NAME

       systemd-oomd.service, systemd-oomd - A userspace out-of-memory (OOM) killer

SYNOPSIS

       systemd-oomd.service

       /usr/lib/systemd/systemd-oomd

DESCRIPTION

       systemd-oomd is a system service that uses cgroups-v2 and pressure stall information (PSI) to monitor and
       take corrective action before an OOM occurs in the kernel space.

       You can enable monitoring and actions on units by setting ManagedOOMSwap= and ManagedOOMMemoryPressure=
       in the unit configuration, see systemd.resource-control(5).  systemd-oomd retrieves information about
       such units from systemd(1) when it starts and watches for subsequent changes.

       Cgroups of units with ManagedOOMSwap= or ManagedOOMMemoryPressure= set to kill will be monitored.
       systemd-oomd periodically polls PSI statistics for the system and those cgroups to decide when to take
       action. If the configured limits are exceeded, systemd-oomd will select a cgroup to terminate, and send
       SIGKILL to all processes in it. Note that only descendant cgroups are eligible candidates for killing;
       the unit with its property set to kill is not a candidate (unless one of its ancestors set their property
       to kill). Also only leaf cgroups and cgroups with memory.oom.group set to 1 are eligible candidates; see
       OOMPolicy= in systemd.service(5).

       oomctl(1) can be used to list monitored cgroups and pressure information.

       See oomd.conf(5) for more information about the configuration of this service.

SYSTEM REQUIREMENTS AND CONFIGURATION

       The system must be running systemd with a full unified cgroup hierarchy for the expected cgroups-v2
       features. Furthermore, memory accounting must be turned on for all units monitored by systemd-oomd. The
       easiest way to turn on memory accounting is by ensuring the value for DefaultMemoryAccounting= is set to
       true in systemd-system.conf(5).

       The kernel must be compiled with PSI support. This is available in Linux 4.20 and above.

       It is highly recommended for the system to have swap enabled for systemd-oomd to function optimally. 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"[1] for more details on swap.
       Any swap-based actions on systems without swap will be ignored. While systemd-oomd can perform
       pressure-based actions on such a system, the pressure increases will be more abrupt and may require more
       tuning to get the desired thresholds and behavior.

       Be aware that if you intend to enable monitoring and actions on user.slice, 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 or KDE, it already spawns many session components with the systemd user manager.

USAGE RECOMMENDATIONS

       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 sense.

       ManagedOOMMemoryPressure= tends to work better on the cgroups below the root 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%.

OPTIONS

       --dry-run
           Do a dry run of systemd-oomd: when a kill is triggered, print it to the log instead of killing the
           cgroup.

           Added in version 253.

       -h, --help
           Print a short help text and exit.

       --version
           Print a short version string and exit.

SEE ALSO

       systemd(1), systemd-system.conf(5), systemd.resource-control(5), oomd.conf(5), oomctl(1)

NOTES

        1. "In defence of swap: common misconceptions"
           https://chrisdown.name/2018/01/02/in-defence-of-swap.html