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

NAME

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

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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

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.

RETURN VALUE

       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 is set to indicate
       the error.

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 thread according to any restrictions that may be
              imposed by cpuset cgroups or the "cpuset" mechanism described in cpuset(7).

       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

       Linux.

HISTORY

       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 size_t.

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.

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
of 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 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) 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) 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 number of bytes placed
copied into the mask buffer; this will be the minimum of cpusetsize and 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);

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)). 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) 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) (rather than iterating over the number of bits requested to be allocated).

EXAMPLES

       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 | 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);
           }
       }