving charges associated with a task along with task migration. Charging is a way of giving a penalty to cgroups which access shared pages too often. These penalties, also called charges, are by default not moved when a task migrates from one cgroup to another. The pages allocated from the original cgroup still remain charged to it; the charge is dropped when the page is freed or reclaimed.
With memory.move_charge_at_immigrate enabled, the pages associated with a task are taken from the old cgroup and charged to the new cgroup. The following example shows how to enable memory.move_charge_at_immigrate:
~]# echo 1 > /cgroup/memory/lab1/memory.move_charge_at_immigrate
Charges are moved only when the moved task is a leader of a thread group. If there is not enough memory for the task in the destination cgroup, an attempt to reclaim memory is performed. If the reclaim is not successful, the task migration is aborted.
To disable memory.move_charge_at_immigrate, execute:
~]# echo 0 > /cgroup/memory/lab1/memory.move_charge_at_immigrate
memory.use_hierarchy
contains a flag (0 or 1) that specifies whether memory usage should be accounted for throughout a hierarchy of cgroups. If enabled (1), the memory subsystem reclaims memory from the children of and process that exceeds its memory limit. By default (0), the subsystem does not reclaim memory from a task's children.
memory.oom_control
contains a flag (0 or 1) that enables or disables the Out of Memory killer for a cgroup. If enabled (0), tasks that attempt to consume more memory than they are allowed are immediately killed by the OOM killer. The OOM killer is enabled by default in every cgroup using the memory subsystem; to disable it, write 1 to the memory.oom_control file:
      ~]# echo 1 > /cgroup/memory/lab1/memory.oom_control
When the OOM killer is disabled, tasks that attempt to use more memory than they are allowed are paused until additional memory is freed.
The memory.oom_control file also reports the OOM status of the current cgroup under the under_oom entry. If the cgroup is out of memory and tasks in it are paused, the under_oom entry reports the value 1.
The memory.oom_control file is capable of reporting an occurrence of an OOM situation using the notification API. For more information, refer to Section 2.13, “Using the Notification API” and Example 3.3, “OOM Control and Notifications”.

3.7.1. Example Usage

Example 3.3. OOM Control and Notifications

The following example demonstrates how the OOM killer takes action when a task in a cgroup attempts to use more memory than allowed, and how a notification handler can report OOM situations:
  1. Attach the memory subsystem to a hierarchy and create a cgroup:
    ~]# mount -t memory -o memory memory /cgroup/memory
    ~]# mkdir /cgroup/memory/blue
  2. Set the amount of memory which tasks in the blue cgroup can use to 100 MB:
    ~]# echo 104857600 > memory.limit_in_bytes
  3. Change into the blue directory and make sure the OOM killer is enabled:
    ~]# cd /cgroup/memory/blue
    blue]# cat memory.oom_control
    oom_kill_disable 0
    under_oom 0
    
  4. Move the current shell process into the tasks file of the blue cgroup so that all other processes started in this shell are automatically moved to the blue cgroup:
    blue]# echo $$ > tasks
  5. Start a test program that attempts to allocate a large amount of memory exceeding the limit you set in step 2. As soon as the blue cgroup runs out of free memory, the OOM killer kills the test program and reports Killed to the standard output:
    blue]# ~/mem-hog
    Killed
    
    The following is an example of such a test program[5]:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <unistd.h>
    
    #define KB (1024)
    #define MB (1024 * KB)
    #define GB (1024 * MB)
    
    int main(int argc, char *argv[])
    {
    	char *p;
    
    again:
    	while ((p = (char *)malloc(GB)))
    		memset(p, 0, GB);
    
    	while ((p = (char *)malloc(MB)))
    		memset(p, 0, MB);
    
    	while ((p = (char *)malloc(KB)))
    		memset(p, 0,
    				KB);
    
    	sleep(1);
    
    	goto again;
    
    	return 0;
    }
    
  6. Disable the OOM killer and rerun the test program. This time, the test program remains paused waiting for additional memory to be freed:
    blue]# echo 1 > memory.oom_control
    blue]# ~/mem-hog
  7. While the test program is paused, note that the under_oom state of the cgroup has changed to indicate that the cgroup is out of available memory:
    ~]# cat /cgroup/memory/blue/memory.oom_control
    oom_kill_disable 1
    under_oom 1
    
