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  par-
       ticular  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  appropri-
       ately.

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 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 cpuset-
       size 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 restric-
       tions 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 allo-
       cate 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 3.25 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/.



Linux                                                      2008-11-14                                       SCHED_SETAFFINITY(2)