Linux Manipulating Real-time Attributes Of A Process

Last time I wrote about how-to set or retrieve the CPU affinity of a running process given its PID or to launch a new COMMAND with a given CPU affinity.

If you are looking to sets or retrieves the real-time scheduling attributes of an existing PID or want to runs COMMAND with the given attributes then I recommend to use chrt command.

nice command run a program with modified scheduling priority however, chrt is much more than nice command. The chrt command allows to set your scheduling policy as well as priority. Currently, the following three scheduling policies are supported under Linux:

• SCHED_FIFO: First In-First Out scheduling
• SCHED_RR: Round Robin scheduling
• SCHED_OTHER: Default Linux time-sharing scheduling

But what is a scheduler?

The scheduler is the Linux kernel part that decides which runnable process will be executed by the CPU next. As I said earlier Linux offers three scheduling policies. One is for normal process and can be changes using nice or chrt command (or use system calls under C). Discussion related to Linux scheduler and policies are beyond the scope of this blog post but, I recommend the following two textbooks for in-depth understanding of this topic:

1. Programming for the real world - POSIX.4 by Bill O. Gallmeister, O'Reilly & Associates, Inc.
2. Understanding the Linux Kernel, Second Edition by Daniel P. Bovet, Marco Cesati, O'Reilly & Associates, Inc.

Examples

You can retrieve the real-time attributes of an existing task/pid with the following command:
$ chrt -p 1
Output:

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

Set process (PID 5124) to 5 priority: remember priority range from -20 (most favorable scheduling) to 19 (least favorable), run:
$ chrt -p 5 5124
You can retrieve minimum and maximum valid priorities with the following command:
$ chrt -m
Sample outputs:

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

Set scheduling policy to SCHED_RR, enter:
$ chrt -p -r 5124
Where:

• -f: Set scheduling policy to SCHED_FIFO
• -o: Set scheduling policy to SCHED_OTHER
• -r: Set scheduling policy to SCHED_RR
• -p: Operate on an existing PID and do not launch a new tas

See also:

• man page chrt(1)
• Please note that chrt command may be more useful if you are using real time Linux kernel.

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