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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
       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.


       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
              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

       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  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

       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.  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).


       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)

       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),
       pthread_setaffinity_np(3), sched_getcpu(3), capabilities(7), cpuset(7),


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