systemd-oomd.service, systemd-oomd — A userspace out-of-memory (OOM) killer
systemd-oomd is a system service that uses cgroups-v2 and pressure stall information (PSI) to monitor and take action on processes before an OOM occurs in kernel space.
You can enable monitoring and actions on units by setting
ManagedOOMMemoryPressure= to the appropriate value. systemd-oomd will
periodically poll enabled units' cgroup data to detect when corrective action needs to occur. When an action needs
to happen, it will only be performed on the descendant cgroups of the enabled units. More precisely, only cgroups with
memory.oom.group set to
1 and leaf cgroup nodes are eligible candidates.
Action will be taken recursively on all of the processes under the chosen candidate.
See oomd.conf(5) for more information about the configuration of this service.
The system must be running systemd with a full unified cgroup hierarchy for the expected cgroups-v2 features.
Furthermore, resource accounting must be turned on for all units monitored by systemd-oomd.
The easiest way to turn on resource accounting is by ensuring the values for
DefaultTasksAccounting are set to
You will need a kernel compiled with PSI support. This is available in Linux 4.20 and above.
The system must also have swap enabled for systemd-oomd to function correctly. With swap enabled, the system spends enough time swapping pages to let systemd-oomd react. Without swap, the system enters a livelocked state much more quickly and may prevent systemd-oomd from responding in a reasonable amount of time. See "In defence of swap: common misconceptions" for more details on swap.
Be aware that if you intend to enable monitoring and actions on
user-$UID.slice, or their ancestor cgroups, it is highly recommended that your programs be
managed by the systemd user manager to prevent running too many processes under the same session scope (and thus
avoid a situation where memory intensive tasks trigger systemd-oomd to kill everything under the
cgroup). If you're using a desktop environment like GNOME, it already spawns many session components with the
systemd user manager.
ManagedOOMSwap= works with the system-wide swap values, so setting it on the root slice
-.slice, and allowing all descendant cgroups to be eligible candidates may make the most
ManagedOOMMemoryPressure= tends to work better on the cgroups below the root slice
-.slice. For units which tend to have processes that are less latency sensitive (e.g.
system.slice), a higher limit like the default of 60% may be acceptable, as those processes
can usually ride out slowdowns caused by lack of memory without serious consequences. However, something like
user@$UID.service may prefer a much lower value like 40%.