Using the control groups Linux kernel feature, you can organize processes into hierarchically ordered groups called cgroups. You define the hierarchy by providing structure to the cgroups virtual file system, mounted by default on the /sys/fs/cgroup/ directory.

The systemd service manager uses cgroups to organize all units and services that it governs. Manually, you can manage the hierarchies of cgroups by creating and removing sub-directories in the /sys/fs/cgroup/ directory.

The resource controllers in the kernel then modify the behavior of processes in cgroups by limiting, prioritizing or allocating system resources, of those processes. These resources include the following:

• CPU time
• Memory
• Network bandwidth
• Combinations of these resources

The primary use case of cgroups is aggregating system processes and dividing hardware resources among applications and users. This makes it possible to increase the efficiency, stability, and security of your environment.

Control groups version 1

Control groups version 1 (cgroups-v1) provides a separate hierarchy for each resource controller. Resources such as CPU, memory, or I/O has its own control group hierarchy. You can combine different control group hierarchies so that one controller can coordinate with another in managing their individual resources. However, when the two controllers belong to different process hierarchies, the coordination is limited.

The cgroups-v1 controllers were developed across a large time span, resulting in inconsistent behavior and naming of their control files.

Control groups version 2

Control groups version 2 (cgroups-v2) provides a single control group hierarchy against which all resource controllers are mounted.

The control file behavior and naming is consistent among different controllers.

Important

RHEL 10, by default, mounts and uses cgroups-v2.

For more details about cgroups-v1 and cgroups-v2, install the kernel-doc RPM package. After installation, the documentation is in the /usr/share/doc/kernel-doc-<version>/Documentation directory on the local system. The cgroups-v1 documentation files are in the Documentation/admin-guide/cgroup-v1/ directory. This directory has multiple files for different controllers. The cgroups-v2 documentation is in the Documentation/admin-guide/cgroup-v2.rst file.