# oomd.conf systemd oomd.conf 5 oomd.conf oomd.conf.d Global systemd-oomd configuration files `/etc/systemd/oomd.conf`, `/run/systemd/oomd.conf`, `/usr/local/lib/systemd/oomd.conf`, `/usr/lib/systemd/oomd.conf`, `/etc/systemd/oomd.conf.d/*.conf`, `/run/systemd/oomd.conf.d/*.conf`, `/usr/local/lib/systemd/oomd.conf.d/*.conf`, `/usr/lib/systemd/oomd.conf.d/*.conf` # Description These files configure the various parameters of the `systemd(1)` userspace out-of-memory (OOM) killer, `systemd-oomd.service(8)`. See `systemd.syntax(7)` for a general description of the syntax. # Prekill event `systemd-oomd` supports notifying external components before killing a control group. This is done by sending a notification over varlink to all sockets found in `/run/systemd/oomd.prekill.hook/` folder. Each socket should implement the `io.systemd.oom.Prekill` interface. The notification contains the control group path to allow the hook to identify which control group is being killed. This allows external components to perform any necessary cleanup or logging before the control group is terminated. The hook is not intended as a way to avoid the kill, but rather as a notification mechanism. Note that this is a privileged option as, even if it has a timeout, is synchronous and delays the kill, so use with care. The typically preferable mechanism to process memory pressure is to do what [Resource Pressure Handling](https://systemd.io/PRESSURE) describes which is unprivileged, asynchronous and does not delay the kill. # OOM Rulesets `systemd-oomd` supports custom rulesets that define conditions and actions for OOM handling on a per-unit basis. Ruleset files use the `.oomrule` extension and are loaded from `/etc/systemd/oomd/rules.d/`, `/run/systemd/oomd/rules.d/`, `/usr/local/lib/systemd/oomd/rules.d/`, and `/usr/lib/systemd/oomd/rules.d/`. Units opt into rulesets via the `OOMRules=` setting in `systemd.resource-control(5)`, which takes a space-separated list of ruleset names (the file name without the `.oomrule` extension). Each ruleset file contains a `[Rule]` section with the following options. At least one of `MemoryPressureAbove=` or `SwapUsageMax=` must be configured; rulesets with no conditions are ignored. If both are set, the conditions are combined with AND, i.e. the action is only triggered when both thresholds are exceeded simultaneously. `MemoryPressureAbove=` Sets the memory pressure threshold above which the rule's action will be triggered. The memory pressure represents the fraction of time in a 10 second window in which all tasks in the control group were delayed (PSI `full avg10`). Takes a value specified in percent (when suffixed with `%`), permille (`‰`) or permyriad (`‱`), between 0% and 100%, inclusive. If unset, this condition is not evaluated. A value of `100%` can never be exceeded and is therefore rejected with a warning; a value of `0%` makes the condition true on any observed pressure, which is usually not useful. `SwapUsageMax=` Sets the system-wide swap usage threshold above which the rule's action will be triggered. Takes a value specified in percent (when suffixed with `%`), permille (`‰`) or permyriad (`‱`), between 0% and 100%, inclusive. If unset, this condition is not evaluated. A value of `100%` can never be exceeded and is therefore rejected with a warning; a value of `0%` fires as soon as any swap is in use, which is usually not useful. `Action=` Specifies the action to take when the rule's conditions are met. Takes one of `kill-all`, `kill-by-pgscan`, or `kill-by-swap`. This setting is mandatory; rulesets without `Action=` are ignored. - `kill-all` sends `SIGKILL` to every process in the unit's cgroup hierarchy, including any descendant cgroups. - `kill-by-pgscan` selects and kills the descendant cgroup with the highest recent page scan (reclaim) rate. - `kill-by-swap` selects and kills the descendant cgroup with the highest swap usage. `LastingSec=` Sets the duration the conditions must be continuously met before the action is taken. Takes a time span value, see `systemd.time(7)` for details on the permitted syntax. Defaults to 0, i.e. the action is taken immediately when the conditions are met. # \[OOM\] Section Options The following options are available in the \[OOM\] section: `SwapUsedLimit=` Sets the limit for memory and swap usage on the system before `systemd-oomd` will take action. If the fraction of memory used and the fraction of swap used on the system are both more than what is defined here, `systemd-oomd` will act on eligible descendant control groups with swap usage greater than 5% of total swap, starting from the ones with the highest swap usage. Which control groups are monitored and what action gets taken depends on what the unit has configured for `ManagedOOMSwap=`. Takes a value specified in percent (when suffixed with "%"), permille ("‰") or permyriad ("‱"), between 0% and 100%, inclusive. Defaults to 90%. `DefaultMemoryPressureLimit=` Sets the limit for memory pressure on the unit's control group before `systemd-oomd` will take action. A unit can override this value with `ManagedOOMMemoryPressureLimit=`. The memory pressure for this property represents the fraction of time in a 10 second window in which all tasks in the control group were delayed. For each monitored control group, if the memory pressure on that control group exceeds the limit set for longer than the duration set by `DefaultMemoryPressureDurationSec=`, `systemd-oomd` will act on eligible descendant control groups, starting from the ones with the most reclaim activity to the least reclaim activity. Which control groups are monitored and what action gets taken depends on what the unit has configured for `ManagedOOMMemoryPressure=`. Takes a fraction specified in the same way as `SwapUsedLimit=` above. Defaults to 60%. `DefaultMemoryPressureDurationSec=` Sets the amount of time a unit's control group needs to have exceeded memory pressure limits before `systemd-oomd` will take action. A unit can override this value with `ManagedOOMMemoryPressureDurationSec=`. Memory pressure limits are defined by `DefaultMemoryPressureLimit=` and `ManagedOOMMemoryPressureLimit=`. Must be set to 0, or at least 1 second. Defaults to 30 seconds when unset or 0. `PrekillHookTimeoutSec=` Sets the amount of time `systemd-oomd` will wait for pre-kill hooks to complete, before proceeding with the control group termination. Pre-kill hooks work by placing varlink socket to `/run/systemd/oomd.prekill.hook/` folder. Each socket should implement interface for notification to work. `systemd-oomd` sends a notification before killing a control group for each discovered socket. The timeout is intended to be global and not per hook. If all hooks return earlier, the kill is performed as soon as possible. The timeout must be at least 1s. Defaults to 0, which means `systemd-oomd` will not wait and no notifications will be sent. # See Also `systemd(1)`, `systemd.resource-control(5)`, `systemd-oomd.service(8)`, `oomctl(1)`