NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

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

NAME         top

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

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.  On
       error, -1 is returned, and errno is set appropriately.

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 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.

       ESRCH  The thread whose ID is pid could not be found.

VERSIONS         top

       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         top

       These system calls are Linux-specific.

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 /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/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);

       will 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         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 | 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         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),
       pthread_setaffinity_np(3), sched_getcpu(3), capabilities(7),
       cpuset(7), sched(7)

COLOPHON         top

       This page is part of release 4.07 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                            2016-03-15             SCHED_SETAFFINITY(2)