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chrt(1) manpage

Copyright (C) 2004 Robert Love
Copyright (C) 2015 Karel Zak <kzak@redhat.com>

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version 2, as published by the Free Software Foundation.

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with this program; if not, write to the Free Software Foundation, Inc.,
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= chrt(1)
:doctype: manpage
:man manual: User Commands
:man source: util-linux {release-version}
:page-layout: base
:command: chrt
:colon: :

== NAME

chrt - manipulate the real-time attributes of a process

== SYNOPSIS

*chrt* [options] _priority command argument_ ...

*chrt* [options] *-p* [_priority_] _PID_

== DESCRIPTION

*chrt* sets or retrieves the real-time scheduling attributes of an existing _PID_, or runs _command_ with the given attributes.

== POLICIES

*-o*, *--other*::
Set scheduling policy to *SCHED_OTHER* (time-sharing scheduling). This is the default Linux scheduling policy.

*-f*, *--fifo*::
Set scheduling policy to *SCHED_FIFO* (first in-first out).

*-r*, *--rr*::
Set scheduling policy to *SCHED_RR* (round-robin scheduling). When no policy is defined, the *SCHED_RR* is used as the default.

*-b*, *--batch*::
Set scheduling policy to *SCHED_BATCH* (scheduling batch processes). Linux-specific, supported since 2.6.16. The priority argument has to be set to zero.

*-i*, *--idle*::
Set scheduling policy to *SCHED_IDLE* (scheduling very low priority jobs). Linux-specific, supported since 2.6.23. The priority argument has to be set to zero.

*-d*, *--deadline*::
Set scheduling policy to *SCHED_DEADLINE* (sporadic task model deadline scheduling). Linux-specific, supported since 3.14. The priority argument has to be set to zero. See also *--sched-runtime*, *--sched-deadline* and *--sched-period*. The relation between the options required by the kernel is runtime <= deadline <= period. *chrt* copies _period_ to _deadline_ if *--sched-deadline* is not specified and _deadline_ to _runtime_ if *--sched-runtime* is not specified. It means that at least *--sched-period* has to be specified. See *sched*(7) for more details.

== SCHEDULING OPTIONS

*-T*, *--sched-runtime* _nanoseconds_::
Specifies runtime parameter for *SCHED_DEADLINE* and custom slice length for *SCHED_OTHER* and *SCHED_BATCH* policies (Linux-specific). Note that custom slice length via the runtime parameter is supported since Linux 6.12.

*-P*, *--sched-period* _nanoseconds_::
Specifies period parameter for *SCHED_DEADLINE* policy (Linux-specific). Note that the kernel's lower limit is 100 milliseconds.

*-D*, *--sched-deadline* _nanoseconds_::
Specifies deadline parameter for *SCHED_DEADLINE* policy (Linux-specific).

*-R*, *--reset-on-fork*::
Use *SCHED_RESET_ON_FORK* or *SCHED_FLAG_RESET_ON_FORK* flag. Linux-specific, supported since 2.6.31.
+
Each thread has a _reset-on-fork_ scheduling flag. When this flag is set, children created by *fork*(2) do not inherit privileged scheduling policies. After the _reset-on-fork_ flag has been enabled, it can be reset only if the thread has the *CAP_SYS_NICE* capability. This flag is disabled in child processes created by *fork*(2).
+
More precisely, if the _reset-on-fork_ flag is set, the following rules apply for subsequently created children:
+
* If the calling thread has a scheduling policy of *SCHED_FIFO* or *SCHED_RR*, the policy is reset to *SCHED_OTHER* in child processes.
+
* If the calling process has a negative nice value, the nice value is reset to zero in child processes.

== OPTIONS

*-a*, *--all-tasks*::
Set or retrieve the scheduling attributes of all the tasks (threads) for a given PID.

*-m*, *--max*::
Show minimum and maximum valid priorities, then exit.

*-p*, *--pid*::
Operate on an existing PID and do not launch a new task.

*-v*, *--verbose*::
Show status information.

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== EXAMPLES

//TRANSLATORS: Keep {colon} untranslated
The default behavior is to run a new command{colon}::
____
*chrt* _priority_ _command_ [_arguments_]
____
//TRANSLATORS: Keep {colon} untranslated
You can also retrieve the real-time attributes of an existing task{colon}::
____
*chrt -p* _PID_
____
//TRANSLATORS: Keep {colon} untranslated
Or set them{colon}::
____
*chrt -r -p* _priority PID_
____
This, for example, sets real-time scheduling to priority _30_ for the process _PID_ with the *SCHED_RR* (round-robin) class{colon}::
____
*chrt -r -p 30* _PID_
____
Reset priorities to default for a process{colon}::
____
*chrt -o -p 0* _PID_
____
See *sched*(7) for a detailed discussion of the different scheduler classes and how they interact.

== PERMISSIONS

A user must possess *CAP_SYS_NICE* to change the scheduling attributes of a process. Any user can retrieve the scheduling information.

== NOTES

Only *SCHED_FIFO*, *SCHED_OTHER* and *SCHED_RR* are part of POSIX 1003.1b Process Scheduling. The other scheduling attributes may be ignored on some systems.

Linux' default scheduling policy is *SCHED_OTHER*.

== AUTHORS

mailto:rml@tech9.net[Robert Love],
mailto:kzak@redhat.com[Karel Zak]

== SEE ALSO

*nice*(1),
*renice*(1),
*taskset*(1),
*sched*(7)

See *sched_setscheduler*(2) for a description of the Linux scheduling scheme.

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