    Reenabling the OOM killer immediately kills the test program.
  8. To receive notifications about every OOM situation, create a program as specified in Section 2.13, “Using the Notification API”. For example[6]:
    #include <sys/types.h>
    #include <sys/stat.h>
    #include <fcntl.h>
    #include <sys/eventfd.h>
    #include <errno.h>
    #include <string.h>
    #include <stdio.h>
    #include <stdlib.h>
    
    static inline void die(const char *msg)
    {
    	fprintf(stderr, "error: %s: %s(%d)\n", msg, strerror(errno), errno);
    	exit(EXIT_FAILURE);
    }
    
    static inline void usage(void)
    {
    	fprintf(stderr, "usage: oom_eventfd_test <cgroup.event_control> <memory.oom_control>\n");
    	exit(EXIT_FAILURE);
    }
    
    #define BUFSIZE 256
    
    int main(int argc, char *argv[])
    {
    	char buf[BUFSIZE];
    	int efd, cfd, ofd, rb, wb;
    	uint64_t u;
    
    	if (argc != 3)
    		usage();
    
    	if ((efd = eventfd(0, 0)) == -1)
    		die("eventfd");
    
    	if ((cfd = open(argv[1], O_WRONLY)) == -1)
    		die("cgroup.event_control");
    
    	if ((ofd = open(argv[2], O_RDONLY)) == -1)
    		die("memory.oom_control");
    
    	if ((wb = snprintf(buf, BUFSIZE, "%d %d", efd, ofd)) >= BUFSIZE)
    		die("buffer too small");
    
    	if (write(cfd, buf, wb) == -1)
    		die("write cgroup.event_control");
    
    	if (close(cfd) == -1)
    		die("close cgroup.event_control");
    
    	for (;;) {
    		if (read(efd, &u, sizeof(uint64_t)) != sizeof(uint64_t))
    			die("read eventfd");
    
    		printf("mem_cgroup oom event received\n");
    	}
    
    	return 0;
    }
    
    The above program detects OOM situations in a cgroup specified as an argument on the command line and reports them using the mem_cgroup oom event received string to the standard output.
  9. Run the above notification handler program in a separate console, specifying the blue cgroup's control files as arguments:
    ~]$ ./oom_notification /cgroup/memory/blue/cgroup.event_control /cgroup/memory/blue/memory.oom_control
  10. In a different console, run the mem_hog test program to create an OOM situation to see the oom_notification program report it in the standard output:
    blue]# ~/mem-hog

3.8. net_cls

The net_cls subsystem tags network packets with a class identifier (classid) that allows the Linux traffic controller (tc) to identify packets originating from a particular cgroup. The traffic controller can be configured to assign different priorities to packets from different cgroups.
net_cls.classid
net_cls.classid contains a single value that indicates a traffic control handle. The value of classid read from the net_cls.classid file is presented in the decimal format while the value to be written to the file is expected in the hexadecimal format. For example, 0x100001 represents the handle conventionally written as 10:1 in the format used by the ip utility. In the net_cls.classid file, it would be represented by the number 1048577.
The format for these handles is: 0xAAAABBBB, where AAAA is the major number in hexadecimal and BBBB is the minor number in hexadecimal. You can omit any leading zeroes; 0x10001 is the same as 0x00010001, and represents 1:1. The following is an example of setting a 10:1 handle in the net_cls.classid file:
~]# echo 0x100001 > /cgroup/net_cls/red/net_cls.classid
~]# cat /cgroup/net_cls/red/net_cls.classid
1048577
Refer to the man page for tc to learn how to configure the traffic controller to use the handles that the net_cls adds to network packets.

