Control Groups (cgroups)

Background

Control Groups are a Linux feature for organizing processes in hierarchical groups and applying resources limits to them. Each rkt pod is placed in a different cgroup to separate the processes of the pod from the processes of the host. Memory and CPU isolators are also implemented with cgroups.

What cgroup does rkt use?

Every pod and application within that pod is run within its own cgroup.

`rkt` started from the command line

When a recent version of systemd is running on the host and rkt is not started as a systemd service (typically, from the command line), rkt will call systemd-nspawn with --register=true. This will cause systemd-nspawn to call the D-Bus method CreateMachineWithNetwork on systemd-machined and the cgroup /machine.slice/machine-rkt... will be created. This requires systemd v216+ as detected by the machinedRegister function in stage1's init.

When systemd is not running on the host, or the systemd version is too old (< v216), rkt uses systemd-nspawn with the --register=false parameter. In this case, systemd-nspawn or other systemd components will not create new cgroups for rkt. Instead, rkt creates a new cgroup for each pod under the current cgroup, like $CALLER_CGROUP/machine-some-id.slice.

`rkt` started as a systemd service

rkt is able to detect if it is started as a systemd service (from a .service file or from systemd-run). In that case, systemd-nspawn is started with the --keep-unit parameter. This will cause systemd-nspawn to use the D-Bus method call RegisterMachineWithNetwork instead of CreateMachineWithNetwork and the pod will remain in the cgroup of the service. By default, the slice is systemd.slice but users are advised to select machine.slice with systemd-run --slice=machine or Slice=machine.slice in the .service file. It will result in /machine.slice/servicename.service when the user select that slice.

Summary

/machine.slice/machine-rkt... when started on the command line with systemd v216+.
/$SLICE.slice/servicename.service when started from a systemd service.
$CALLER_CGROUP/machine-some-id.slice without systemd, or with systemd pre-v216

For example, a simple pod run interactively on a system with systemd would look like:

├─machine.slice
│ └─machine-rkt\x2df28d074b\x2da8bb\x2d4246\x2d96a5\x2db961e1fe7035.scope
│   ├─init.scope
│   │ └─/usr/lib/systemd/systemd
│   └─system.slice
│     ├─alpine-sh.service
│     │ ├─/bin/sh 
│     └─systemd-journald.service
│       └─/usr/lib/systemd/systemd-journald

What subsystems does rkt use?

Right now, rkt uses the cpu, cpuset, and memory subsystems.

How are they mounted?

When the stage1 starts, it mounts /sys . Then, for every subsystem, it:

Mounts the subsystem (on <rootfs>/sys/fs/cgroup/<subsystem>)
Bind-mounts the subcgroup on top of itself (e.g <rootfs>/sys/fs/cgroup/memory/machine.slice/machine-rkt-UUID.scope/)
Remounts the subsystem readonly

This is so that the pod itself cannot escape the cgroup. Currently the cgroup filesystems are not accessible to applications within the pod, but that may change.

(N.B. rkt prior to v1.23 mounted each individual knob read-write. E.g. .../memory/machine.slice/machine-rkt-UUID.scope/system.slice/etcd.service/{memory.limit_in_bytes, cgroup.procs})

Future work

Unified hierarchy and cgroup2

Unified hierarchy and cgroup2 is a new feature in Linux that will be available in Linux 4.4.

This is tracked by #1757.

CGroup Namespaces

CGroup Namespaces is a new feature being developed in Linux.