man: document systemd-oomd and related items
This commit is contained in:
Anita Zhang
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<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1+ -->
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<refentry id="oomctl" conditional='ENABLE_OOMD'
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>oomctl</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>oomctl</refentrytitle>
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<manvolnum>1</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>oomctl</refname>
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<refpurpose>Analyze the state stored in <command>systemd-oomd</command></refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<cmdsynopsis>
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<command>oomctl</command>
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<arg choice="opt" rep="repeat">OPTIONS</arg>
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<arg choice="req">COMMAND</arg>
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</cmdsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><command>oomctl</command> may be used to get information about the various contexts read in by
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the <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> userspace
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out-of-memory (OOM) killer,
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<citerefentry><refentrytitle>systemd-oomd</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
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</refsect1>
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<refsect1>
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<title>Commands</title>
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<para>The following commands are understood:</para>
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<variablelist>
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<varlistentry>
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<term><command>dump</command></term>
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<listitem><para>Show the current state of the cgroup(s) and system context(s) stored by
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<command>systemd-oomd</command>.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Options</title>
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<para>The following options are understood:</para>
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<variablelist>
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<xi:include href="standard-options.xml" xpointer="help" />
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<xi:include href="standard-options.xml" xpointer="version" />
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<xi:include href="standard-options.xml" xpointer="no-pager" />
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Exit status</title>
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<para>On success, 0 is returned, a non-zero failure code otherwise.</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-oomd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>oomd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1+ -->
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<refentry id="oomd.conf" conditional='ENABLE_OOMD'
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>oomd.conf</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>oomd.conf</refentrytitle>
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<manvolnum>5</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>oomd.conf</refname>
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<refname>oomd.conf.d</refname>
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<refpurpose>Global <command>systemd-oomd</command> configuration files</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>/etc/systemd/oomd.conf</filename></para>
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<para><filename>/etc/systemd/oomd.conf.d/*.conf</filename></para>
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<para><filename>/usr/lib/systemd/oomd.conf.d/*.conf</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>These files configure the various parameters of the
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> userspace
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out-of-memory (OOM) killer,
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<citerefentry><refentrytitle>systemd-oomd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
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See <citerefentry><refentrytitle>systemd.syntax</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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for a general description of the syntax.</para>
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</refsect1>
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<xi:include href="standard-conf.xml" xpointer="main-conf" />
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<refsect1>
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<title>[OOM] Section Options</title>
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<para>The following options are available in the [OOM] section:</para>
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<variablelist class='config-directives'>
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<varlistentry>
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<term><varname>SwapUsedLimitPercent=</varname></term>
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<listitem><para>Sets the limit for swap usage on the system before <command>systemd-oomd</command> will
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take action. If the percentage of swap used on the system is more than what is defined here,
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<command>systemd-oomd</command> will act on eligible descendant cgroups, starting from the ones with the
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highest swap usage to the lowest swap usage. Which cgroups are monitored and what
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action gets taken depends on what the unit has configured for <varname>ManagedOOMSwap=</varname>.
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Takes a percentage value between 0% and 100%, inclusive. Defaults to 90%.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>DefaultMemoryPressureLimitPercent=</varname></term>
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<listitem><para>Sets the limit for memory pressure on the unit's cgroup before <command>systemd-oomd</command>
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will take action. A unit can override this value with <varname>ManagedOOMMemoryPressureLimitPercent=</varname>.
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The memory pressure for this property represents the fraction of time in a 10 second window in which all tasks
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in the cgroup were delayed. For each monitored cgroup, if the memory pressure on that cgroup exceeds the
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limit set for more than 30 seconds, <command>systemd-oomd</command> will act on eligible descendant cgroups,
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starting from the ones with the most reclaim activity to the least reclaim activity. Which cgroups are
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monitored and what action gets taken depends on what the unit has configured for
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<varname>ManagedOOMMemoryPressure=</varname>. Takes a percentage value between 0% and 100%, inclusive.
