Chapter 2. Using Control Groups

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The following sections provide an overview of tasks related to creation and management of control groups. This guide focuses on utilities provided by systemd that are preferred as a way of cgroup management and will be supported in the future. Previous versions of Red Hat Enterprise Linux used the libcgroup package for creating and managing cgroups. This package is still available to assure backward compatibility (see Warning), but it will not be supported in future versions of Red Hat Enterprise Linux.

2.1. Creating Control Groups

From the systemd's perspective, a cgroup is bound to a system unit configurable with a unit file and manageable with systemd's command-line utilities. Depending on the type of application, your resource management settings can be transient or persistent.
To create a transient cgroup for a service, start the service with the systemd-run command. This way, it is possible to set limits on resources consumed by the service during its runtime. Applications can create transient cgroups dynamically by using API calls to systemd. See the section called “Online Documentation” for API reference. Transient unit is removed automatically as soon as the service is stopped.
To assign a persistent cgroup to a service, edit its unit configuration file. The configuration is preserved after the system reboot, so it can be used to manage services that are started automatically. Note that scope units cannot be created in this way.

2.1.1. Creating Transient Cgroups with systemd-run

The systemd-run command is used to create and start a transient service or scope unit and run a custom command in the unit. Commands executed in service units are started asynchronously in the background, where they are invoked from the systemd process. Commands run in scope units are started directly from the systemd-run process and thus inherit the execution environment of the caller. Execution in this case is synchronous.
To run a command in a specified cgroup, type as root:
~]# systemd-run --unit=name --scope --slice=slice_name command
  • The name stands for the name you want the unit to be known under. If --unit is not specified, a unit name will be generated automatically. It is recommended to choose a descriptive name, since it will represent the unit in the systemctl output. The name has to be unique during runtime of the unit.
  • Use the optional --scope parameter to create a transient scope unit instead of service unit that is created by default.
  • With the --slice option, you can make your newly created service or scope unit a member of a specified slice. Replace slice_name with the name of an existing slice (as shown in the output of systemctl -t slice), or create a new slice by passing a unique name. By default, services and scopes are created as members of the system.slice.
  • Replace command with the command you wish to execute in the service unit. Place this command at the very end of the systemd-run syntax, so that the parameters of this command are not confused for parameters of systemd-run.
Besides the above options, there are several other parameters available for systemd-run. For example, --description creates a description of the unit, --remain-after-exit allows to collect runtime information after terminating the service's process. The --machine option executes the command in a confined container. See the systemd-run(1) manual page to learn more.

Example 2.1. Starting a New Service with systemd-run

Use the following command to run the top utility in a service unit in a new slice called test. Type as root:
~]# systemd-run --unit=toptest --slice=test top -b
The following message is displayed to confirm that you started the service successfully:
Running as unit toptest.service
Now, the name toptest.service can be used to monitor or to modify the cgroup with systemctl commands.

2.1.2. Creating Persistent Cgroups

To configure a unit to be started automatically on system boot, execute the systemctl enable command (see the chapter called Managing Services with systemd in Red Hat Enterprise Linux 7 System Administrators Guide). Running this command automatically creates a unit file in the /usr/lib/systemd/system/ directory. To make persistent changes to the cgroup, add or modify configuration parameters in its unit file. For more information, see Section 2.3.2, “Modifying Unit Files”.
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