mirror of https://github.com/mkerrisk/man-pages
233 lines
6.9 KiB
Groff
233 lines
6.9 KiB
Groff
.\" Copyright (C) 2002 Robert Love
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.\" and Copyright (C) 2006 Michael Kerrisk
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.\"
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.\" %%%LICENSE_START(GPLv2+_DOC_FULL)
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.\" This is free documentation; you can redistribute it and/or
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.\" modify it under the terms of the GNU General Public License as
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.\" published by the Free Software Foundation; either version 2 of
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.\" the License, or (at your option) any later version.
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.\"
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.\" The GNU General Public License's references to "object code"
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.\" and "executables" are to be interpreted as the output of any
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.\" document formatting or typesetting system, including
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.\" intermediate and printed output.
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.\"
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.\" This manual is distributed in the hope that it will be useful,
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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.\" GNU General Public License for more details.
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.\"
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.\" You should have received a copy of the GNU General Public
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.\" License along with this manual; if not, see
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.\" <http://www.gnu.org/licenses/>.
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.\" %%%LICENSE_END
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.\"
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.\" 2002-11-19 Robert Love <rml@tech9.net> - initial version
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.\" 2004-04-20 mtk - fixed description of return value
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.\" 2004-04-22 aeb - added glibc prototype history
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.\" 2005-05-03 mtk - noted that sched_setaffinity may cause thread
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.\" migration and that CPU affinity is a per-thread attribute.
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.\" 2006-02-03 mtk -- Major rewrite
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.\" 2008-11-12, mtk, removed CPU_*() macro descriptions to a
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.\" separate CPU_SET(3) page.
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.\"
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.TH SCHED_SETAFFINITY 2 2013-02-11 "Linux" "Linux Programmer's Manual"
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.SH NAME
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sched_setaffinity, sched_getaffinity \- \
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set and get a process's CPU affinity mask
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.SH SYNOPSIS
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.nf
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.BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
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.B #include <sched.h>
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.sp
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.BI "int sched_setaffinity(pid_t " pid ", size_t " cpusetsize ,
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.BI " cpu_set_t *" mask );
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.sp
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.BI "int sched_getaffinity(pid_t " pid ", size_t " cpusetsize ,
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.BI " cpu_set_t *" mask );
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.fi
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.SH DESCRIPTION
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A process's CPU affinity mask determines the set of CPUs on which
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it is eligible to run.
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On a multiprocessor system, setting the CPU affinity mask
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can be used to obtain performance benefits.
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For example,
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by dedicating one CPU to a particular process
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(i.e., setting the affinity mask of that process to specify a single CPU,
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and setting the affinity mask of all other processes to exclude that CPU),
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it is possible to ensure maximum execution speed for that process.
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Restricting a process to run on a single CPU also avoids
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the performance cost caused by the cache invalidation that occurs
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when a process ceases to execute on one CPU and then
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recommences execution on a different CPU.
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A CPU affinity mask is represented by the
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.I cpu_set_t
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structure, a "CPU set", pointed to by
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.IR mask .
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A set of macros for manipulating CPU sets is described in
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.BR CPU_SET (3).
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.BR sched_setaffinity ()
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sets the CPU affinity mask of the process whose ID is
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.I pid
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to the value specified by
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.IR mask .
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If
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.I pid
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is zero, then the calling process is used.
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The argument
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.I cpusetsize
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is the length (in bytes) of the data pointed to by
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.IR mask .
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Normally this argument would be specified as
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.IR "sizeof(cpu_set_t)" .
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If the process specified by
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.I pid
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is not currently running on one of the CPUs specified in
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.IR mask ,
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then that process is migrated to one of the CPUs specified in
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.IR mask .
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.BR sched_getaffinity ()
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writes the affinity mask of the process whose ID is
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.I pid
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into the
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.I cpu_set_t
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structure pointed to by
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.IR mask .
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The
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.I cpusetsize
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argument specifies the size (in bytes) of
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.IR mask .
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If
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.I pid
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is zero, then the mask of the calling process is returned.
