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NAME

       sched_setaffinity,  sched_getaffinity  -  set  and  get a process’s CPU
       affinity mask

SYNOPSIS

       #define _GNU_SOURCE
       #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 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.

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),              sched_getcpu(3),              capabilities(7),
       pthread_setaffinity_np(3), cpuset(7)

COLOPHON

       This  page  is  part of release 3.21 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/.