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systemd cgroup driver

By default, runc creates cgroups and sets cgroup limits on its own (this mode is known as fs cgroup driver). When --systemd-cgroup global option is given (as in e.g. runc --systemd-cgroup run ...), runc switches to systemd cgroup driver. This document describes its features and peculiarities.

systemd unit name and placement

When creating a container, runc requests systemd (over dbus) to create a transient unit for the container, and place it into a specified slice.

The name of the unit and the containing slice is derived from the container runtime spec in the following way:

  1. If Linux.CgroupsPath is set, it is expected to be in the form [slice]:[prefix]:[name].

    Here slice is a systemd slice under which the container is placed. If empty, it defaults to system.slice, except when cgroup v2 is used and rootless container is created, in which case it defaults to user.slice.

    Note that slice can contain dashes to denote a sub-slice (e.g. user-1000.slice is a correct notation, meaning a subslice of user.slice), but it must not contain slashes (e.g. user.slice/user-1000.slice is invalid).

    A slice of - represents a root slice.

    Next, prefix and name are used to compose the unit name, which is <prefix>-<name>.scope, unless name has .slice suffix, in which case prefix is ignored and the name is used as is.

  2. If Linux.CgroupsPath is not set or empty, it works the same way as if it would be set to :runc:<container-id>. See the description above to see what it transforms to.

As described above, a unit being created can either be a scope or a slice. For a scope, runc specifies its parent slice via a Slice= systemd property, and also sets Delegate=true. For a slice, runc specifies a weak dependency on the parent slice via a Wants= property.

Resource limits

runc always enables accounting for all controllers, regardless of any limits being set. This means it unconditionally sets the following properties for the systemd unit being created:

  • CPUAccounting=true
  • IOAccounting=true (BlockIOAccounting for cgroup v1)
  • MemoryAccounting=true
  • TasksAccounting=true

The resource limits of the systemd unit are set by runc by translating the runtime spec resources to systemd unit properties.

Such translation is by no means complete, as there are some cgroup properties that can not be set via systemd. Therefore, runc systemd cgroup driver is backed by fs driver (in other words, cgroup limits are first set via systemd unit properties, and when by writing to cgroupfs files).

The set of runtime spec resources which is translated by runc to systemd unit properties depends on kernel cgroup version being used (v1 or v2), and on the systemd version being run. If an older systemd version (which does not support some resources) is used, runc do not set those resources.

The following tables summarize which properties are translated.

cgroup v1

runtime spec resource systemd property name min systemd version
memory.limit MemoryLimit
cpu.shares CPUShares
blockIO.weight BlockIOWeight
pids.limit TasksMax
cpu.cpus AllowedCPUs v244
cpu.mems AllowedMemoryNodes v244

cgroup v2

runtime spec resource systemd property name min systemd version
memory.limit MemoryMax
memory.reservation MemoryLow
memory.swap MemorySwapMax
cpu.shares CPUWeight
pids.limit TasksMax
cpu.cpus AllowedCPUs v244
cpu.mems AllowedMemoryNodes v244
unified.cpu.max CPUQuota, CPUQuotaPeriodSec v242
unified.cpu.weight CPUWeight
unified.cpu.idle CPUWeight v252
unified.cpuset.cpus AllowedCPUs v244
unified.cpuset.mems AllowedMemoryNodes v244
unified.memory.high MemoryHigh
unified.memory.low MemoryLow
unified.memory.min MemoryMin
unified.memory.max MemoryMax
unified.memory.swap.max MemorySwapMax
unified.pids.max TasksMax

For documentation on systemd unit resource properties, see systemd.resource-control(5) man page.

Auxiliary properties

Auxiliary properties of a systemd unit (as shown by systemctl show <unit-name> after the container is created) can be set (or overwritten) by adding annotations to the container runtime spec (config.json).

For example:

        "annotations": {
                "org.systemd.property.TimeoutStopUSec": "uint64 123456789",
                "org.systemd.property.CollectMode":"'inactive-or-failed'"
        },

The above will set the following properties:

  • TimeoutStopSec to 2 minutes and 3 seconds;
  • CollectMode to "inactive-or-failed".

The values must be in the gvariant text format, as described in gvariant documentation.

To find out which type systemd expects for a particular parameter, please consult systemd sources.