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/.