sched_setaffinity

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



NAME
       sched_setaffinity, sched_getaffinity - 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);

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
       dedicating 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 avoids 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.  A set of macros for manipulating CPU sets is
       described in CPU_SET(3).

       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 calling
       process 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 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
              currently physically on the system and permitted to the process
              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 process does not have
              appropriate privileges.  The caller needs an effective user ID
              equal to the user ID or effective user ID of the process
              identified 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
       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 process
       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 process runs if the
       "cpuset" mechanism described in cpuset(7) is being used.  These
       restrictions on the actual set of CPUs on which the process will run are
       silently imposed by the kernel.

       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 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 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_priority_max(2), sched_get_priority_min(2),
       sched_getscheduler(2), sched_setscheduler(2), setpriority(2), CPU_SET(3),
       sched_getcpu(3), capabilities(7), pthread_setaffinity_np(3), cpuset(7)

COLOPHON
       This page is part of release 3.22 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                              2008-11-14               SCHED_SETAFFINITY(2)