sched_getaffinity man page on Archlinux

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SCHED_SETAFFINITY(2)	   Linux Programmer's Manual	  SCHED_SETAFFINITY(2)

       sched_setaffinity,  sched_getaffinity  -	 set  and  get	a thread's CPU
       affinity mask

       #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);

       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 dedi‐
       cating  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	 call‐
       ing  thread  is used.  The argument cpusetsize is the length (in bytes)
       of the data pointed to by mask.	Normally this argument would be speci‐
       fied 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.

       On success, sched_setaffinity() and sched_getaffinity() return  0.   On
       error, -1 is returned, and errno is set appropriately.

       EFAULT A supplied memory address was invalid.

       EINVAL The  affinity bit mask mask contains no processors that are cur‐
	      rently 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 appropri‐
	      ate  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.

       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.

       These system calls are Linux-specific.

       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 inde‐
       pendently 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

       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  suc‐
       cess,  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.

       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)

       This page is part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can
       be found at

Linux				  2013-09-17		  SCHED_SETAFFINITY(2)

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