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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.

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

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       project, and information about reporting bugs, can be found at  http://man7.org/linux/man-
       pages/.