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, USB, etc.).
  • 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.

Subsystems are also known as resource controllers

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] 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.