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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.   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 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).

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

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       project, information about reporting bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.