SCHED_SETAFFINITY(2)                Linux Programmer's Manual                SCHED_SETAFFINITY(2)



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,
                             const 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  perfor‐
       mance  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 cpuset‐
       size  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 cpuset‐
       size  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(7) 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).

   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 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), sched(7)

COLOPHON
       This page is part of release 3.74 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.



Linux                                       2014-08-19                       SCHED_SETAFFINITY(2)