3.9. net_prio

The Network Priority (net_prio) subsystem provides a way to dynamically set the priority of network traffic per each network interface for applications within various cgroups. A network's priority is a number assigned to network traffic and used internally by the system and network devices. Network priority is used to differentiate packets that are sent, queued, or dropped. The tc command may be used to set a network's priority (setting the network priority via the tc command is outside the scope of this guide; for more information, refer to the tc man page).
Typically, an application sets the priority of its traffic via the SO_PRIORITY socket option. However, applications are often not coded to set the priority value, or the application's traffic is site-specific and does not provide a defined priority.
Using the net_prio subsystem in a cgroup allows the administrator to assign a process to a specific cgroup which defines the priority of outgoing traffic on a given network interface.

Note

The net_prio subsystem is not compiled in the kernel like the rest of the subsystems, rather it is a module that has to be loaded before attempting to mount it. To load this module, type as root:
modprobe netprio_cgroup
The following tunable parameters are available for the memory subsystem.
net_prio.prioidx
a read-only file which contains a unique integer value that the kernel uses as an internal representation of this cgroup.
net_prio.ifpriomap
contains a map of priorities assigned to traffic originating from processes in this group and leaving the system on various interfaces. This map is represented by a list of pairs in the form <network_interface> <priority>:
~]# cat /cgroup/net_prio/iscsi/net_prio.ifpriomap
eth0 5
eth1 4
eth2 6
Contents of the net_prio.ifpriomap file can be modified by echoing a string into the file using the above format, for example:
~]# echo "eth0 5" > /cgroup/net_prio/iscsi/net_prio.ifpriomap
The above command forces any traffic originating from processes belonging to the iscsi net_prio cgroup, and with traffic outgoing on the eth0 network interface, to have the priority set to the value of 5. The parent cgroup also has a writable net_prio.ifpriomap file that can be used to set a system default priority.

3.10. ns

The ns subsystem provides a way to group processes into separate namespaces. Within a particular namespace, processes can interact with each other but are isolated from processes running in other namespaces. These separate namespaces are sometimes referred to as containers when used for operating-system-level virtualization.

3.11. perf_event

When the perf_event subsystem is attached to a hierarchy, all cgroups in that hierarchy can be used to group processes and threads which can then be monitored with the perf tool, as opposed to monitoring each process or thread separately or per-CPU. Cgroups which use the perf_event subsystem do not contain any special tunable parameters other than the common parameters listed in Section 3.12, “Common Tunable Parameters”.
For additional information on how tasks in a cgroup can be monitored using the perf tool, refer to the Red Hat Enterprise Linux Developer Guide, available from http://access.redhat.com/site/documentation/Red_Hat_Enterprise_Linux/.

3.12. Common Tunable Parameters

The following parameters are present in every created cgroup, regardless of the subsystem that the cgroup is using:
tasks
contains a list of processes, represented by their PIDs, that are running in a cgroup. The list of PIDs is not guaranteed to be ordered or unique (that is, it may contain duplicate entries). Writing a PID into the tasks file of a cgroup moves that process into that cgroup.
cgroup.procs
contains a list of thread groups, represented by their TGIDs, that are running in a cgroup. The list of TGIDs is not guaranteed to be ordered or unique (that is, it may contain duplicate entries). Writing a TGID into the cgroup.procs file of a cgroup moves that thread group into that cgroup.
cgroup.event_control
along with the cgroup notification API, allows notifications to be sent about a changing status of a cgroup.
notify_on_release
contains a Boolean value, 1 or 0, that either enables or disables the execution of the release agent. If the notify_on_release parameter is enabled, the kernel executes the contents of the release_agent file when a cgroup no longer contains any tasks (that is, the cgroup's tasks file contained some PIDs and those PIDs were removed, leaving the file empty). A path to the empty cgroup is provided as an argument to the release agent.
The default value of the notify_on_release parameter in the root cgroup is 0. All non-root cgroups inherit the value in notify_on_release from their parent cgroup.
release_agent (present in the root cgroup only)
contains a command to be executed when a notify on release is triggered. Once a cgroup is emptied of all processes, and the notify_on_release flag is enabled, the kernel runs the command in the release_agent file and supplies it with a relative path (relative to the root cgroup) to the emptied cgroup as an argument. The release agent can be used, for example, to automatically remove empty cgroups; for more information, see Example 3.4, “Automatically removing empty cgroups”.

