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

       sched_setaffinity, sched_getaffinity - set and get a process'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  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  perfor-
       mance 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 per-
       formance  cost  caused by the cache invalidation that occurs when a process ceases to exe-
       cute 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 argu-
       ment 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.

       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 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 capabil-

       ESRCH  The process 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 cpuset-
       size 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 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

       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 3.53 of the Linux man-pages project.  A description of the
       project, and information about reporting bugs, can be found at

Linux					    2013-02-11			     SCHED_SETAFFINITY(2)
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