2.9. Starting a Process in a Control Group

Important — Mandatory parameters

Some controllers have mandatory parameters that you must set before you run a task in a hierarchy that includes those controllers. For example, before you use the cpuset controller, cpuset.cpus and cpuset.mems must be defined.

The examples in this section illustrate the correct syntax for the command, but only work on systems on which the relevant mandatory parameters have been set for any controllers used in the examples. If you have not already configured the relevant controllers, you cannot copy example commands directly from this section and expect them to work on your system.

Refer to Section 3.10, “Additional Resources” for a description of which parameters are mandatory for given subsystems.

Launch processes in a control group by running the cgexec command. For example, this command launches the lynx web browser within the group1 control group, subject to the limitations imposed on that group by the cpu subsystem:

cgexec -g cpu:group1 lynx http://www.redhat.com

The syntax for cgexec is: cgexec -g subsystems:path_to_cgroup command arguements , where:

subsystems is a comma-separated list of subsystems, or * to launch the process in the hierarchies associated with all available subsystems. Note that, as with cgset described in Section 2.7, “Setting Parameters”, if control groups of the same name exist in multiple hierarchies, the -g option creates processes in each of those groups. Ensure that the cgroup exists within each of the hierarchies whose subsystems you specify here.
path_to_cgroup is the path to the control group relative to the hierarchy.
command is the command to run
arguements are any arguements for the command

You can also add the --sticky option before the command to keep any child processes in the same control group. If you do not set this option and the cgred daemon is running, child processes will be allocated to control groups based on the settings found in /etc/cgrules.conf. The process itself, however, will remain in the control group in which you started it.

Alternative method

When you start a new process, it inherits the group of its parent process. Therefore, an alternative method for starting a process in a particular control group is to move your shell process to that group (refer to Section 2.8, “Moving a Process to a Control Group”), and then launch the process from that shell. For example:

echo $$ > /cgroup/lab1/group1/tasks
lynx

Note that after exiting lynx, your existing shell is still in the group1 control group. Therefore, an even better way would be:

sh -c "echo \$$ > /cgroup/lab1/group1/tasks && lynx"

2.9.1. Starting a Service in a Control Group

You can start some services in a control group. Services that can be started in control groups must:

use a /etc/sysconfig/servicename file
use the daemon() function from /etc/init.d/functions to start the service

To make an eligible service start in a control group, edit its file in the /etc/sysconfig directory to include an entry in the form CGROUP_DAEMON="subsystem:control_group" where subsystem is a subsystem associated with a particular hierarchy, and control_group is a control group in that hierarchy. For example:

CGROUP_DAEMON="cpuset:daemons/sql"