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     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);


     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 dedi-
     cating  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  call-
     ing  thread  is used.  The argument cpusetsize is the length (in bytes)
     of the data pointed to by mask.  Normally this argument would be speci-
     fied 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.


     On success, sched_setaffinity() and sched_getaffinity() return  0.   On
     error, -1 is returned, and errno is set appropriately.


     EFAULT A supplied memory address was invalid.
     EINVAL The  affinity bit mask mask contains no processors that are cur-
            rently 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 appropri-
            ate  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.


     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.


     These system calls are Linux-specific.


     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 direc-
     tories 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  inde-
     pendently  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
     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.  Follow-
     ing  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 Docu-
     mentation/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 suc-
     cess, 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 repre-
     sented 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 num-
     ber of bits requested to be allocated).


     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",
         parentCPU = atoi(argv[1]);
         childCPU = atoi(argv[2]);
         nloops = atoi(argv[3]);
         switch (fork()) {
         case -1:            /* Error */
         case 0:             /* Child */
             CPU_SET(childCPU, &set);
             if (sched_setaffinity(getpid(), sizeof(set), &set) == -1)
             for (j = 0; j < nloops; j++)
         default:            /* Parent */
             CPU_SET(parentCPU, &set);
             if (sched_setaffinity(getpid(), sizeof(set), &set) == -1)
             for (j = 0; j < nloops; j++)
             wait(NULL);     /* Wait for child to terminate */
         } }


     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)


     This page is part of release 4.16 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

Linux 2017-09-15 SCHED_SETAFFINITY(2)

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