chrt command: Set / Manipulate Real Time Attributes of a Linux Process

by on March 6, 2008 · 5 comments· LAST UPDATED March 6, 2008

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Q. How do I set the real time scheduling priority of a process under Linux operating system using a shell prompt?

A. You can use chrt command to set or retrieve the real-time scheduling attributes / scheduling priority of an existing PID. You can also run COMMAND with the given attributes.

Understanding Linux Scheduling Priorities

From the man page:
The scheduler is the kernel part that decides which runnable process will be executed by the CPU next. The Linux scheduler offers three different scheduling policies, one for normal processes and two for real-time applications.

  1. SCHED_OTHER - the default universal time-sharing scheduler policy used by most processes.
  2. SCHED_FIFO or SCHED_RR - intended for special time-critical applications that need precise control over the way in which runnable processes are selected for execution
  3. SCHED_BATCH - intended for "batch" style execution of processes
WARNING! These examples requires proper understanding of UNIX / Linux scheduling algorithms; don't try this on production systems; if you are not familiar with Linux scheduling algorithms.

Scheduling Algorithm

  • SCHED_FIFO uses First In-First Out scheduling algorithm
  • SCHED_RR uses Round Robin scheduling algorithm
  • SCHED_OTHER uses Default Linux time-sharing scheduling algorithm
  • SCHED_BATCH use Scheduling batch processes algorithm
WARNING! Standard Linux kernel is used in as a general-purpose operating system and can handle background processes, interactive applications, and soft real-time applications (applications that need to usually meet timing deadlines). chrt command is directed at these kinds of applications.

util-linux package

chrt command is part of util-linux package - low-level system utilities that are necessary for a Linux system to function. It is installed by default under Debian / Ubuntu / CentOS / RHEL / Fedora and almost all other Linux distributions.

How do I use chrt command to get real time attributes of a Linux process?

To get / retrieve the real-time attributes of an existing task / PID, enter:
# chrt -p pid
# chrt -p 112
# chrt -p 1

Output:

pid 1's current scheduling policy: SCHED_OTHER
pid 1's current scheduling priority: 0

Any user can retrieve the scheduling information. No special privileges required.

How do I use chrt command to set real time attributes of a Linux process (already running processes)?

Use the syntax as follows to set new priority:
# chrt -p prio pid
# chrt -p 1025
# chrt -p 55 1025
# chrt -p 1025

Before setting new scheduling policy, you need to find out minimum and maximum valid priorities for each scheduling algorithm, enter:
# chrt -m
Output:

SCHED_OTHER min/max priority    : 0/0
SCHED_FIFO min/max priority     : 1/99
SCHED_RR min/max priority       : 1/99
SCHED_BATCH min/max priority    : 0/0

How do I set SCHED_BATCH scheduling policy?

To set scheduling policy to SCHED_BATCH, enter:
# chrt -b -p 0 {pid}
# chrt -b -p 0 1024

How do I set SCHED_FIFO scheduling policy?

To set scheduling policy to SCHED_FIFO, enter:
# chrt -f -p [1..99] {pid}
Set policy to SCHED_FIFO with 50 priority:
# chrt -f -p 50 1024
# chrt -p 1024

How do I set SCHED_OTHER scheduling policy?

To set policy scheduling policy to SCHED_OTHER, enter:
# chrt -o -p 0 {pid}
# chrt -o -p 0 1024
# chrt -p 1024

How do I set SCHED_RR scheduling policy?

To set scheduling policy to SCHED_RR, enter:
# chrt -r -p [1..99] {pid}
Set policy to SCHED_RR scheduling with 20 priority:
# chrt -r -p 20 1024
# chrt -p 1024

Further readings:

If you run CPU-intensive processes, you should familiarize yourself with other tools:

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{ 5 comments… read them below or add one }

1 PradeepR March 22, 2010 at 4:38 am

Hi,

Thanks for the info it was very useful, but i had a problem where i had to set RT scheduling for processes without root privilages .
i overcame the problem by adding the following to /etc/security/limits.conf

@users – rtprio 99

if the 2.6.x kernel is built with the following option

Security options —>
[*] Enable different security models

this works with out any problem

thanks
PradeepR

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2 Nick April 21, 2010 at 9:07 am

I am having a problem with this setting a processes attributes, to real time. I get the message:

xxxxx@xxxxxx:/usr/hlds# chrt -f -p 98 hlds_amd
sched_setscheduler: Operation not permitted
failed to set pid 0’s policy

What would cause this?

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3 Ben April 10, 2012 at 6:02 pm

This article is barely more useful than the man page. I was hoping for more. When would I want to use one of these over another? What are the differences betwen SCHED_RR and SCHED_FIFO? SCHED_IDLE vs SCHED_BATCH? How about an example of a task that would be a better candidate for SCHED_BATCH than SCHED_IDLE, etc? Can I make sure that a task NEVER consumes CPU resources while any other task wants CPU? How can I play highly compressed HD video smoothly while running 10 concurrent CPU-bound Matlab jobs?

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4 Rahim Azizarab October 26, 2012 at 10:55 am

This command can be used to specify which processor(s) a task must run on on a multi core system. Has anyone experimented with it in such a manner?

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5 Naresh Pirati August 22, 2014 at 6:15 am

what is the default scheduling algorithm present in linux kernel?

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