Example 3.4. Automatically removing empty cgroups

Follow these steps to configure automatic removal of any emptied cgroup from the cpu cgroup:
  1. Create a shell script that removes empty cpu cgroups, place it in, for example, /usr/local/bin, and make it executable.
    ~]# cat /usr/local/bin/remove-empty-cpu-cgroup.sh
    #!/bin/sh
    rmdir /cgroup/cpu/$1
    ~]# chmod +x /usr/local/bin/remove-empty-cpu-cgroup.sh
    The $1 variable contains a relative path to the emptied cgroup.
  2. In the cpu cgroup, enable the notify_on_release flag:
    ~]# echo 1 > /cgroup/cpu/notify_on_release
  3. In the cpu cgroup, specify a release agent to be used:
    ~]# echo "/usr/local/bin/remove-empty-cpu-cgroup.sh" > /cgroup/cpu/release_agent
  4. Test your configuration to make sure emptied cgroups are properly removed:
    cpu]# pwd; ls
    /cgroup/cpu
    cgroup.event_control  cgroup.procs  cpu.cfs_period_us  cpu.cfs_quota_us  cpu.rt_period_us  cpu.rt_runtime_us  cpu.shares  cpu.stat  libvirt  notify_on_release  release_agent  tasks
    cpu]# cat notify_on_release 
    1
    cpu]# cat release_agent 
    /usr/local/bin/remove-empty-cpu-cgroup.sh
    cpu]# mkdir blue; ls
    blue  cgroup.event_control  cgroup.procs  cpu.cfs_period_us  cpu.cfs_quota_us  cpu.rt_period_us  cpu.rt_runtime_us  cpu.shares  cpu.stat  libvirt  notify_on_release  release_agent  tasks
    cpu]# cat blue/notify_on_release 
    1
    cpu]# cgexec -g cpu:blue dd if=/dev/zero of=/dev/null bs=1024k &
    [1] 8623
    cpu]# cat blue/tasks 
    8623
    cpu]# kill -9 8623
    cpu]# ls
    cgroup.event_control  cgroup.procs  cpu.cfs_period_us  cpu.cfs_quota_us  cpu.rt_period_us  cpu.rt_runtime_us  cpu.shares  cpu.stat  libvirt  notify_on_release  release_agent  tasks
    

3.13. Additional Resources

Subsystem-Specific Kernel Documentation

All of the following files are located under the /usr/share/doc/kernel-doc-<kernel_version>/Documentation/cgroups/ directory (provided by the kernel-doc package).
  • blkio subsystem — blkio-controller.txt
  • cpuacct subsystem — cpuacct.txt
  • cpuset subsystem — cpusets.txt
  • devices subsystem — devices.txt
  • freezer subsystem — freezer-subsystem.txt
  • memory subsystem — memory.txt
  • net_prio subsystem — net_prio.txt
Additionally, refer to the following files for further information about the cpu subsystem:
  • Real-Time scheduling — /usr/share/doc/kernel-doc-<kernel_version>/Documentation/scheduler/sched-rt-group.txt
  • CFS scheduling — /usr/share/doc/kernel-doc-<kernel_version>/Documentation/scheduler/sched-bwc.txt


[5] Source code provided by Red Hat Engineer František Hrbata.
[6] Source code provided by Red Hat Engineer František Hrbata.

Chapter 4. Control Group Application Examples

This chapter provides application examples that take advantage of the cgroup functionality.

