NAME

       sched_setaffinity,  sched_getaffinity  -  set  and  get a process'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 process'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 process (i.e., setting the affinity
       mask of that process to specify a single CPU, and setting the  affinity
       mask  of  all  other  processes to exclude that CPU), it is possible to
       ensure maximum execution speed for that process.  Restricting a process
       to  run  on a single CPU also avoids the performance cost caused by the
       cache invalidation that occurs when a process 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 process whose ID
       is pid to the value specified by  mask.   If  pid  is  zero,  then  the
       calling  process  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 process specified by pid is not currently running on one of the
       CPUs specified in mask, then that process is migrated  to  one  of  the
       CPUs specified in mask.

       sched_getaffinity() writes the affinity mask of the process 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 process 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  process
              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  process   does   not   have
              appropriate  privileges.   The caller needs an effective user ID
              equal to the real user ID or effective user ID  of  the  process
              identified   by   pid,  or  it  must  possess  the  CAP_SYS_NICE
              capability.

       ESRCH  The process 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
       process  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 process
       runs if the "cpuset" mechanism described in cpuset(7)  is  being  used.
       These  restrictions on the actual set of CPUs on which the process will
       run are silently imposed by the kernel.

       sched_setscheduler(2) has a description of the Linux scheduling scheme.

       The affinity mask is  actually  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.

       The cpu_set_t affinity mask size provided by glibc only allows for upto
       1024 CPUs. It is possible to build Linux kernels with greater than 1024
       CPUs. Any application using the statically sized  cpu_set_t  will  fail
       with  EINVAL  on such kernels. It is thus recommended that applications
       avoid  using  the  statically  sized  cpu_set_t   type,   and   instead
       dynamically  allocate  a mask using the CPU_*_S macros described in the
       CPU_SET(3) man page. Since it is not possible  to  determine  ahead  of
       time what NR_CPUS value the kernel was built with, applications must be
       prepared  to  catch  EINVAL,  and  retry  the  command  with  a  larger
       dynamically  allocated  mask.   The  example  that  follows illustrates
       portable usage.

EXAMPLE

          #define _GNU_SOURCE

          #include <sched.h>
          #include <stdio.h>
          #include <errno.h>

          int main(void)
          {
               cpu_set_t *mask;
               size_t size;
               int i;
               int nrcpus = 1024;

       realloc:
               mask = CPU_ALLOC(nrcpus);
               size = CPU_ALLOC_SIZE(nrcpus);
               CPU_ZERO_S(size, mask);
               if ( sched_getaffinity(0, size, mask) == -1 ) {
                       CPU_FREE(mask);
                       if (errno == EINVAL &&
                           nrcpus < (1024 << 8)) {
                              nrcpus = nrcpus << 2;
                              goto realloc;
                       }
                       perror("sched_getaffinity");
                       return -1;
               }

               for ( i = 0; i < nrcpus; i++ ) {
                       if ( CPU_ISSET_S(i, size, mask) ) {
                               printf("CPU %d is set\n", (i+1));
                       }
               }

               CPU_FREE(mask);

               return 0;
          }

SEE ALSO

       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 3.32 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/.