OpenRC/CGroups

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Cgroups overview

General info about Linux cgroups: Wikipedia

OpenRC's cgroup implementation follows the FDO recommendations.

OpenRC creates it's 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 it's process inclusive all it's 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

rc_controller_cgroups="YES"

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

rc_cgroup_${CONTROLLER_NAME}="${CONTROLLER_NAME}.${OPTION_NAME} ${OPTION VALUE}"

or

File/etc/conf.d/${service_name}.confSet cgroup controller option for one service

rc_cgroup_${CONTROLLER_NAME}="${CONTROLLER_NAME}.${OPTION_NAME} ${OPTION VALUE}"

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

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

rc_cgroup_cpu="cpu.shares 512"

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}.confan example of triggering cgroup cleanup

rc_cgroup_cleanup="yes" 

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