sched_setaffinity(2)
NAME
sched_setaffinity, sched_getaffinity - set and get a process's CPU
affinity mask
SYNOPSIS
#define _GNU_SOURCE
#include <sched.h>
int sched_setaffinity(pid_t pid, size_t cpusetsize,
cpu_set_t *mask);
int sched_getaffinity(pid_t pid, size_t cpusetsize,
cpu_set_t *mask);
DESCRIPTION
A process'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 process (i.e., setting the affinity mask
of that process to specify a single CPU, and setting the affinity mask
of all other processes to exclude that CPU), it is possible to ensure
maximum execution speed for that process. Restricting a process to run
on a single CPU also avoids the performance cost caused by the cache
invalidation that occurs when a process 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 process whose ID
is pid to the value specified by mask. If pid is zero, then the calling process 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 process specified by pid is not currently running on one of the
CPUs specified in mask, then that process is migrated to one of the
CPUs specified in mask.
sched_getaffinity() writes the affinity mask of the process 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 process is returned.
RETURN VALUE
On success, sched_setaffinity() and sched_getaffinity() return 0. On
error, -1 is returned, and errno is set appropriately.
ERRORS
EFAULT A supplied memory address was invalid.
- EINVAL The affinity bit mask mask contains no processors that are cur
- rently physically on the system and permitted to the process 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 process does not have appro
- priate privileges. The caller needs an effective user ID equal to the user ID or effective user ID of the process identified by pid, or it must possess the CAP_SYS_NICE capability.
- ESRCH The process whose ID is pid could not be found.
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 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.
CONFORMING TO
These system calls are Linux-specific.
NOTES
After a call to sched_setaffinity(), the set of CPUs on which the
process 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 process
runs if the "cpuset" mechanism described in cpuset(7) is being used.
These restrictions on the actual set of CPUs on which the process will
run are silently imposed by the kernel.
sched_setscheduler(2) has a description of the Linux scheduling scheme.
The affinity mask is actually 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().)
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.
SEE ALSO
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), sched_getcpu(3), capabilities(7), pthread_setaffinity_np(3), cpuset(7)
COLOPHON
- This page is part of release 3.25 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/.