By default, runsc 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. runsc --systemd-cgroup run ...), runsc switches to systemd cgroup driver. This document describes its features and peculiarities. Runsc requires the host systemd version to be at least 244 and for unified cgroups (aka cgroupv2) to be enabled.
When creating a container, runsc 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:
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.
If Linux.CgroupsPath is not set or empty, it works the same way as if it would be set to :runsc:<container-id>. See the description above to see what it transforms to.
As described above, a unit will be created as a systemd scope. For a scope, runsc specifies its parent slice via a Slice= systemd property, and also sets Delegate=true.
runsc 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:
The resource limits of the systemd unit are set by runsc 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, runsc 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 runsc 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, runsc does not set those resources.
The following tables summarize which properties are translated.
| 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 | |
| cpu.mems | AllowedMemoryNodes | |
| unified.cpu.max | CPUQuota, CPUQuotaPeriodSec | |
| unified.cpu.weight | CPUWeight | |
| unified.cpu.idle | CPUWeight | v252 |
| unified.cpuset.cpus | AllowedCPUs | |
| unified.cpuset.mems | AllowedMemoryNodes | |
| 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.