.\" Copyright (C) Tom Bjorkholm & Markus Kuhn, 1996
.\"
.\" SPDX-License-Identifier: GPL-2.0-or-later
.\"
.\" 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 2023-10-31 "Linux man-pages 6.7"
.SH NAME
sched_rr_get_interval \- get the SCHED_RR interval for the named process
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sched.h>
.P
.BI "int sched_rr_get_interval(pid_t " pid ", struct timespec *" tp );
.fi
.SH DESCRIPTION
.BR sched_rr_get_interval ()
writes into the
.BR timespec (3)
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.
.P
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 to indicate the error.
.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 VERSIONS
.SS Linux
Linux 3.9 added
.\" commit ce0dbbbb30aee6a835511d5be446462388ba9eee
a new mechanism for adjusting (and viewing) the
.B 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 STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001.
.SS Linux
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.
.SH NOTES
POSIX systems on which
.BR sched_rr_get_interval ()
is available define
.B _POSIX_PRIORITY_SCHEDULING
in
.IR <unistd.h> .
.\" .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 timespec (3),
.BR sched (7)