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



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  execu-
       tion  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  pro-
       cess 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  per-
              mitted  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.

       The cpu_set_t affinity mask size provided by glibc only allows for upto 1024 CPUs.  It  is  possible  to  build
       Linux  kernels with greater than 1024 CPUs. Any application using the statically sized cpu_set_t will fail with
       EINVAL on such kernels. It is thus recommended that applications avoid using  the  statically  sized  cpu_set_t
       type,  and  instead  dynamically allocate a mask using the CPU_*_S macros described in the CPU_SET(3) man page.
       Since it is not possible to determine ahead of time what NR_CPUS value the kernel was built with,  applications
       must  be prepared to catch EINVAL, and retry the command with a larger dynamically allocated mask.  The example
       that follows illustrates portable usage.


EXAMPLE
          #define _GNU_SOURCE

          #include <sched.h>
          #include <stdio.h>
          #include <errno.h>

          int main(void)
          {
               cpu_set_t *mask;
               size_t size;
               int i;
               int nrcpus = 1024;

       realloc:
               mask = CPU_ALLOC(nrcpus);
               size = CPU_ALLOC_SIZE(nrcpus);
               CPU_ZERO_S(size, mask);
               if ( sched_getaffinity(0, size, mask) == -1 ) {
                       CPU_FREE(mask);
                       if (errno == EINVAL &&
                           nrcpus < (1024 << 8)) {
                              nrcpus = nrcpus << 2;
                              goto realloc;
                       }
                       perror("sched_getaffinity");
                       return -1;
               }

               for ( i = 0; i < nrcpus; i++ ) {
                       if ( CPU_ISSET_S(i, size, mask) ) {
                               printf("CPU %d is set\n", (i+1));
                       }
               }

               CPU_FREE(mask);

               return 0;
          }


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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-11-14              SCHED_SETAFFINITY(2)