sched_setaffinity(2) - Linux manual page (original) (raw)

schedsetaffinity(2) System Calls Manual schedsetaffinity(2)

NAME top

   sched_setaffinity, sched_getaffinity - set and get a thread's CPU
   affinity mask

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#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_**);**

DESCRIPTION top

   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 _cpusett_ structure, a
   "CPU set", pointed to by _mask_.  A set of macros for manipulating
   CPU sets is described in [CPU_SET(3)](../man3/CPU%5FSET.3.html).

   **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(cpusett)_.

   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 _cpusett_ 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 top

   On success, **sched_setaffinity**() and **sched_getaffinity**() return 0
   (but see "C library/kernel differences" below, which notes that
   the underlying **sched_getaffinity**() differs in its return value).
   On failure, -1 is returned, and _[errno](../man3/errno.3.html)_ is set to indicate the
   error.

ERRORS top

   **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
          _cpuset_ cgroups or the "cpuset" mechanism described in
          [cpuset(7)](../man7/cpuset.7.html).

   **EINVAL** (**sched_getaffinity**() and, before Linux 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** capability in the user namespace of the thread
          _pid_.

   **ESRCH** The thread whose ID is _pid_ could not be found.

STANDARDS top

   Linux.

HISTORY top

   Linux 2.5.8, 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
   _sizet_.

NOTES top

   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)](../man7/cpuset.7.html)
   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)](../man3/sysconf.3.html) to obtain the values of the
   **_SC_NPROCESSORS_CONF** and **_SC_NPROCESSORS_ONLN** parameters; and
   inspecting the list of CPU directories under
   _/sys/devices/system/cpu/_.

   [sched(7)](../man7/sched.7.html) 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)](../man2/gettid.2.html) 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)](../man2/getpid.2.html) 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)](../man3/pthread%5Fsetaffinity%5Fnp.3.html) instead of **sched_setaffinity**().)

   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)](../man7/cpuset.7.html) mechanism.  For further information, see the kernel
   source file _Documentation/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)](../man2/fork.2.html) inherits its parent's CPU affinity
   mask.  The affinity mask is preserved across an [execve(2)](../man2/execve.2.html).

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
   number of bytes placed copied into the _mask_ buffer; this will be
   the minimum of _cpusetsize_ and the size (in bytes) of the _cpumaskt_
   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 cpusett 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);

   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)](../man3/CPU%5FALLOC.3.html)).
   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)](../man3/CPU%5FALLOC.3.html) 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)](../man3/CPU%5FCOUNT.3.html) (rather than iterating over the number of
   bits requested to be allocated).

EXAMPLES top

   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)](../man1/lscpu.1.html) to determine that this (x86) system has
   two cores, each with two CPUs:

       $ **lscpu | egrep -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 <err.h>
   #include <sched.h>
   #include <stdio.h>
   #include <stdlib.h>
   #include <sys/wait.h>
   #include <unistd.h>

   int
   main(int argc, char *argv[])
   {
       int           parentCPU, childCPU;
       cpu_set_t     set;
       unsigned int  nloops;

       if (argc != 4) {
           fprintf(stderr, "Usage: %s parent-cpu child-cpu num-loops\n",
                   argv[0]);
           exit(EXIT_FAILURE);
       }

       parentCPU = atoi(argv[1]);
       childCPU = atoi(argv[2]);
       nloops = atoi(argv[3]);

       CPU_ZERO(&set);

       switch (fork()) {
       case -1:            /* Error */
           err(EXIT_FAILURE, "fork");

       case 0:             /* Child */
           CPU_SET(childCPU, &set);

           if (sched_setaffinity(getpid(), sizeof(set), &set) == -1)
               err(EXIT_FAILURE, "sched_setaffinity");

           for (unsigned int j = 0; j < nloops; j++)
               getppid();

           exit(EXIT_SUCCESS);

       default:            /* Parent */
           CPU_SET(parentCPU, &set);

           if (sched_setaffinity(getpid(), sizeof(set), &set) == -1)
               err(EXIT_FAILURE, "sched_setaffinity");

           for (unsigned int j = 0; j < nloops; j++)
               getppid();

           wait(NULL);     /* Wait for child to terminate */
           exit(EXIT_SUCCESS);
       }
   }

COLOPHON top

   This page is part of the _man-pages_ (Linux kernel and C library
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Linux man-pages 6.10 2024-07-23 schedsetaffinity(2)

Pages that refer to this page:systemd-nspawn(1), taskset(1), getcpu(2), gettid(2), sched_get_priority_max(2), sched_setattr(2), sched_setparam(2), sched_setscheduler(2), syscalls(2), CPU_SET(3), numa(3), pthread_attr_setaffinity_np(3), pthread_create(3), pthread_setaffinity_np(3), systemd.exec(5), capabilities(7), cpuset(7), credentials(7), pthreads(7), sched(7), migratepages(8), numactl(8)