4.1. Prioritizing Database I/O

Running each instance of a database server inside its own dedicated virtual guest allows you to allocate resources per database based on their priority. Consider the following example: a system is running two database servers inside two KVM guests. One of the databases is a high priority database and the other one a low priority database. When both database servers are run simultaneously, the I/O throughput is decreased to accommodate requests from both databases equally; Figure 4.1, “I/O throughput without resource allocation” indicates this scenario — once the low priority database is started (around time 45), I/O throughput is the same for both database servers.
I/O throughput without resource allocation

Figure 4.1. I/O throughput without resource allocation

To prioritize the high priority database server, it can be assigned to a cgroup with a high number of reserved I/O operations, whereas the low priority database server can be assigned to a cgroup with a low number of reserved I/O operations. To achieve this, follow the steps in Procedure 4.1, “I/O throughput prioritization”, all of which are performed on the host system.

Procedure 4.1. I/O throughput prioritization

  1. Attach the blkio subsystem to the /cgroup/blkio cgroup:
    ~]# mkdir /cgroup/blkio
    ~]# mount -t cgroup -o blkio blkio /cgroup/blkio
  2. Create a high and low priority cgroup:
    ~]# mkdir /cgroup/blkio/high_prio
    ~]# mkdir /cgroup/blkio/low_prio
  3. Acquire the PIDs of the processes that represent both virtual guests (in which the database servers are running) and move them to their specific cgroup. In our example, VM_high represents a virtual guest running a high priority database server, and VM_low represents a virtual guest running a low priority database server. For example:
    ~]# ps -eLf | grep qemu | grep VM_high | awk '{print $4}' | while read pid; do echo $pid >> /cgroup/blkio/high_prio/tasks; done
    ~]# ps -eLf | grep qemu | grep VM_low | awk '{print $4}' | while read pid; do echo $pid >> /cgroup/blkio/low_prio/tasks; done
  4. Set a ratio of 10:1 for the high_prio and low_prio cgroups. Processes in those cgroups (that is, processes running the virtual guests that have been added to those cgroups in the previous step) will immediately use only the resources made available to them.
    ~]# echo 1000 > /cgroup/blkio/high_prio/blkio.weight
    ~]# echo 100 > /cgroup/blkio/low_prio/blkio.weight
    In our example, the low priority cgroup permits the low priority database server to use only about 10% of the I/O operations, whereas the high priority cgroup permits the high priority database server to use about 90% of the I/O operations.
Figure 4.2, “I/O throughput with resource allocation” illustrates the outcome of limiting the low priority database and prioritizing the high priority database. As soon as the database servers are moved to their appropriate cgroups (around time 75), I/O throughput is divided among both servers with the ratio of 10:1.
I/O throughput with resource allocation

Figure 4.2. I/O throughput with resource allocation

Alternatively, block device I/O throttling can be used for the low priority database to limit its number of read and write operation. For more information on the blkio subsystem, refer to Section 3.1, “blkio”.

4.2. Prioritizing Network Traffic

When running multiple network-related services on a single server system, it is important to define network priorities between these services. Defining these priorities ensures that packets originating from certain services have a higher priority than packets originating from other services. For example, such priorities are useful when a server system simultaneously functions as an NFS and Samba server. The NFS traffic must be of high priority as users expect high throughput. The Samba traffic can be deprioritized to allow better performance of the NFS server.
The net_prio subsystem can be used to set network priorities for processes in cgroups. These priorities are then translated into Type Of Service (TOS) bits and embedded into every packet. Follow the steps in Procedure 4.2, “Setting Network Priorities for File Sharing Services” to configure prioritization of two file sharing services (NFS and Samba).

