|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
|
|
|
sched_setaffinity(2) System Calls Manual sched_setaffinity(2)
sched_setaffinity, sched_getaffinity - set and get a thread's CPU
affinity mask
Standard C library (libc, -lc)
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sched.h>
int sched_setaffinity(pid_t pid, size_t cpusetsize,
const cpu_set_t *mask);
int sched_getaffinity(pid_t pid, size_t cpusetsize,
cpu_set_t *mask);
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.
A CPU affinity mask is represented by the cpu_set_t structure, a
"CPU set", pointed to by mask. A set of macros for manipulating
CPU sets is described in CPU_SET(3).
sched_setaffinity() sets the CPU affinity mask of the thread whose
ID is pid to the value specified by mask. If pid is zero, then
the calling thread is used. The argument cpusetsize is the length
(in bytes) of the data pointed to by mask. Normally this argument
would be specified as sizeof(cpu_set_t).
If the thread specified by pid is not currently running on one of
the CPUs specified in mask, then that thread is migrated to one of
the CPUs specified in mask.
sched_getaffinity() writes the affinity mask of the thread whose
ID is pid into the cpu_set_t structure pointed to by mask. The
cpusetsize argument specifies the size (in bytes) of mask. If pid
is zero, then the mask of the calling thread is returned.
On success, sched_setaffinity() and sched_getaffinity() return 0
(but see "C library/kernel differences" below, which notes that
the underlying sched_getaffinity() differs in its return value).
On failure, -1 is returned, and errno is set to indicate the
error.
EFAULT A supplied memory address was invalid.
EINVAL The affinity bit mask 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
cpuset cgroups or the "cpuset" mechanism described in
cpuset(7).
EINVAL (sched_getaffinity() and, before Linux 2.6.9,
sched_setaffinity()) cpusetsize is smaller than the size of
the affinity mask used by the kernel.
EPERM (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 pid, or it must possess the
CAP_SYS_NICE capability in the user namespace of the thread
pid.
ESRCH The thread whose ID is pid could not be found.
Linux.
Linux 2.5.8, glibc 2.3.
Initially, the glibc interfaces included a cpusetsize argument,
typed as unsigned int. In glibc 2.3.3, the cpusetsize argument
was removed, but was then restored in glibc 2.3.4, with type
size_t.
After a call to sched_setaffinity(), the set of CPUs on which the
thread will actually run is the intersection of the set specified
in the 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 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.
There are various ways of determining the number of CPUs available
on the system, including: inspecting the contents of
/proc/cpuinfo; using sysconf(3) to obtain the values of the
_SC_NPROCESSORS_CONF and _SC_NPROCESSORS_ONLN parameters; and
inspecting the list of CPU directories under
/sys/devices/system/cpu/.
sched(7) has a description of the Linux scheduling scheme.
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 gettid(2) can be passed in the
argument pid. Specifying pid as 0 will set the attribute for the
calling thread, and passing the value returned from a call to
getpid(2) will set the attribute for the main thread of the thread
group. (If you are using the POSIX threads API, then use
pthread_setaffinity_np(3) instead of sched_setaffinity().)
The isolcpus boot option can be used to isolate one or more CPUs
at boot time, so that no processes are scheduled onto those CPUs.
Following the use of this boot option, the only way to schedule
processes onto the isolated CPUs is via sched_setaffinity() or the
cpuset(7) mechanism. For further information, see the kernel
source file Documentation/admin-guide/kernel-parameters.txt. As
noted in that file, isolcpus is the preferred mechanism of
isolating CPUs (versus the alternative of manually setting the CPU
affinity of all processes on the system).
A child created via fork(2) inherits its parent's CPU affinity
mask. The affinity mask is preserved across an execve(2).
C library/kernel differences
This manual page describes the glibc interface for the CPU
affinity calls. The actual system call interface is slightly
different, with the mask being typed as unsigned long *,
reflecting the fact that the underlying implementation of CPU sets
is a simple bit mask.
On success, the raw sched_getaffinity() system call returns the
number of bytes placed copied into the mask buffer; this will be
the minimum of cpusetsize and the size (in bytes) of the cpumask_t
data type that is used internally by the kernel to represent the
CPU set bit mask.
