NAME

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

SYNOPSIS

       systemd-oomd.service

       /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.

       -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