sched_getaffinity

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

NAME
       sched_setaffinity,  sched_getaffinity  -  set  and  get  a thread'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,
                             const cpu_set_t *mask);

       int sched_getaffinity(pid_t pid, size_t cpusetsize,
                             cpu_set_t *mask);

DESCRIPTION
       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.

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 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.

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
       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.

       There  are  various ways of determining the number of CPUs available on
       the system, including: inspecting the contents of /proc/cpuinfo;  using
       sysconf(3)  to  obtain  the  values  of  the  _SC_NPROCESSORS_CONF  and
       _SC_NPROCESSORS_ONLN parameters; and inspecting the list  CPU  directo-
       ries under /sys/devices/system/cpu/.

       sched(7) 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
       sched_setaffinity().)

       The isolcpus boot option can be used to isolate one  or  more  CPUs  at
       boot time, so that no processes are scheduled onto those CPUs.  Follow-
       ing the use of this boot option, the only  way  to  schedule  processes
       onto  the  isolated  CPUs  is  via sched_setaffinity() or the cpuset(7)
       mechanism.  For further information, see the kernel source  file  Docu-
       mentation/kernel-parameters.txt.   As  noted  in that file, isolcpus is
       the preferred mechanism of isolating CPUs (versus  the  alternative  of
       manually setting the CPU affinity of all processes on the system).

       A  child  created  via fork(2) inherits its parent's CPU affinity mask.
       The affinity mask is preserved across an execve(2).

   C library/kernel differences
       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.

   Handling systems with large CPU affinity masks
       The underlying system calls (which represent CPU masks as bit masks  of
       type  unsigned  long *)  impose  no  restriction on the size of the CPU
       mask.  However, the cpu_set_t data type used by glibc has a fixed  size
       of  128  bytes,  meaning that the maximum CPU number that can be repre-
       sented is 1023.  If the kernel CPU affinity mask is larger  than  1024,
       then calls of the form:

           sched_getaffinity(pid, sizeof(cpu_set_t), &mask);

       will  fail  with the error EINVAL, the error produced by the underlying
       system call for the case where the mask size specified in cpusetsize is
       smaller  than  the  size  of  the  affinity  mask  used  by the kernel.
       (Depending on the system CPU topology, the kernel affinity mask can  be
       substantially larger than the number of active CPUs in the system.)

       When  working on systems with large kernel CPU affinity masks, one must
       dynamically allocate the mask argument.  Currently, the only way to  do
       this   is   by  probing  for  the  size  of  the  required  mask  using
       sched_getaffinity() calls with increasing mask sizes  (until  the  call
       does not fail with the error EINVAL).

EXAMPLE
       The  program  below creates a child process.  The parent and child then
       each assign themselves to a specified CPU and execute  identical  loops
       that  consume  some CPU time.  Before terminating, the parent waits for
       the child to complete.  The program takes three command-line arguments:
       the  CPU  number  for the parent, the CPU number for the child, and the
       number of loop iterations that both processes should perform.

       As the sample runs below demonstrate, the amount of real and  CPU  time
       consumed  when  running  the  program will depend on intra-core caching
       effects and whether the processes are using the same CPU.

       We first employ lscpu(1) to determine that this (x86)  system  has  two
       cores, each with two CPUs:

           $ lscpu | grep -i 'core.*:|socket'
           Thread(s) per core:    2
           Core(s) per socket:    2
           Socket(s):             1

       We then time the operation of the example program for three cases: both
       processes running on the same CPU; both processes running on  different
       CPUs  on the same core; and both processes running on different CPUs on
       different cores.

           $ time -p ./a.out 0 0 100000000
           real 14.75
           user 3.02
           sys 11.73
           $ time -p ./a.out 0 1 100000000
           real 11.52
           user 3.98
           sys 19.06
           $ time -p ./a.out 0 3 100000000
           real 7.89
           user 3.29
           sys 12.07

   Program source

       #define _GNU_SOURCE
       #include <sched.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <sys/wait.h>

       #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                               } while (0)

       int
       main(int argc, char *argv[])
       {
           cpu_set_t set;
           int parentCPU, childCPU;
           int nloops, j;

           if (argc != 4) {
               fprintf(stderr, "Usage: %s parent-cpu child-cpu num-loops\n",
                       argv[0]);
               exit(EXIT_FAILURE);
           }

           parentCPU = atoi(argv[1]);
           childCPU = atoi(argv[2]);
           nloops = atoi(argv[3]);

           CPU_ZERO(&set);

           switch (fork()) {
           case -1:            /* Error */
               errExit("fork");

           case 0:             /* Child */
               CPU_SET(childCPU, &set);

               if (sched_setaffinity(getpid(), sizeof(set), &set) == -1)
                   errExit("sched_setaffinity");

               for (j = 0; j < nloops; j++)
                   getppid();

               exit(EXIT_SUCCESS);

           default:            /* Parent */
               CPU_SET(parentCPU, &set);

               if (sched_setaffinity(getpid(), sizeof(set), &set) == -1)
                   errExit("sched_setaffinity");

               for (j = 0; j < nloops; j++)
                   getppid();

               wait(NULL);     /* Wait for child to terminate */
               exit(EXIT_SUCCESS);
           }
       }

SEE ALSO
       lscpu(1), nproc(1), 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),
       sched(7)

COLOPHON
       This page is part of release 4.04 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       http://www.kernel.org/doc/man-pages/.

Linux                             2015-07-23              SCHED_SETAFFINITY(2)
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