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[thirdparty/man-pages.git] / man2 / sched_setaffinity.2

.\" Copyright (C) 2002 Robert Love
.\" and Copyright (C) 2006, 2015 Michael Kerrisk
.\"
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.\" 2002-11-19 Robert Love <rml@tech9.net> - initial version
.\" 2004-04-20 mtk - fixed description of return value
.\" 2004-04-22 aeb - added glibc prototype history
.\" 2005-05-03 mtk - noted that sched_setaffinity may cause thread
.\"	migration and that CPU affinity is a per-thread attribute.
.\" 2006-02-03 mtk -- Major rewrite
.\" 2008-11-12, mtk, removed CPU_*() macro descriptions to a
.\" separate CPU_SET(3) page.
.\"
.TH SCHED_SETAFFINITY 2 2019-10-10 "Linux" "Linux Programmer's Manual"
.SH NAME
sched_setaffinity, sched_getaffinity \- \
set and get a thread's CPU affinity mask
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
.B #include <sched.h>
.PP
.BI "int sched_setaffinity(pid_t " pid ", size_t " cpusetsize ,
.BI "                      const cpu_set_t *" mask );
.PP
.BI "int sched_getaffinity(pid_t " pid ", size_t " cpusetsize ,
.BI "                      cpu_set_t *" mask );
.fi
.SH DESCRIPTION
A thread's CPU affinity mask determines the set of CPUs on which
it is eligible to run.
On a multiprocessor system, setting the CPU affinity mask
can be used to obtain performance benefits.
For example,
by dedicating one CPU to a particular thread
(i.e., setting the affinity mask of that thread to specify a single CPU,
and setting the affinity mask of all other threads to exclude that CPU),
it is possible to ensure maximum execution speed for that thread.
Restricting a thread to run on a single CPU also avoids
the performance cost caused by the cache invalidation that occurs
when a thread ceases to execute on one CPU and then
recommences execution on a different CPU.
.PP
A CPU affinity mask is represented by the
.I cpu_set_t
structure, a "CPU set", pointed to by
.IR mask .
A set of macros for manipulating CPU sets is described in
.BR CPU_SET (3).
.PP
.BR sched_setaffinity ()
sets the CPU affinity mask of the thread whose ID is
.I pid
to the value specified by
.IR mask .
If
.I pid
is zero, then the calling thread is used.
The argument
.I cpusetsize
is the length (in bytes) of the data pointed to by
.IR mask .
Normally this argument would be specified as
.IR "sizeof(cpu_set_t)" .
.PP
If the thread specified by
.I pid
is not currently running on one of the CPUs specified in
.IR mask ,
then that thread is migrated to one of the CPUs specified in
.IR mask .
.PP
.BR sched_getaffinity ()
writes the affinity mask of the thread whose ID is
.I pid
into the
.I cpu_set_t
structure pointed to by
.IR mask .
The
.I cpusetsize
argument specifies the size (in bytes) of
.IR mask .
If
.I pid
is zero, then the mask of the calling thread is returned.
.SH RETURN VALUE
On success,
.BR sched_setaffinity ()
and
.BR sched_getaffinity ()
return 0 (but see "C library/kernel differences" below,
which notes that the underlying
.BR sched_getaffinity ()
differs in its return value).
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EFAULT
A supplied memory address was invalid.
.TP
.B EINVAL
The affinity bit mask
.I mask
contains no processors that are currently physically on the system
and permitted to the thread according to any restrictions that
may be imposed by
.I cpuset
cgroups or the "cpuset" mechanism described in
.BR cpuset (7).
.TP
.B EINVAL
.RB ( sched_getaffinity ()
and, in kernels before 2.6.9,
.BR sched_setaffinity ())
.I cpusetsize
is smaller than the size of the affinity mask used by the kernel.
.TP
.B EPERM
.RB ( sched_setaffinity ())
The calling thread does not have appropriate privileges.
The caller needs an effective user ID equal to the real user ID
or effective user ID of the thread identified by
.IR pid ,
or it must possess the
.B CAP_SYS_NICE
capability in the user namespace of the thread
.IR pid .
.TP
.B ESRCH
The thread whose ID is \fIpid\fP could not be found.
.SH VERSIONS
The CPU affinity system calls were introduced in Linux kernel 2.5.8.
The system call wrappers were introduced in glibc 2.3.
Initially, the glibc interfaces included a
.I cpusetsize
argument, typed as
.IR "unsigned int" .
In glibc 2.3.3, the
.I cpusetsize
argument was removed, but was then restored in glibc 2.3.4, with type
.IR size_t .
.SH CONFORMING TO
These system calls are Linux-specific.
.SH NOTES
After a call to
.BR sched_setaffinity (),
the set of CPUs on which the thread will actually run is
the intersection of the set specified in the
.I mask
argument and the set of CPUs actually present on the system.
The system may further restrict the set of CPUs on which the thread
runs if the "cpuset" mechanism described in
.BR cpuset (7)
is being used.
These restrictions on the actual set of CPUs on which the thread
will run are silently imposed by the kernel.
.PP
There are various ways of determining the number of CPUs
available on the system, including: inspecting the contents of
.IR /proc/cpuinfo ;
using
.BR sysconf (3)
to obtain the values of the
.BR _SC_NPROCESSORS_CONF
and
.BR _SC_NPROCESSORS_ONLN
parameters; and inspecting the list of CPU directories under
.IR /sys/devices/system/cpu/ .
.PP
.BR sched (7)
has a description of the Linux scheduling scheme.
.PP
The affinity mask is a per-thread attribute that can be
adjusted independently for each of the threads in a thread group.
The value returned from a call to
.BR gettid (2)
can be passed in the argument
.IR pid .
Specifying
.I pid
as 0 will set the attribute for the calling thread,
and passing the value returned from a call to
.BR getpid (2)
will set the attribute for the main thread of the thread group.
(If you are using the POSIX threads API, then use