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diff --git a/man/systemd-oomd.service.xml b/man/systemd-oomd.service.xml new file mode 100644 index 0000000..11c9237 --- /dev/null +++ b/man/systemd-oomd.service.xml @@ -0,0 +1,114 @@ +<?xml version='1.0'?> +<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" + "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> +<!-- SPDX-License-Identifier: LGPL-2.1-or-later --> + +<refentry id="systemd-oomd.service" conditional='ENABLE_OOMD'> + + <refentryinfo> + <title>systemd-oomd.service</title> + <productname>systemd</productname> + </refentryinfo> + + <refmeta> + <refentrytitle>systemd-oomd.service</refentrytitle> + <manvolnum>8</manvolnum> + </refmeta> + + <refnamediv> + <refname>systemd-oomd.service</refname> + <refname>systemd-oomd</refname> + <refpurpose>A userspace out-of-memory (OOM) killer</refpurpose> + </refnamediv> + + <refsynopsisdiv> + <para><filename>systemd-oomd.service</filename></para> + <para><filename>/usr/lib/systemd/systemd-oomd</filename></para> + </refsynopsisdiv> + + <refsect1> + <title>Description</title> + + <para><command>systemd-oomd</command> 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.</para> + + <para>You can enable monitoring and actions on units by setting <varname>ManagedOOMSwap=</varname> and + <varname>ManagedOOMMemoryPressure=</varname> in the unit configuration, see + <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>. + <command>systemd-oomd</command> retrieves information about such units from <command>systemd</command> + when it starts and watches for subsequent changes.</para> + + <para>Cgroups of units with <varname>ManagedOOMSwap=</varname> or + <varname>ManagedOOMMemoryPressure=</varname> set to <option>kill</option> will be monitored. + <command>systemd-oomd</command> periodically polls PSI statistics for the system and those cgroups to + decide when to take action. If the configured limits are exceeded, <command>systemd-oomd</command> will + select a cgroup to terminate, and send <constant>SIGKILL</constant> to all processes in it. Note that + only descendant cgroups are eligible candidates for killing; the unit with its property set to + <option>kill</option> is not a candidate (unless one of its ancestors set their property to + <option>kill</option>). Also only leaf cgroups and cgroups with <filename>memory.oom.group</filename> set + to <constant>1</constant> are eligible candidates; see <varname>OOMPolicy=</varname> in + <citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>. + </para> + + <para><citerefentry><refentrytitle>oomctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> can + be used to list monitored cgroups and pressure information.</para> + + <para>See <citerefentry><refentrytitle>oomd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> + for more information about the configuration of this service.</para> + </refsect1> + + <refsect1> + <title>System requirements and configuration</title> + + <para>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 <command>systemd-oomd</command>. + The easiest way to turn on memory accounting is by ensuring the value for <varname>DefaultMemoryAccounting=</varname> + is set to <constant>true</constant> in + <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para> + + <para>The kernel must be compiled with PSI support. This is available in Linux 4.20 and above.</para> + + <para>It is highly recommended for the system to have swap enabled for <command>systemd-oomd</command> to + function optimally. With swap enabled, the system spends enough time swapping pages to let + <command>systemd-oomd</command> react. Without swap, the system enters a livelocked state much more + quickly and may prevent <command>systemd-oomd</command> from responding in a reasonable amount of + time. See <ulink url="https://chrisdown.name/2018/01/02/in-defence-of-swap.html">"In defence of swap: + common misconceptions"</ulink> for more details on swap. Any swap-based actions on systems without swap + will be ignored. While <command>systemd-oomd</command> 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.</para> + + <para>Be aware that if you intend to enable monitoring and actions on <filename>user.slice</filename>, + <filename>user-$UID.slice</filename>, or their ancestor cgroups, it is highly recommended that your + programs be managed by the systemd user manager to prevent running too many processes under the same + session scope (and thus avoid a situation where memory intensive tasks trigger + <command>systemd-oomd</command> to kill everything under the cgroup). If you're using a desktop + environment like GNOME or KDE, it already spawns many session components with the systemd user manager. + </para> + </refsect1> + + <refsect1> + <title>Usage Recommendations</title> + + <para><varname>ManagedOOMSwap=</varname> works with the system-wide swap values, so setting it on the root slice + <filename>-.slice</filename>, and allowing all descendant cgroups to be eligible candidates may make the most + sense.</para> + + <para><varname>ManagedOOMMemoryPressure=</varname> tends to work better on the cgroups below the root + slice. For units which tend to have processes that are less latency sensitive (e.g. + <filename>system.slice</filename>), a higher limit like the default of 60% may be acceptable, as those + processes can usually ride out slowdowns caused by lack of memory without serious consequences. However, + something like <filename>user@$UID.service</filename> may prefer a much lower value like 40%.</para> + </refsect1> + + <refsect1> + <title>See Also</title> + <para> + <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, + <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, + <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>, + <citerefentry><refentrytitle>oomd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>, + <citerefentry><refentrytitle>oomctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> + </para> + </refsect1> +</refentry> |