2.2. Creating a Hierarchy and Attaching Subsystems

Effects on running systems

The following instructions, which cover creating a new hierarchy and attaching subsystems to it, assume that cgroups are not already configured on your system. In this case, these instructions will not affect the operation of the system. Changing the tunable parameters in a cgroup with tasks, however, may immediately affect those tasks. This guide alerts you the first time it illustrates changing a tunable cgroup parameter that may affect one or more tasks.

On a system on which cgroups are already configured (either manually, or by the cgconfig service) these commands will fail unless you first unmount existing hierarchies, which will affect the operation of the system. Do not experiment with these instructions on production systems.

To create a hierarchy and attach subsystems to it, edit the mount section of the /etc/cgconfig.conf file as root. Entries in the mount section have the following format:

subsystem = /cgroup/hierarchy;

When cgconfig next starts, it will create the hierarchy and attach the subsystems to it.

The following example creates a hierarchy called cpu_and_mem and attaches the cpu, cpuset, cpuacct, and memory subsystems to it.

mount {
    cpuset  = /cgroup/cpu_and_mem;
    cpu     = /cgroup/cpu_and_mem;
    cpuacct = /cgroup/cpu_and_mem;
    memory  = /cgroup/cpu_and_mem;
}

Alternative method

You can also use shell commands and utilities to create hierarchies and attach subsystems to them.

Create a mount point for the hierarchy as root. Include the name of the cgroup in the mount point:

~]# mkdir /cgroup/name

For example:

~]# mkdir /cgroup/cpu_and_mem

Next, use the mount command to mount the hierarchy and simultaneously attach one or more subsystems. For example:

~]# mount -t cgroup -o subsystems name /cgroup/name

Where subsystems is a comma-separated list of subsystems and name is the name of the hierarchy. Brief descriptions of all available subsystems are listed in Available Subsystems in Red Hat Enterprise Linux, and Chapter 3, Subsystems and Tunable Parameters provides a detailed reference.

Example 2.3. Using the mount command to attach subsystems

In this example, a directory named /cgroup/cpu_and_mem already exists, which will serve as the mount point for the hierarchy that we create. We will attach the cpu, cpuset and memory subsystems to a hierarchy we name cpu_and_mem, and mount the cpu_and_mem hierarchy on /cgroup/cpu_and_mem:

~]# mount -t cgroup -o cpu,cpuset,memory cpu_and_mem /cgroup/cpu_and_mem

You can list all available subsystems along with their current mount points (i.e. where the hierarchy they are attached to is mounted) with the lssubsys ^[3] command:

~]# lssubsys -am
cpu,cpuset,memory /cgroup/cpu_and_mem
net_cls
ns
cpuacct
devices
freezer
blkio

This output indicates that:

the cpu, cpuset and memory subsystems are attached to a hierarchy mounted on /cgroup/cpu_and_mem, and
the net_cls, ns, cpuacct, devices, freezer and blkio subsystems are as yet unattached to any hierarchy, as illustrated by the lack of a corresponding mount point.

^[3] The lssubsys command is one of the utilities provided by the libcgroup package. You must install libcgroup to use it: refer to Chapter 2, Using Control Groups if you are unable to run lssubsys.