"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
+ SPDX-License-Identifier: LGPL-2.1+
+
This file is part of systemd.
Copyright 2010 Lennart Poettering
<filename>foo.timer</filename> activates a matching service
<filename>foo.service</filename>. The unit to activate may be
controlled by <varname>Unit=</varname> (see below).</para>
+
+ <para>Note that in case the unit to activate is already active at the time the timer elapses it is not restarted,
+ but simply left running. There is no concept of spawning new service instances in this case. Due to this, services
+ with <varname>RemainAfterExit=</varname> set (which stay around continuously even after the service's main process
+ exited) are usually not suitable for activation via repetitive timers, as they will only be activated once, and
+ then stay around forever.</para>
+ </refsect1>
+
+ <refsect1>
+ <title>Implicit Dependencies</title>
+
+ <para>The following dependencies are implicitly added:</para>
+
+ <itemizedlist>
+ <listitem><para>Timer units automatically gain a <varname>Before=</varname>
+ dependency on the service they are supposed to activate.</para></listitem>
+ </itemizedlist>
</refsect1>
<refsect1>
- <title>Automatic Dependencies</title>
-
- <para>Timer units automatically gain a <varname>Before=</varname>
- dependency on the service they are supposed to activate.</para>
-
- <para>Unless <varname>DefaultDependencies=</varname> is set to
- <option>false</option>, all timer units will implicitly have
- dependencies of type <varname>Requires=</varname> and
- <varname>After=</varname> on <filename>sysinit.target</filename>,
- a dependency of type <varname>Before=</varname> on
- <filename>timers.target</filename>, as well as
- <varname>Conflicts=</varname> and <varname>Before=</varname> on
- <filename>shutdown.target</filename> to ensure that they are
- stopped cleanly prior to system shutdown. Timer units with at
- least one <varname>OnCalendar=</varname> directive will have an
- additional <varname>After=</varname> dependency on
- <filename>timer-sync.target</filename> to avoid being started
- before the system clock has been correctly set. Only timer units
- involved with early boot or late system shutdown should disable
- the <varname>DefaultDependencies=</varname> option.</para>
+ <title>Default Dependencies</title>
+
+ <para>The following dependencies are added unless <varname>DefaultDependencies=no</varname> is set:</para>
+
+ <itemizedlist>
+ <listitem><para>Timer units will automatically have dependencies of type <varname>Requires=</varname> and
+ <varname>After=</varname> on <filename>sysinit.target</filename>, a dependency of type <varname>Before=</varname>
+ on <filename>timers.target</filename>, as well as <varname>Conflicts=</varname> and <varname>Before=</varname> on
+ <filename>shutdown.target</filename> to ensure that they are stopped cleanly prior to system shutdown. Only timer
+ units involved with early boot or late system shutdown should disable the
+ <varname>DefaultDependencies=</varname> option.</para></listitem>
+
+ <listitem><para>Timer units
+ with at least one <varname>OnCalendar=</varname> directive will have an additional <varname>After=</varname>
+ dependency on <filename>time-sync.target</filename> to avoid being started before the system clock has been
+ correctly set.</para></listitem>
+ </itemizedlist>
</refsect1>
<refsect1>
<para>Note that timers do not necessarily expire at the
precise time configured with this setting, as it is subject to
the <varname>AccuracySec=</varname> setting
- below.</para></listitem>
+ below.</para>
+
+ <para>May be specified more than once.</para></listitem>
</varlistentry>
<varlistentry>
<varname>OnUnitInactiveSec=</varname> and ending the time
configured with <varname>AccuracySec=</varname> later. Within
this time window, the expiry time will be placed at a
- host-specific, randomized but stable position that is
+ host-specific, randomized, but stable position that is
synchronized between all local timer units. This is done in
- order to distribute the wake-up time in networked
- installations, as well as optimizing power consumption to
- suppress unnecessary CPU wake-ups. To get best accuracy, set
- this option to 1us. Note that the timer is still subject to
- the timer slack configured via
+ order to optimize power consumption to suppress unnecessary
+ CPU wake-ups. To get best accuracy, set this option to
+ 1us. Note that the timer is still subject to the timer slack
+ configured via
<citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>'s
<varname>TimerSlackNSec=</varname> setting. See
<citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
this value as high as possible and as low as
necessary.</para></listitem>
</varlistentry>
+
+ <varlistentry>
+ <term><varname>RandomizedDelaySec=</varname></term>
+
+ <listitem><para>Delay the timer by a randomly selected, evenly
+ distributed amount of time between 0 and the specified time
+ value. Defaults to 0, indicating that no randomized delay
+ shall be applied. Each timer unit will determine this delay
+ randomly each time it is started, and the delay will simply be
+ added on top of the next determined elapsing time. This is
+ useful to stretch dispatching of similarly configured timer
+ events over a certain amount time, to avoid that they all fire
+ at the same time, possibly resulting in resource
+ congestion. Note the relation to
+ <varname>AccuracySec=</varname> above: the latter allows the
+ service manager to coalesce timer events within a specified
+ time range in order to minimize wakeups, the former does the
+ opposite: it stretches timer events over a time range, to make
+ it unlikely that they fire simultaneously. If
+ <varname>RandomizedDelaySec=</varname> and
+ <varname>AccuracySec=</varname> are used in conjunction, first
+ the randomized delay is added, and then the result is
+ possibly further shifted to coalesce it with other timer
+ events happening on the system. As mentioned above
+ <varname>AccuracySec=</varname> defaults to 1min and
+ <varname>RandomizedDelaySec=</varname> to 0, thus encouraging
+ coalescing of timer events. In order to optimally stretch
+ timer events over a certain range of time, make sure to set
+ <varname>RandomizedDelaySec=</varname> to a higher value, and
+ <varname>AccuracySec=1us</varname>.</para></listitem>
+ </varlistentry>
+
<varlistentry>
<term><varname>Unit=</varname></term>
during the time when the timer was inactive. This is useful to
catch up on missed runs of the service when the machine was
off. Note that this setting only has an effect on timers
- configured with <varname>OnCalendar=</varname>.
+ configured with <varname>OnCalendar=</varname>. Defaults
+ to <varname>false</varname>.
</para></listitem>
</varlistentry>
unloaded. Turning this off is particularly useful for
transient timer units that shall disappear after they first
elapse. Note that this setting has an effect on repeatedly
- starting the a timer unit that only elapses once: if
+ starting a timer unit that only elapses once: if
<varname>RemainAfterElapse=</varname> is on, it will not be
started again, and is guaranteed to elapse only once. However,
- if <varname>RemainAfterLeapse=</varname> is off, it might be
+ if <varname>RemainAfterElapse=</varname> is off, it might be
started again if it is already elapsed, and thus be triggered
multiple times. Defaults to
<varname>yes</varname>.</para></listitem>
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