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Defaults to 60%.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-oomd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>oomctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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@ -45,6 +45,8 @@ manpages = [
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['nss-mymachines', '8', ['libnss_mymachines.so.2'], 'ENABLE_NSS_MYMACHINES'],
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['nss-resolve', '8', ['libnss_resolve.so.2'], 'ENABLE_NSS_RESOLVE'],
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['nss-systemd', '8', ['libnss_systemd.so.2'], 'ENABLE_NSS_SYSTEMD'],
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['oomctl', '1', [], 'ENABLE_OOMD'],
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['oomd.conf', '5', ['oomd.conf.d'], 'ENABLE_OOMD'],
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['org.freedesktop.LogControl1', '5', [], ''],
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['org.freedesktop.home1', '5', [], 'ENABLE_HOMED'],
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['org.freedesktop.hostname1', '5', [], 'ENABLE_HOSTNAMED'],
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@ -907,6 +909,7 @@ manpages = [
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['systemd-networkd.service', '8', ['systemd-networkd'], 'ENABLE_NETWORKD'],
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['systemd-notify', '1', [], ''],
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['systemd-nspawn', '1', [], ''],
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['systemd-oomd.service', '8', ['systemd-oomd'], 'ENABLE_OOMD'],
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['systemd-path', '1', [], ''],
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['systemd-portabled.service', '8', ['systemd-portabled'], 'ENABLE_PORTABLED'],
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['systemd-pstore.service', '8', ['systemd-pstore'], 'ENABLE_PSTORE'],
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@ -0,0 +1,98 @@
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<?xml version='1.0'?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1+ -->
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<refentry id="systemd-oomd.service" conditional='ENABLE_OOMD'>
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<refentryinfo>
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<title>systemd-oomd.service</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd-oomd.service</refentrytitle>
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<manvolnum>8</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd-oomd.service</refname>
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<refname>systemd-oomd</refname>
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<refpurpose>A userspace out-of-memory (OOM) killer</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>systemd-oomd.service</filename></para>
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<para><filename>/usr/lib/systemd/systemd-oomd</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><command>systemd-oomd</command> is a system service that uses cgroups-v2 and pressure stall information (PSI)
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to monitor and take action on processes before an OOM occurs in kernel space.</para>
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<para>You can enable monitoring and actions on units by setting <varname>ManagedOOMSwap=</varname> and/or
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<varname>ManagedOOMMemoryPressure=</varname> to the appropriate value. <command>systemd-oomd</command> will
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periodically poll enabled units' cgroup data to detect when corrective action needs to occur. When an action needs
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to happen, it will only be performed on the descendant cgroups of the enabled units. More precisely, only cgroups with
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<filename>memory.oom.group</filename> set to <constant>1</constant> and leaf cgroup nodes are eligible candidates.
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Action will be taken recursively on all of the processes under the chosen candidate.</para>
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<para>See
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<citerefentry><refentrytitle>oomd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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for more information about the configuration of this service.</para>
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</refsect1>
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<refsect1>
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<title>Setup Information</title>
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<para>The system must be running systemd with a full unified cgroup hierarchy for the expected cgroups-v2 features.
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Furthermore, resource accounting must be turned on for all units monitored by <command>systemd-oomd</command>.
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The easiest way to turn on resource accounting is by ensuring the values for <varname>DefaultCPUAccounting</varname>,
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<varname>DefaultIOAccounting</varname>, <varname>DefaultMemoryAccounting</varname>, and
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<varname>DefaultTasksAccounting</varname> are set to <constant>true</constant> in
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<citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
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<para>You will need a kernel compiled with PSI support. This is available in Linux 4.20 and above.</para>
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<para>The system must also have swap enabled for <command>systemd-oomd</command> to function correctly. With swap
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enabled, the system spends enough time swapping pages to let <command>systemd-oomd</command> react.
