OOMD.CONF(5)

📦
version 261.3 · 15 versions
, in ubuntu stonking

💖 NAME 💖

oomd.conf, oomd.conf.d — the global systemd-oomd configuration files. Yep bestie, this is OOMD.CONF(5), and we’re about to make config files make sense 💅✨.

🌸 SYNOPSIS 🌸

/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) 🫶. For the general description of the syntax, check out systemd.syntax(7) — that’s your grammar bestie for this whole situation.

✨ CONFIGURATION DIRECTORIES AND PRECEDENCE ✨

Okay so the default configuration is set during compilation, which means you only need configuration when you actually need to deviate from those defaults. The main configuration file is loaded from one of the listed directories in order of priority, and only the first file found is used 👑:

  1. /etc/systemd/
  2. /run/systemd/
  3. /usr/local/lib/systemd/ [1]
  4. /usr/lib/systemd/

The vendor version of the file contains commented out entries showing the defaults as a guide to the administrator. Local overrides can also be created by creating drop-ins, as described below. The main configuration file can also be edited for this purpose (or a copy in /etc/ if it is shipped under /usr/), however using drop-ins for local configuration is recommended over modifications to the main configuration file 💖.

In addition to the main configuration file, drop-in configuration snippets are read from /usr/lib/systemd/*.conf.d/, /usr/local/lib/systemd/*.conf.d/, and /etc/systemd/*.conf.d/. Those drop-ins have higher precedence and override the main configuration file. Files in the *.conf.d/ configuration subdirectories are sorted by their filename in lexicographic order, regardless of in which of the subdirectories they reside. When multiple files specify the same option, for options which accept just a single value, the entry in the file sorted last takes precedence, and for options which accept a list of values, entries are collected as they occur in the sorted files 🦋.

When packages need to customize the configuration, they can install drop-ins under /usr/. Files in /etc/ are reserved for the local administrator, who may use this logic to override the configuration files installed by vendor packages. Drop-ins have to be used to override package drop-ins, since the main configuration file has lower precedence. It is recommended to prefix all filenames in those subdirectories with a two-digit number and a dash, to simplify the ordering. This also defines a concept of drop-in priorities to allow OS vendors to ship drop-ins within a specific range lower than the range used by users. This should lower the risk of package drop-ins accidentally overriding drop-ins defined by users. It is recommended to use the range 10-40 for drop-ins in /usr/ and the range 60-90 for drop-ins in /etc/ and /run/, to make sure that local and transient drop-ins take priority over drop-ins shipped by the OS vendor 🌈.

To disable a configuration file supplied by the vendor, the recommended way is to place a symlink to /dev/null in the configuration directory in /etc/, with the same filename as the vendor configuration file 💀.

🧚 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[2] 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.

Added in version 261.

  • 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.

Added in version 261.

  • 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.

Added in version 261.

  • 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.

Added in version 261.

💅 [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%.

Added in version 247.

  • 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%.

Added in version 247.

  • 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.

Added in version 248.

  • 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.

Added in version 260.

🦋 SEE ALSO 🦋

systemd(1), systemd.resource-control(5), systemd-oomd.service(8), oomctl(1)

🎀 NOTES 🎀

  1. 💣💥🧨💥💥💣 Please note that those configuration files must be available at all times. If /usr/local/ is a separate partition, it may not be available during early boot, and must not be used for configuration.
  2. Resource Pressure Handling — https://systemd.io/PRESSURE

And that’s the tea, bestie: this page is OOMD.CONF(5) from systemd 261.3 🌈💖. Footer version accounted for, no extra date invented, and the facts stayed iconic — periodt 💅✨.

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