man2/sched_rr_get_interval.2

   1 .\" Copyright (C) Tom Bjorkholm & Markus Kuhn, 1996
   2 .\"
   3 .\" SPDX-License-Identifier: GPL-2.0-or-later
   4 .\"
   5 .\" 1996-04-01 Tom Bjorkholm <tomb@mydata.se>
   6 .\"            First version written
   7 .\" 1996-04-10 Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
   8 .\"            revision
   9 .\"
  10 .TH SCHED_RR_GET_INTERVAL 2 2021-03-22 "Linux" "Linux Programmer's Manual"
  11 .SH NAME
  12 sched_rr_get_interval \- get the SCHED_RR interval for the named process
  13 .SH LIBRARY
  14 Standard C library
  15 .RI ( libc ", " \-lc )
  16 .SH SYNOPSIS
  17 .nf
  18 .B #include <sched.h>
  19 .PP
  20 .BI "int sched_rr_get_interval(pid_t " pid ", struct timespec *" tp );
  21 .fi
  22 .SH DESCRIPTION
  23 .BR sched_rr_get_interval ()
  24 writes into the
  25 .BR timespec (3)
  26 structure pointed to by
  27 .I tp
  28 the round-robin time quantum for the process identified by
  29 .IR pid .
  30 The specified process should be running under the
  31 .B SCHED_RR
  32 scheduling policy.
  33 .PP
  34 If
  35 .I pid
  36 is zero, the time quantum for the calling process is written into
  37 .IR *tp .
  38 .\" FIXME . On Linux, sched_rr_get_interval()
  39 .\" returns the timeslice for SCHED_OTHER processes -- this timeslice
  40 .\" is influenced by the nice value.
  41 .\" For SCHED_FIFO processes, this always returns 0.
  42 .\"
  43 .\" The round-robin time quantum value is not alterable under Linux
  44 .\" 1.3.81.
  45 .\"
  46 .SH RETURN VALUE
  47 On success,
  48 .BR sched_rr_get_interval ()
  49 returns 0.
  50 On error, \-1 is returned, and
  51 .I errno
  52 is set to indicate the error.
  53 .SH ERRORS
  54 .TP
  55 .B EFAULT
  56 Problem with copying information to user space.
  57 .TP
  58 .B EINVAL
  59 Invalid pid.
  60 .TP
  61 .B ENOSYS
  62 The system call is not yet implemented (only on rather old kernels).
  63 .TP
  64 .B ESRCH
  65 Could not find a process with the ID
  66 .IR pid .
  67 .SH CONFORMING TO
  68 POSIX.1-2001, POSIX.1-2008.
  69 .SH NOTES
  70 POSIX systems on which
  71 .BR sched_rr_get_interval ()
  72 is available define
  73 .B _POSIX_PRIORITY_SCHEDULING
  74 in
  75 .IR <unistd.h> .
  76 .SS Linux notes
  77 POSIX does not specify any mechanism for controlling the size of the
  78 round-robin time quantum.
  79 Older Linux kernels provide a (nonportable) method of doing this.
  80 The quantum can be controlled by adjusting the process's nice value (see
  81 .BR setpriority (2)).
  82 Assigning a negative (i.e., high) nice value results in a longer quantum;
  83 assigning a positive (i.e., low) nice value results in a shorter quantum.
  84 The default quantum is 0.1 seconds;
  85 the degree to which changing the nice value affects the
  86 quantum has varied somewhat across kernel versions.
  87 This method of adjusting the quantum was removed
  88 .\" commit a4ec24b48ddef1e93f7578be53270f0b95ad666c
  89 starting with Linux 2.6.24.
  90 .PP
  91 Linux 3.9 added
  92 .\" commit ce0dbbbb30aee6a835511d5be446462388ba9eee
  93 a new mechanism for adjusting (and viewing) the
  94 .BR SCHED_RR
  95 quantum: the
  96 .I /proc/sys/kernel/sched_rr_timeslice_ms
  97 file exposes the quantum as a millisecond value, whose default is 100.
  98 Writing 0 to this file resets the quantum to the default value.
  99 .\" .SH BUGS
 100 .\" As of Linux 1.3.81
 101 .\" .BR sched_rr_get_interval ()
 102 .\" returns with error
 103 .\" ENOSYS, because SCHED_RR has not yet been fully implemented and tested
 104 .\" properly.
 105 .SH SEE ALSO
 106 .BR timespec (3),
 107 .BR sched (7)