sched_setaffinity, sched_getaffinity - set and get a thread's CPU
#define _GNU_SOURCE /* See feature_test_macros(7) */
int sched_setaffinity(pid_t pid, size_t cpusetsize,
int sched_getaffinity(pid_t pid, size_t cpusetsize,
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. On
error, -1 is returned, and errno is set appropriately.
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 the
"cpuset" mechanism described in cpuset(7).
EINVAL (sched_getaffinity() and, in kernels before 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
ESRCH The thread whose ID is pid could not be found.
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 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.
These system calls are Linux-specific.
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.
sched_setscheduler(2) 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
A child created via fork(2) inherits its parent's CPU affinity mask.
The affinity mask is preserved across an execve(2).
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 size (in
bytes) of the cpumask_t data type that is used internally by the kernel
to represent the CPU set bit mask.
taskset(1), clone(2), getcpu(2), getpriority(2), gettid(2), nice(2),
sched_getscheduler(2), sched_setscheduler(2), setpriority(2),
CPU_SET(3), pthread_setaffinity_np(3), sched_getcpu(3),
This page is part of release 3.65 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.