Handling systems with large CPU affinity masks
The underlying system calls (which represent CPU masks as bit
masks of type unsigned long *) impose no restriction on the size
of the CPU mask. However, the cpu_set_t data type used by glibc
has a fixed size of 128 bytes, meaning that the maximum CPU number
that can be represented is 1023. If the kernel CPU affinity mask
is larger than 1024, then calls of the form:
sched_getaffinity(pid, sizeof(cpu_set_t), &mask);
fail with the error EINVAL, the error produced by the underlying
system call for the case where the mask size specified in
cpusetsize is smaller than the size of the affinity mask used by
the kernel. (Depending on the system CPU topology, the kernel
affinity mask can be substantially larger than the number of
active CPUs in the system.)
When working on systems with large kernel CPU affinity masks, one
must dynamically allocate the mask argument (see CPU_ALLOC(3)).
Currently, the only way to do this is by probing for the size of
the required mask using sched_getaffinity() calls with increasing
mask sizes (until the call does not fail with the error EINVAL).
Be aware that CPU_ALLOC(3) may allocate a slightly larger CPU set
than requested (because CPU sets are implemented as bit masks
allocated in units of sizeof(long)). Consequently,
sched_getaffinity() can set bits beyond the requested allocation
size, because the kernel sees a few additional bits. Therefore,
the caller should iterate over the bits in the returned set,
counting those which are set, and stop upon reaching the value
returned by CPU_COUNT(3) (rather than iterating over the number of
bits requested to be allocated).
The program below creates a child process. The parent and child
then each assign themselves to a specified CPU and execute
identical loops that consume some CPU time. Before terminating,
the parent waits for the child to complete. The program takes
three command-line arguments: the CPU number for the parent, the
CPU number for the child, and the number of loop iterations that
both processes should perform.
As the sample runs below demonstrate, the amount of real and CPU
time consumed when running the program will depend on intra-core
caching effects and whether the processes are using the same CPU.
We first employ lscpu(1) to determine that this (x86) system has
two cores, each with two CPUs:
$ lscpu | egrep -i 'core.*:|socket';
Thread(s) per core: 2
Core(s) per socket: 2
Socket(s): 1
We then time the operation of the example program for three cases:
both processes running on the same CPU; both processes running on
different CPUs on the same core; and both processes running on
different CPUs on different cores.
$ time -p ./a.out 0 0 100000000;
real 14.75
user 3.02
sys 11.73
$ time -p ./a.out 0 1 100000000;
real 11.52
user 3.98
sys 19.06
$ time -p ./a.out 0 3 100000000;
real 7.89
user 3.29
sys 12.07
Program source
#define _GNU_SOURCE
#include <err.h>
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>
int
main(int argc, char *argv[])
{
int parentCPU, childCPU;
cpu_set_t set;
unsigned int nloops;
if (argc != 4) {
fprintf(stderr, "Usage: %s parent-cpu child-cpu num-loops\n",
argv[0]);
exit(EXIT_FAILURE);
}
parentCPU = atoi(argv[1]);
childCPU = atoi(argv[2]);
nloops = atoi(argv[3]);
CPU_ZERO(&set);
switch (fork()) {
case -1: /* Error */
err(EXIT_FAILURE, "fork");
case 0: /* Child */
CPU_SET(childCPU, &set);
if (sched_setaffinity(0, sizeof(set), &set) == -1)
err(EXIT_FAILURE, "sched_setaffinity");
for (unsigned int j = 0; j < nloops; j++)
getppid();
exit(EXIT_SUCCESS);
default: /* Parent */
CPU_SET(parentCPU, &set);
if (sched_setaffinity(0, sizeof(set), &set) == -1)
err(EXIT_FAILURE, "sched_setaffinity");
for (unsigned int j = 0; j < nloops; j++)
getppid();
wait(NULL); /* Wait for child to terminate */
exit(EXIT_SUCCESS);
}
}
lscpu(1), nproc(1), taskset(1), clone(2), getcpu(2),
getpriority(2), gettid(2), nice(2), sched_get_priority_max(2),
sched_get_priority_min(2), sched_getscheduler(2),
sched_setscheduler(2), setpriority(2), CPU_SET(3), get_nprocs(3),
pthread_setaffinity_np(3), sched_getcpu(3), capabilities(7),
cpuset(7), sched(7), numactl(8)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-30 sched_setaffinity(2)
Pages that refer to this page: systemd-nspawn(1), taskset(1), getcpu(2), gettid(2), sched_get_priority_max(2), sched_setattr(2), sched_setparam(2), sched_setscheduler(2), syscalls(2), CPU_SET(3), numa(3), pthread_attr_setaffinity_np(3), pthread_create(3), pthread_setaffinity_np(3), systemd.exec(5), capabilities(7), cpuset(7), credentials(7), pthreads(7), sched(7), migratepages(8), numactl(8)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|