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100 lines
5.5 KiB
XML
100 lines
5.5 KiB
XML
<?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-or-later -->
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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, memory accounting must be turned on for all units monitored by <command>systemd-oomd</command>.
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The easiest way to turn on memory accounting is by ensuring the value for <varname>DefaultMemoryAccounting=</varname>
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is 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>It is highly recommended for the system to have swap enabled for <command>systemd-oomd</command> to function
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optimally. With swap 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. Any swap-based actions on systems without swap will be ignored. While
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<command>systemd-oomd</command> can perform pressure-based actions on a system without swap, the pressure increases
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will be more abrupt and may require more tuning to get the desired thresholds and behavior.</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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