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    Command:

    sched_getaffinity

    
    
    
    

    SYNOPSIS

           #define _GNU_SOURCE             /* See feature_test_macros(7) */
           #include <sched.h>
    
           int sched_setaffinity(pid_t pid, size_t cpusetsize,
                                 cpu_set_t *mask);
    
           int sched_getaffinity(pid_t pid, size_t cpusetsize,
                                 cpu_set_t *mask);
    
    
    

    DESCRIPTION

           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.
    
    
    

    RETURN VALUE

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

    ERRORS

           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 the
    
           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

           These system calls are Linux-specific.
    
    
    

    NOTES

           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.
    
           sched_setscheduler(2) 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
           sched_setaffinity().)
    
           A  child  created  via fork(2) inherits its parent's CPU affinity mask.
           The affinity mask is preserved across an execve(2).
    
           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.
    
    
    

    SEE ALSO

           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)
    
    
    

    Linux 2013-09-17 SCHED_SETAFFINITY(2)

    
    
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