What is cgroup?

• cgroup = control group.
• System for resource management on Linux.
• Directory hierarchy at /sys/fs/cgroup, called the root control group. This root cgroup is the cgroup to which all processes belong.
• Limit, throttle, and account for resource usage per control group.
• Each resource interface is provided by a controller.
• Used to constrain resources that are allocated to processes.

When systemd is chosen as the init system for a Linux distribution, the init process generates and consumes a root control group (cgroup) and acts as a cgroup manager. systemd has a tight integration with cgroups and allocates a cgroup per systemd unit.

cgroup v1 vs cgroup v2

cgroupv1 has a hierarchy per-resource (resource = cpu, devices, memory, pids, etc):

• Separate hierarchy/cgroups for each resource.
• cgroups can be nested inside each other.
• Limits and accounting are performed per-cgroup.
• One PID is in exactly one cgroup per resource.

/sys/fs/cgroup => resource => cgroup => pid

cgroupv2 has a unified hierarchy, Each cgroup can support multiple resource domains.

• cgroups are "global": not limited to one resource.
• Resources are now opt-in for cgroups.
• Granularity at TGID (PID), not TID level.
• Focus on simplicity/clarity over ultimate flexibility.

/sys/fs/cgroup => cgroup => (cgroup) => pid => (resource)

How to identify the cgroup version?

To check which cgroup version your distribution uses:

$ stat -fc %T /sys/fs/cgroup/

• For cgroup v2, the output is cgroup2fs.
• For cgroup v1, the output is tmpfs.

To check the layout:

$ ls /sys/fs/cgroup