OpenRC/CGroups

Cgroups overview

General info about Linux cgroups: Wikipedia

OpenRC's cgroup implementation follows the FDO recommendations.

OpenRC creates its own cgroup controller named openrc in which the service processes are put. If set in the service's options a new cgroup controller named openrc_${service_name} is created to hold its process inclusive all its child services' processes. The result is a hierarchical tree of cgroups containing the service processes.

Turning the cgroup feature support on

The cgroup feature is only available on Linux.

To use cgroups in OpenRC turn on the following option in the main rc configuration file:

FILE /etc/rc.confTurn on the cgroup feature support

<syntaxhighlight lang="bash">rc_controller_cgroups="YES"</syntaxhighlight>

Each cgroup controller is mounted at: /sys/fs/cgroup

All other features described in this wiki page will work only if you have set this option to "YES".

Setting limits

For each cgroup controller one can set options in the form of:

FILE /etc/rc.confSet cgroup controller option globally for all services

<syntaxhighlight lang="bash">rc_cgroup_${CONTROLLER_NAME}="${CONTROLLER_NAME}.${OPTION_NAME} ${OPTION_VALUE}"</syntaxhighlight>

or

FILE /etc/conf.d/${service_name}Set cgroup controller option for one service

<syntaxhighlight lang="bash">rc_cgroup_${CONTROLLER_NAME}="${CONTROLLER_NAME}.${OPTION_NAME} ${OPTION_VALUE}"</syntaxhighlight>

For example, you would use this to set the cpu.shares setting in the cpu controller to 512 for the service.

FILE /etc/conf.d/${service_name}

<syntaxhighlight lang="bash">rc_cgroup_cpu="cpu.shares 512"</syntaxhighlight>

For more information about the adjustments that can be made with cgroups, see Documentation/cgroups/* in the Linux kernel source tree.

Service cleanup

It is possible to trigger the killing of the whole service's process tree inside the same process cgroup if the service gets stopped or restarted. This is achieved by setting the following option in the service's rc script:

FILE /etc/conf.d/${service_name}an example of triggering cgroup cleanup

<syntaxhighlight lang="bash">rc_cgroup_cleanup="yes"</syntaxhighlight>

This option should not be set globally as it will kill all service's child processes e.g. SSH connections or apache workers, as OpenRC doesn't support automatic process cgroup-relocation as it is done in logind services.

Service death notification

TBD

Possible improvements

System notification about cleared cgroups (can be done via sh user script).
Grouping cgroups subsystems together.
Support for userlevel cgroups.

OpenRC cgroup architecture

TODO: shortly describe architecture

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