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NAME

       sched_setaffinity,  sched_getaffinity  -  set  and  get  a thread's CPU
       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
       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.

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
              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
              capability.

       ESRCH  The thread 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
       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
       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

       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 3.65 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/.



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