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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 dedicating 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  calling  thread  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 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  (but  see  "C
       library/kernel differences" below, which notes  that  the  underlying  sched_getaffinity()
       differs in its return value).  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 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 appropriate 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 directories 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 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().)

       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.  Following 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
       Documentation/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 success, the raw sched_getaffinity() system call returns the  number  of  bytes  placed
       copied  into  the  mask  buffer;  this  will be the minimum of cpusetsize and 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 represented 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 number of bits requested to be allocated).

EXAMPLES

       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 | egrep -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;

           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 (int 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 (int 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 5.10 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/.