sched_getaffinity

       affinity mask

SYNOPSIS
       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #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 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 dedi-
       cating  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  call-
       ing  thread  is used.  The argument cpusetsize is the length (in bytes)
       of the data pointed to by mask.  Normally this argument would be speci-
       fied 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.

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  thread
              according  to  any  restrictions  that  may  be  imposed  by the
              "cpuset" mechanism described in cpuset(7).

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
       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 inde-
       pendently 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  suc-
       cess,  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
       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), pthread_setaffinity_np(3), sched_getcpu(3),
       capabilities(7), cpuset(7)

COLOPHON
       This page is part of release 3.54 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/.
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