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.SH RETURN VALUE
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On success,
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.BR sched_setaffinity ()
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and
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.BR sched_getaffinity ()
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return 0.
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On error, \-1 is returned, and
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.I errno
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is set appropriately.
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.SH ERRORS
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.TP
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.B EFAULT
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A supplied memory address was invalid.
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.TP
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.B EINVAL
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The affinity bit mask
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.I mask
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contains no processors that are currently physically on the system
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and permitted to the process according to any restrictions that
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may be imposed by the "cpuset" mechanism described in
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.BR cpuset (7).
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.TP
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.B EINVAL
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.RB ( sched_getaffinity ()
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and, in kernels before 2.6.9,
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.BR sched_setaffinity ())
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.I cpusetsize
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is smaller than the size of the affinity mask used by the kernel.
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.TP
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.B EPERM
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.RB ( sched_setaffinity ())
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The calling process does not have appropriate privileges.
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The caller needs an effective user ID equal to the real user ID
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or effective user ID of the process identified by
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.IR pid ,
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or it must possess the
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.B CAP_SYS_NICE
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capability.
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.TP
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.B ESRCH
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The process whose ID is \fIpid\fP could not be found.
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.SH VERSIONS
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The CPU affinity system calls were introduced in Linux kernel 2.5.8.
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The system call wrappers were introduced in glibc 2.3.
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Initially, the glibc interfaces included a
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.I cpusetsize
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argument, typed as
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.IR "unsigned int" .
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In glibc 2.3.3, the
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.I cpusetsize
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argument was removed, but was then restored in glibc 2.3.4, with type
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.IR size_t .
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.SH CONFORMING TO
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These system calls are Linux-specific.
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.SH NOTES
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After a call to
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.BR sched_setaffinity (),
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the set of CPUs on which the process will actually run is
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the intersection of the set specified in the
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.I mask
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argument and the set of CPUs actually present on the system.
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The system may further restrict the set of CPUs on which the process
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runs if the "cpuset" mechanism described in
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.BR cpuset (7)
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is being used.
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These restrictions on the actual set of CPUs on which the process
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will run are silently imposed by the kernel.
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.BR sched_setscheduler (2)
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has a description of the Linux scheduling scheme.
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.PP
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The affinity mask is actually a per-thread attribute that can be
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adjusted independently for each of the threads in a thread group.
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The value returned from a call to
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.BR gettid (2)
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can be passed in the argument
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.IR pid .
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Specifying
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.I pid
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as 0 will set the attribute for the calling thread,
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and passing the value returned from a call to
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.BR getpid (2)
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will set the attribute for the main thread of the thread group.
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(If you are using the POSIX threads API, then use
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.BR pthread_setaffinity_np (3)
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instead of
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.BR sched_setaffinity ().)
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A child created via
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.BR fork (2)
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inherits its parent's CPU affinity mask.
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The affinity mask is preserved across an
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.BR execve (2).
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This manual page describes the glibc interface for the CPU affinity calls.
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The actual system call interface is slightly different, with the
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.I mask
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being typed as
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.IR "unsigned long *" ,
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reflecting the fact that the underlying implementation of CPU
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sets is a simple bit mask.
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On success, the raw
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.BR sched_getaffinity ()
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system call returns the size (in bytes) of the
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.I cpumask_t
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data type that is used internally by the kernel to
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represent the CPU set bit mask.
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.SH SEE ALSO
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.ad l
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.nh
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.BR taskset (1),
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.BR clone (2),
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.BR getcpu (2),
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.BR getpriority (2),
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.BR gettid (2),
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.BR nice (2),
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.BR sched_get_priority_max (2),
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.BR sched_get_priority_min (2),
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.BR sched_getscheduler (2),
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.BR sched_setscheduler (2),
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.BR setpriority (2),
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.BR CPU_SET (3),
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.BR pthread_setaffinity_np (3),
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.BR sched_getcpu (3),
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.BR capabilities (7),
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.BR cpuset (7)
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