Procedure 4.2. Setting Network Priorities for File Sharing Services

  1. The net_prio subsystem is not compiled in the kernel, it is a module that must be loaded manually. To do so, type:
    ~]# modprobe net_prio
  2. Attach the net_prio subsystem to the /cgroup/net_prio cgroup:
    ~]# mkdir /cgroup/net_prio
    ~]# mount -t cgroup -o net_prio net_prio /cgroup/net_prio
  3. Create two cgroups, one for each service:
    ~]# mkdir /cgroup/net_prio/nfs_high
    ~]# mkdir /cgroup/net_prio/samba_low
  4. To automatically move the nfs services to the nfs_high cgroup, add the following line to the /etc/sysconfig/nfs file:
    CGROUP_DAEMON="net_prio:nfs_high"
    This configuration ensures that nfs service processes are moved to the nfs_high cgroup when the nfs service is started or restarted. For more information about moving service processes to cgroups, refer to Section 2.9.1, “Starting a Service in a Control Group”.
  5. The smbd daemon does not have a configuration file in the /etc/sysconfig directory. To automatically move the smbd daemon to the samba_low cgroup, add the following line to the /etc/cgrules.conf file:
    *:smbd                net_prio                samba_low
    Note that this rule moves every smbd daemon, not only /usr/sbin/smbd, into the samba_low cgroup.
    You can define rules for the nmbd and winbindd daemons to be moved to the samba_low cgroup in a similar way.
  6. Start the cgred service to load the configuration from the previous step:
    ~]# service cgred start
    Starting CGroup Rules Engine Daemon:                       [  OK  ]
    
  7. For the purposes of this example, let us assume both services use the eth1 network interface. Define network priorities for each cgroup, where 1 denotes low priority and 10 denotes high priority:
    ~]# echo "eth1 1" > /cgroup/net_prio/samba_low
    ~]# echo "eth1 10" > /cgroup/net_prio/nfs_high
  8. Start the nfs and smb services and check whether their processes have been moved into the correct cgroups:
    ~]# service smb start
    Starting SMB services:                                     [  OK  ]
    ~]# cat /cgroup/net_prio/samba_low
    16122
    16124
    ~]# service nfs start
    Starting NFS services:                                     [  OK  ]
    Starting NFS quotas:                                       [  OK  ]
    Starting NFS mountd:                                       [  OK  ]
    Stopping RPC idmapd:                                       [  OK  ]
    Starting RPC idmapd:                                       [  OK  ]
    Starting NFS daemon:                                       [  OK  ]
    ~]# cat /cgroup/net_prio/nfs_high
    16321
    16325
    16376
    
    Network traffic originating from NFS now has higher priority than traffic originating from Samba.
Similar to Procedure 4.2, “Setting Network Priorities for File Sharing Services”, the net_prio subsystem can be used to set network priorities for client applications, for example, Firefox.

4.3. Per-group Division of CPU and Memory Resources

When a large amount of users use a single system, it is practical to provide certain users with more resources than others. Consider the following example: in a hypothetical company, there are three departments — finance, sales, and engineering. Because engineers use the system and its resources for more tasks than the other departments, it is logical that they have more resources available in case all departments are running CPU and memory intensive tasks.
Cgroups provide a way to limit the resources per each system group of users. For this example, assume that the following users have been created on the system:
~]$ grep home /etc/passwd
martin:x:500:500::/home/martin:/bin/bash
john:x:501:501::/home/john:/bin/bash
mark:x:502:502::/home/mark:/bin/bash
peter:x:503:503::/home/peter:/bin/bash
jenn:x:504:504::/home/jenn:/bin/bash
mike:x:505:505::/home/mike:/bin/bash
These users have been assigned to the following system groups:
~]$ grep -e "50[678]" /etc/group
finance:x:506:jenn,john
sales:x:507:mark,martin
engineering:x:508:peter,mike
For this example to work properly, you must have the libcgroup package installed. Using the /etc/cgconfig.conf and /etc/cgrules.conf files, you can create a hierarchy and a set of rules which determine the amount of resources for each user. To achieve this, follow the steps in Procedure 4.3, “Per-group CPU and memory resource management”.

