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

       sched_setaffinity, sched_getaffinity - set and get a thread's CPU
       affinity mask

       #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);

       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
       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.

       set", pointed to by mask.  A set of macros for manipulating CPU sets is
       described in CPU_SET(3).

       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

       CPUs specified in mask, then that thread is migrated to one of the CPUs
       specified in mask.

       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.

       "C library/kernel differences" below, which notes that the underlying
       sched_getaffinity() differs in its return value).  On failure, -1 is
       returned, and errno is set to indicate the error.

       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

       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.

              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.

       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.

       These system calls are Linux-specific.

       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
       silently imposed by the kernel.

       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/.


       independently for each of the threads in a thread group.  The value
       Specifying pid as 0 will set the attribute for the calling thread, and
       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

       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
       isolcpus is the preferred mechanism of isolating CPUs (versus the
       alternative of manually setting the CPU affinity of all processes on 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.

       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
       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:


       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.)

       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).

       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
       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).

       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.

       consumed when running the program will depend on intra-core caching
       effects and whether the processes are using the same CPU.

       cores, each with two CPUs:

           Thread(s) per core:    2
           Core(s) per socket:    2
           Socket(s):             1

       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>

                               } while (0)

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

           if (argc != 4) {

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


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

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




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




       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)

       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                              2021-03-22               SCHED_SETAFFINITY(2)