.\" Copyright (C) Tom Bjorkholm & Markus Kuhn, 1996

.\"

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.\"

.\" 1996-04-01 Tom Bjorkholm <tomb@mydata.se>

.\"            First version written

.\" 1996-04-10 Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>

.\"            revision

.\"

.TH SCHED_RR_GET_INTERVAL 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

sched_rr_get_interval \- get the SCHED_RR interval for the named process

.SH SYNOPSIS

.B #include <sched.h>

.PP

.BI "int sched_rr_get_interval(pid_t " pid ", struct timespec *" tp );

.SH DESCRIPTION

.BR sched_rr_get_interval ()

writes into the

.I timespec

structure pointed to by

.I tp

the round-robin time quantum for the process identified by

.IR pid .

The specified process should be running under the

.B SCHED_RR

scheduling policy.

.PP

The

.I timespec

structure has the following form:

.PP

.in +4n

.EX

struct timespec {

    time_t tv_sec;    /* seconds */

    long   tv_nsec;   /* nanoseconds */

};

.EE

.in

.PP

If

.I pid

is zero, the time quantum for the calling process is written into

.IR *tp .

.\" FIXME . On Linux, sched_rr_get_interval()

.\" returns the timeslice for SCHED_OTHER processes -- this timeslice

.\" is influenced by the nice value.

.\" For SCHED_FIFO processes, this always returns 0.

.\"

.\" The round-robin time quantum value is not alterable under Linux

.\" 1.3.81.

.\"

.SH RETURN VALUE

On success,

.BR sched_rr_get_interval ()

returns 0.

On error, \-1 is returned, and

.I errno

is set appropriately.

.SH ERRORS

.TP

.B EFAULT

Problem with copying information to user space.

.TP

.B EINVAL

Invalid pid.

.TP

.B ENOSYS

The system call is not yet implemented (only on rather old kernels).

.TP

.B ESRCH

Could not find a process with the ID

.IR pid .

.SH CONFORMING TO

POSIX.1-2001, POSIX.1-2008.

.SH NOTES

POSIX systems on which

.BR sched_rr_get_interval ()

is available define

.B _POSIX_PRIORITY_SCHEDULING

in

.IR <unistd.h> .

.SS Linux notes

POSIX does not specify any mechanism for controlling the size of the

round-robin time quantum.

Older Linux kernels provide a (nonportable) method of doing this.

The quantum can be controlled by adjusting the process's nice value (see

.BR setpriority (2)).

Assigning a negative (i.e., high) nice value results in a longer quantum;

assigning a positive (i.e., low) nice value results in a shorter quantum.

The default quantum is 0.1 seconds;

the degree to which changing the nice value affects the

quantum has varied somewhat across kernel versions.

This method of adjusting the quantum was removed

.\" commit a4ec24b48ddef1e93f7578be53270f0b95ad666c

starting with Linux 2.6.24.

.PP

Linux 3.9 added

.\" commit ce0dbbbb30aee6a835511d5be446462388ba9eee

a new mechanism for adjusting (and viewing) the

.BR SCHED_RR

quantum: the

.I /proc/sys/kernel/sched_rr_timeslice_ms

file exposes the quantum as a millisecond value, whose default is 100.

Writing 0 to this file resets the quantum to the default value.

.\" .SH BUGS

.\" As of Linux 1.3.81

.\" .BR sched_rr_get_interval ()

.\" returns with error

.\" ENOSYS, because SCHED_RR has not yet been fully implemented and tested

.\" properly.

.SH SEE ALSO

.BR sched (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.