Procedure 4.3. Per-group CPU and memory resource management

  1. In the /etc/cgconfig.conf file, configure the following subsystems to be mounted and cgroups to be created:
    mount {
        cpu     = /cgroup/cpu_and_mem;
        cpuacct = /cgroup/cpu_and_mem;
        memory  = /cgroup/cpu_and_mem;
    }
    
    group finance {
            cpu {
                    cpu.shares="250";
            }
            cpuacct {
                    cpuacct.usage="0";
            }
            memory {
                    memory.limit_in_bytes="2G";
                    memory.memsw.limit_in_bytes="3G";
            }
    }
    
    group sales {
            cpu {
                    cpu.shares="250";
            }
            cpuacct {
                    cpuacct.usage="0";
            }
            memory {
                    memory.limit_in_bytes="4G";
                    memory.memsw.limit_in_bytes="6G";
            }
    }
    
    group engineering {
            cpu {
                    cpu.shares="500";
            }
            cpuacct {
                    cpuacct.usage="0";
            }
            memory {
                    memory.limit_in_bytes="8G";
                    memory.memsw.limit_in_bytes="16G";
            }
    }
    
    When loaded, the above configuration file mounts the cpu, cpuacct, and memory subsystems to a single cpu_and_mem cgroup. For more information on these subsystems, refer to Chapter 3, Subsystems and Tunable Parameters. Next, it creates a hierarchy in cpu_and_mem which contains three cgroups: sales, finance, and engineering. In each of these cgroups, custom parameters are set for each subsystem:
    • cpu — the cpu.shares parameter determines the share of CPU resources available to each process in all cgroups. Setting the parameter to 250, 250, and 500 in the finance, sales, and engineering cgroups respectively means that processes started in these groups will split the resources with a 1:1:2 ratio. Note that when a single process is running, it consumes as much CPU as necessary no matter which cgroup it is placed in. The CPU limitation only comes into effect when two or more processes compete for CPU resources.
    • cpuacct — the cpuacct.usage="0" parameter is used to reset values stored in the cpuacct.usage and cpuacct.usage_percpu files. These files report total CPU time (in nanoseconds) consumed by all processes in a cgroup.
    • memory — the memory.limit_in_bytes parameter represents the amount of memory that is made available to all processes within a certain cgroup. In our example, processes started in the finance cgroup have 2 GB of memory available, processes in the sales group have 4 GB of memory available, and processes in the engineering group have 8 GB of memory available. The memory.memsw.limit_in_bytes parameter specifies the total amount of memory and swap space processes may use. Should a process in the finance cgroup hit the 2 GB memory limit, it is allowed to use another 1 GB of swap space, thus totaling the configured 3 GB.
  2. To define the rules which the cgrulesengd daemon uses to move processes to specific cgroups, configure the /etc/cgrules.conf in the following way:
    #<user/group>         <controller(s)>         <cgroup>
    @finance              cpu,cpuacct,memory      finance
    @sales                cpu,cpuacct,memory      sales
    @engineering          cpu,cpuacct,memory      engineering
    
    The above configuration creates rules that assign a specific system group (for example, @finance) the resource controllers it may use (for example, cpu, cpuacct, memory) and a cgroup (for example, finance) which contains all processes originating from that system group.
    In our example, when the cgrulesengd daemon, started via the service cgred start command, detects a process that is started by a user that belongs to the finance system group (for example, jenn), that process is automatically moved to the /cgroup/cpu_and_mem/finance/tasks file and is subjected to the resource limitations set in the finance cgroup.
  3. Start the cgconfig service to create the hierarchy of cgroups and set the needed parameters in all created cgroups:
    ~]# service cgconfig start
    Starting cgconfig service:                                 [  OK  ]
    
    Start the cgred service to let the cgrulesengd daemon detect any processes started in system groups configured in the /etc/cgrules.conf file:
    ~]# service cgred start
    Starting CGroup Rules Engine Daemon:                       [  OK  ]
    
