sched_setaffinity(2) — Linux manual page


sched_setaffinity(2)       System Calls Manual      sched_setaffinity(2)

NAME         top

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

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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         top

       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         top

       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 failure, -1 is returned, and errno is set to indicate the

ERRORS         top

       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

       EINVAL (sched_getaffinity() and, before Linux 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.

STANDARDS         top


HISTORY         top

       Linux 2.5.8, 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

NOTES         top

       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

       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         top

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

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

           if (argc != 4) {
               fprintf(stderr, "Usage: %s parent-cpu child-cpu num-loops\n",

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


           switch (fork()) {
           case -1:            /* Error */
               err(EXIT_FAILURE, "fork");

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

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

               for (unsigned int j = 0; j < nloops; j++)


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

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

               for (unsigned int j = 0; j < nloops; j++)

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

SEE ALSO         top

       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)

Linux man-pages (unreleased)     (date)             sched_setaffinity(2)

Pages that refer to this page: systemd-nspawn(1)taskset(1)getcpu(2)gettid(2)sched_get_priority_max(2)sched_setattr(2)sched_setparam(2)sched_setscheduler(2)syscalls(2)CPU_SET(3)numa(3)pthread_attr_setaffinity_np(3)pthread_create(3)pthread_setaffinity_np(3)systemd.exec(5)capabilities(7)cpuset(7)credentials(7)pthreads(7)sched(7)migratepages(8)numactl(8)