sched_setaffinity

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 cpuset
              cgroups or 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 in the
              user namespace of the thread pid.

       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 of CPU direc-
       tories 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/admin-guide/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);

       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 (see CPU_ALLOC(3)).   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).

       Be  aware that CPU_ALLOC(3) may allocate a slightly larger CPU set than
       requested (because CPU sets are implemented as bit masks  allocated  in
       units of sizeof(long)).  Consequently, sched_getaffinity() can set bits
       beyond the requested allocation size, because the  kernel  sees  a  few
       additional bits.  Therefore, the caller should iterate over the bits in
       the returned set, counting those which are set, and stop upon  reaching
       the value returned by CPU_COUNT(3) (rather than iterating over the num-
       ber of bits requested to be allocated).

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), get_nprocs(3),
       pthread_setaffinity_np(3), sched_getcpu(3), capabilities(7), cpuset(7),
       sched(7), numactl(8)

COLOPHON
       This page is part of release 4.15 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15              SCHED_SETAFFINITY(2)
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