    Note that cgred is the name of the service that starts the cgrulesengd daemon.
  4. To make all of the changes above persistent across reboots, configure the cgconfig and cgred services to be started by default:
    ~]# chkconfig cgconfig on
    ~]# chkconfig cgred on
To test whether this setup works, execute a CPU or memory intensive process and observe the results, for example, using the top utility. To test the CPU resource management, execute the following dd command under each user:
~]$ dd if=/dev/zero of=/dev/null bs=1024k
The above command reads the /dev/zero and outputs it to the /dev/null in chunks of 1024 KB. When the top utility is launched, you can see results similar to these:
 PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
8201 peter     20   0  103m 1676  556 R 24.9  0.2   0:04.18 dd
8202 mike      20   0  103m 1672  556 R 24.9  0.2   0:03.47 dd
8199 jenn      20   0  103m 1676  556 R 12.6  0.2   0:02.87 dd
8200 john      20   0  103m 1676  556 R 12.6  0.2   0:02.20 dd
8197 martin    20   0  103m 1672  556 R 12.6  0.2   0:05.56 dd
8198 mark      20   0  103m 1672  556 R 12.3  0.2   0:04.28 dd
⋮
All processes have been correctly assigned to their cgroups and are only allowed to consume CPU resource made available to them. If all but two processes, which belong to the finance and engineering cgroups, are stopped, the remaining resources are evenly split between both processes:
      PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
8202 mike      20   0  103m 1676  556 R 66.4  0.2   0:06.35 dd
8200 john      20   0  103m 1672  556 R 33.2  0.2   0:05.08 dd
⋮

Alternative method

Because the cgrulesengd daemon moves a process to a cgroup only after the appropriate conditions set by the rules in /etc/cgrules.conf have been fulfilled, that process may be running for a few milliseconds in an incorrect cgroup. An alternative way to move processes to their specified cgroups is to use the pam_cgroup.so PAM module. This module moves processes to available cgroups according to rules defined in the /etc/cgrules.conf file. Follow the steps in Procedure 4.4, “Using a PAM module to move processes to cgroups” to configure the pam_cgroup.so PAM module. Note that the libcgroup-pam package that provides this module is available form the Optional subscription channel. Before subscribing to this channel please see the Scope of Coverage Details. If you decide to install packages from the channel, follow the steps documented in the article called How to access Optional and Supplementary channels, and -devel packages using Red Hat Subscription Manager (RHSM)? on Red Hat Customer Portal.

Procedure 4.4. Using a PAM module to move processes to cgroups

  1. Install the libcgroup-pam package from the optional Red Hat Enterprise Linux Yum repository:
    ~]# yum install libcgroup-pam --enablerepo=rhel-6-server-optional-rpms
  2. Ensure that the PAM module has been installed and exists:
    ~]# ls /lib64/security/pam_cgroup.so
    /lib64/security/pam_cgroup.so
    
    Note that on 32-bit systems, the module is placed in the /lib/security directory.
  3. Add the following line to the /etc/pam.d/su file to use the pam_cgroup.so module each time the su command is executed:
    session         optional        pam_cgroup.so
    
  4. Configure the /etc/cgconfig.conf and /etc/cgrules.conf files as in Procedure 4.3, “Per-group CPU and memory resource management”.
  5. Log out all users that are affected by the cgroup settings in the /etc/cgrules.conf file to apply the above configuration.
When using the pam_cgroup.so PAM module, you may disable the cgred service.

Appendix A. Revision History

Revision History
Revision 1.0-35Wed Mar 15 2017Marie Doleželová
Version for 6.9 GA publication.
Revision 1.0-31Thu Apr 28 2016Jana Heves
Red Hat Enterprise Linux 6.8 GA release of the Resource Management Guide
Revision 1.0-29Tue May 12 2015Radek Bíba
Asynchronous update.
Revision 1.0-26Thu Oct 10 2014Peter Ondrejka
Red Hat Enterprise Linux 6.6 GA release of the Resource Management Guide
Revision 1.0-16Thu Feb 21 2013Martin Prpič
Red Hat Enterprise Linux 6.4 GA release of the Resource Management Guide
Revision 1.0-7Wed Jun 20 2012Martin Prpič
Red Hat Enterprise Linux 6.3 GA release of the Resource Management Guide.
Revision 1.0-6Tue Dec 6 2011Martin Prpič
Red Hat Enterprise Linux 6.2 GA release of the Resource Management Guide
Revision 1.0-5Thu May 19 2011Martin Prpič
Red Hat Enterprise Linux 6.1 GA release of the Resource Management Guide
Revision 1.0-0Tue Nov 9 2010Rüdiger Landmann
Feature-complete version for GA