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2.4. Obtaining Information about Control Groups

Use the systemctl command to list system units and to view their status. Also, the systemd-cgls command is provided to view the hierarchy of control groups and systemd-cgtop to monitor their resource consumption in real time.

2.4.1. Listing Units

Use the following command to list all active units on the system:
~]# systemctl list-units
The list-units option is executed by default, which means that you will receive the same output when you omit this option and execute just:
UNIT                     LOAD   ACTIVE SUB     DESCRIPTION
abrt-ccpp.service        loaded active exited  Install ABRT coredump hook
abrt-oops.service        loaded active running ABRT kernel log watcher
abrt-vmcore.service      loaded active exited  Harvest vmcores for ABRT
abrt-xorg.service        loaded active running ABRT Xorg log watcher
The output displayed above contains five columns:
  • UNIT — the name of the unit that also reflects the unit's position in the cgroup tree. As mentioned in the section called “Systemd Unit Types”, three unit types are relevant for resource control: slice, scope, and service. For a complete list of systemd's unit types, see the chapter called Managing Services with systemd in Red Hat Enterprise Linux 7 System Administrators Guide.
  • LOAD — indicates whether the unit configuration file was properly loaded. If the unit file failed to load, the field contains the state error instead of loaded. Other unit load states are: stub, merged, and masked.
  • ACTIVE — the high-level unit activation state, which is a generalization of SUB.
  • SUB — the low-level unit activation state. The range of possible values depends on the unit type.
  • DESCRIPTION — the description of the unit's content and functionality.
By default, systemctl lists only active units (in terms of high-level activations state in the ACTIVE field). Use the --all option to see inactive units too. To limit the amount of information in the output list, use the --type (-t) parameter that requires a comma-separated list of unit types such as service and slice, or unit load states such as loaded and masked.

Example 2.8. Using systemctl list-units

To view a list of all slices used on the system, type:
~]$ systemctl -t slice 
To list all active masked services, type:
~]$ systemctl -t service,masked
To list all unit files installed on your system and their status, type:
~]$ systemctl list-unit-files

2.4.2. Viewing the Control Group Hierarchy

The aforementioned listing commands do not go beyond the unit level to show the actual processes running in cgroups. Also, the output of systemctl does not show the hierarchy of units. You can achieve both by using the systemd-cgls command that groups the running process according to cgroups. To display the whole cgroup hierarchy on your system, type:
~]$ systemd-cgls
When systemd-cgls is issued without parameters, it returns the entire cgroup hierarchy. The highest level of the cgroup tree is formed by slices and can look as follows:
│ ├─1 /usr/lib/systemd/systemd --switched-root --system --deserialize 20  
│ ...
│ ├─user-1000
│ │ └─ ...
│ ├─user-2000
│ │ └─ ...
│ ...
  │ └─ ...
Note that machine slice is present only if you are running a virtual machine or a container. For more information on the cgroup tree, see the section called “Systemd Unit Types”.
To reduce the output of systemd-cgls, and to view a specified part of the hierarchy, execute:
~]$ systemd-cgls name
Replace name with a name of the resource controller you want to inspect.
As an alternative, use the systemctl status command to display detailed information about a system unit. A cgroup subtree is a part of the output of this command.
~]$ systemctl name
To learn more about systemctl status, see the chapter called Managing Services with systemd in Red Hat Enterprise Linux 7 System Administrators Guide.

Example 2.9. Viewing the Control Group Hierarchy

To see a cgroup tree of the memory resource controller, execute:
~]$ systemd-cgls memory
├─    1 /usr/lib/systemd/systemd --switched-root --system --deserialize 23
├─  475 /usr/lib/systemd/systemd-journald
The output of the above command lists the services that interact with the selected controller. A different approach is to view a part of the cgroup tree for a certain service, slice, or scope unit:
~]# systemctl status httpd.service
httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled)
   Active: active (running) since Sun 2014-03-23 08:01:14 MDT; 33min ago
  Process: 3385 ExecReload=/usr/sbin/httpd $OPTIONS -k graceful (code=exited, status=0/SUCCESS)
 Main PID: 1205 (httpd)
   Status: "Total requests: 0; Current requests/sec: 0; Current traffic:   0 B/sec"
   CGroup: /system.slice/httpd.service
           ├─1205 /usr/sbin/httpd -DFOREGROUND
           ├─3387 /usr/sbin/httpd -DFOREGROUND
           ├─3388 /usr/sbin/httpd -DFOREGROUND
           ├─3389 /usr/sbin/httpd -DFOREGROUND
           ├─3390 /usr/sbin/httpd -DFOREGROUND
           └─3391 /usr/sbin/httpd -DFOREGROUND

Besides the aforementioned tools, systemd also provides the machinectl command dedicated to monitoring Linux containers.

2.4.3. Viewing Resource Controllers

The aforementioned systemctl commands enable monitoring the higher-level unit hierarchy, but do not show which resource controllers in Linux kernel are actually used by which processes. This information is stored in dedicated process files, to view it, type as root:
~]# cat proc/PID/cgroup
Where PID stands for the ID of the process you wish to examine. By default, the list is the same for all units started by systemd, since it automatically mounts all default controllers. See the following example:
~]# cat proc/27/cgroup
By examining this file, you can determine if the process has been placed in the correct cgroups as defined by the systemd unit file specifications.

2.4.4. Monitoring Resource Consumption

The systemd-cgls command provides a static snapshot of the cgroup hierarchy. To see a dynamic account of currently running cgroups ordered by their resource usage (CPU, Memory, and IO), use:
~]# systemd-cgtop
The behavior, provided statistics, and control options of systemd-cgtop are akin of those of the top utility. See systemd-cgtop(1) manual page for more information.