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Without swap, the system enters a livelocked state much more quickly and may prevent <command>systemd-oomd</command>
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from responding in a reasonable amount of time. See
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<ulink url="https://chrisdown.name/2018/01/02/in-defence-of-swap.html">"In defence of swap: common misconceptions"</ulink>
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for more details on swap.</para>
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<para>Be aware that if you intend to enable monitoring and actions on <filename>user.slice</filename>,
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<filename>user-$UID.slice</filename>, or their ancestor cgroups, it is highly recommended that your programs be
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managed by the systemd user manager to prevent running too many processes under the same session scope (and thus
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avoid a situation where memory intensive tasks trigger <command>systemd-oomd</command> to kill everything under the
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cgroup). If you're using a desktop environment like GNOME, it already spawns many session components with the
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systemd user manager.</para>
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</refsect1>
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<refsect1>
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<title>Usage Recommendations</title>
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<para><varname>ManagedOOMSwap=</varname> works with the system-wide swap values, so setting it on the root slice
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<filename>-.slice</filename>, and allowing all descendant cgroups to be eligible candidates may make the most
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sense.</para>
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<para><varname>ManagedOOMMemoryPressure=</varname> tends to work better on the cgroups below the root slice
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<filename>-.slice</filename>. For units which tend to have processes that are less latency sensitive (e.g.
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<filename>system.slice</filename>), a higher limit like the default of 60% may be acceptable, as those processes
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can usually ride out slowdowns caused by lack of memory without serious consequences. However, something like
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<filename>user@$UID.service</filename> may prefer a much lower value like 40%.</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>oomd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>oomctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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@ -869,6 +869,49 @@ DeviceAllow=/dev/loop-control
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<xi:include href="supported-controllers.xml" xpointer="controllers-text" />
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>ManagedOOMSwap=auto|kill</varname></term>
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<term><varname>ManagedOOMMemoryPressure=auto|kill</varname></term>
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<listitem>
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<para>Specifies how
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<citerefentry><refentrytitle>systemd-oomd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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will act on this unit's cgroups. Defaults to <option>auto</option>.</para>
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<para>When set to <option>kill</option>, <command>systemd-oomd</command> will actively monitor this unit's
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cgroup metrics to decide whether it needs to act. If the cgroup passes the limits set by
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<citerefentry><refentrytitle>oomd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> or its
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overrides, <command>systemd-oomd</command> will send a <constant>SIGKILL</constant> to all of the processes
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under the chosen candidate cgroup. Note that only descendant cgroups can be eligible candidates for killing;
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the unit that set its property to <option>kill</option> is not a candidate (unless one of its ancestors set
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their property to <option>kill</option>). You can find more details on candidates and kill behavior at
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<citerefentry><refentrytitle>systemd-oomd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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and <citerefentry><refentrytitle>oomd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>. Setting
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either of these properties to <option>kill</option> will also automatically acquire
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<varname>After=</varname> and <varname>Wants=</varname> dependencies on
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<filename>systemd-oomd.service</filename> unless <varname>DefaultDependencies=no</varname>.
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</para>
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<para>When set to <option>auto</option>, <command>systemd-oomd</command> will not actively use this cgroup's
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data for monitoring and detection. However, if an ancestor cgroup has one of these properties set to
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<option>kill</option>, a unit with <option>auto</option> can still be an eligible candidate for
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<command>systemd-oomd</command> to act on.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>ManagedOOMMemoryPressureLimitPercent=</varname></term>
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<listitem>
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<para>Overrides the default memory pressure limit set by
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<citerefentry><refentrytitle>oomd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> for this unit
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(cgroup). Takes a percentage value between 0% and 100%, inclusive. This property is ignored unless
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<varname>ManagedOOMMemoryPressure=</varname><option>kill</option>. Defaults to 0%, which means use the
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default set by <citerefentry><refentrytitle>oomd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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@ -1030,6 +1073,7 @@ DeviceAllow=/dev/loop-control
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<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-oomd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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The documentation for control groups and specific controllers in the Linux kernel:
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<ulink url="https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html">Control Groups v2</ulink>.
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</para>
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