NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON
SCHED_SETAFFINITY(2) Linux Programmer's Manual SCHED_SETAFFINITY(2)
sched_setaffinity, sched_getaffinity - set and get a process's CPU affinity
mask
#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);
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.
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 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 appropriate
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.
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 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.
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)
This page is part of release 3.23 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/.
Linux 2008-11-14 SCHED_SETAFFINITY(2)