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NAME

       sched_setaffinity,   sched_getaffinity,  CPU_CLR,  CPU_ISSET,  CPU_SET,
       CPU_ZERO - set and get a process’s CPU affinity mask

SYNOPSIS

       #define _GNU_SOURCE
       #include <sched.h>

       int sched_setaffinity(pid_t pid, unsigned int cpusetsize,
                             cpu_set_t *mask);

       int sched_getaffinity(pid_t pid, unsigned int cpusetsize,
                             cpu_set_t *mask);

       void CPU_CLR(int cpu, cpu_set_t *set);
       int CPU_ISSET(int cpu, cpu_set_t *set);
       void CPU_SET(int cpu, cpu_set_t *set);
       void CPU_ZERO(cpu_set_t *set);

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 prevents 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.  Four macros are provided to  manipulate  CPU
       sets.   CPU_ZERO()  clears a set.  CPU_SET() and CPU_CLR() respectively
       add and remove a given CPU from a set.  CPU_ISSET() tests to see  if  a
       CPU  is  part  of  the  set;  this  is useful after sched_getaffinity()
       returns.  The first available CPU on the system corresponds  to  a  cpu
       value  of  0,  the next CPU corresponds to a cpu value of 1, and so on.
       The constant CPU_SETSIZE (1024) specifies a value one greater than  the
       maximum CPU number that can be stored in a CPU set.

       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
              physically on the system, or cpusetsize is smaller than the size
              of the affinity mask used by the kernel.

       EPERM  The  calling  process does not have appropriate privileges.  The
              process calling sched_setaffinity() 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 library interfaces were introduced in glibc  2.3.   Initially,  the
       glibc  interfaces  included a cpusetsize argument.  In glibc 2.3.3, the
       cpusetsize argument was removed, but  this  argument  was  restored  in
       glibc 2.3.4.

CONFORMING TO

       These system calls are Linux-specific.

NOTES

       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 current
       thread, and passing the value returned from a call  to  getpid(2)  will
       set the attribute for the main thread of the thread group.

       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 that 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),         getpriority(2),          gettid(2),          nice(2),
       sched_get_priority_max(2),                   sched_get_priority_min(2),
       sched_getscheduler(2),      sched_setscheduler(2),      setpriority(2),
       capabilities(7)

COLOPHON

       This  page  is  part of release 2.77 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/.