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 3.54 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/.