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

NAME
       sched_setaffinity,   sched_getaffinity,  CPU_CLR,  CPU_ISSET,  CPU_SET,
       CPU_ZERO - set and get a process's CPU affinity mask

SYNOPSIS
       #define _GNU_SOURCE
       #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);

       void CPU_CLR(int cpu, cpu_set_t *set);
       int CPU_ISSET(int cpu, cpu_set_t *set);
       void CPU_SET(int cpu, cpu_set_t *set);
       void CPU_ZERO(cpu_set_t *set);

DESCRIPTION
       A process'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 process (i.e., setting the affinity mask
       of that process to specify a single CPU, and setting the affinity  mask
       of  all  other processes to exclude that CPU), it is possible to ensure
       maximum execution speed for that process.  Restricting a process to run
       on  a single CPU also prevents the performance cost caused by the cache
       invalidation that occurs when a process 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.  Four macros are provided to  manipulate  CPU
       sets.   CPU_ZERO()  clears a set.  CPU_SET() and CPU_CLR() respectively
       add and remove a given CPU from a set.  CPU_ISSET() tests to see  if  a
       CPU  is  part  of  the  set;  this  is useful after sched_getaffinity()
       returns.  The first available CPU on the system corresponds  to  a  cpu
       value  of  0,  the next CPU corresponds to a cpu value of 1, and so on.
       The constant CPU_SETSIZE (1024) specifies a value one greater than  the
       maximum CPU number that can be stored in a CPU set.

       sched_setaffinity()  sets the CPU affinity mask of the process whose ID
       is pid to the value specified by mask.  If pid is zero, then the  call-
       ing  process 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 process specified by pid is not currently running on one of the
       CPUs specified in mask, then that process is migrated  to  one  of  the
       CPUs specified in mask.

       sched_getaffinity() writes the affinity mask of the process 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 process 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 phys-
              ically on the system, or cpusetsize is smaller than the size  of
              the affinity mask used by the kernel.

       EPERM  The  calling  process does not have appropriate privileges.  The
              process calling sched_setaffinity() needs an effective  user  ID
              equal to the user ID or effective user ID of the process identi-
              fied by pid, or it must possess the CAP_SYS_NICE capability.

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

VERSIONS
       The CPU affinity system calls were introduced in  Linux  kernel  2.5.8.
       The  library  interfaces  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
       sched_setscheduler(2) has a description of the Linux scheduling scheme.

       The  affinity  mask  is  actually  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  current
       thread,  and  passing  the value returned from a call to getpid(2) will
       set the attribute for the main thread of the thread group.

       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 that 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.

SEE ALSO
       clone(2), getcpu(2), getpriority(2), gettid(2), nice(2), sched_get_pri-
       ority_max(2),     sched_get_priority_min(2),     sched_getscheduler(2),
       sched_setscheduler(2), setpriority(2), capabilities(7), cpuset(7)

COLOPHON
       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                             2008-07-09              SCHED_SETAFFINITY(2)

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