sched_setaffinity, sched_getaffinity - set and get a thread's CPU affinity mask
#define _GNU_SOURCE /* See feature_test_macros(7) */
int sched_setaffinity(pid_t pid, size_t cpusetsize,
const cpu_set_t *mask);
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
"CPU set", pointed to by mask
. A set of macros for
manipulating CPU sets is described in CPU_SET(3)
() sets the CPU affinity mask of the thread whose ID is
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
() writes the affinity mask of the thread whose ID is
into the cpu_set_t
structure pointed to by mask
argument specifies the size (in bytes) of mask
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.
- A supplied memory address was invalid.
- 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).
- (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.
- (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.
- 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.
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
can be passed in the argument 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
instead of sched_setaffinity
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 ABI differences¶
This manual page describes the glibc interface for the CPU affinity calls. The
actual system call interface is slightly different, with the mask
typed as unsigned long *
, reflecting the fact that the
underlying implementation of CPU sets is a simple bit mask. On success, the
() system call returns the size (in bytes) of the
data type that is used internally by the kernel to represent
the CPU set bit mask.
This page is part of release 3.74 of the Linux man-pages
description of the project, information about reporting bugs, and the latest
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