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. On error, −1 is returned, and errno is set appropriately.

ERRORS

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 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/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).

EXAMPLE

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:

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.

#define _GNU_SOURCE
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                        } while (0)

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

    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 */
        errExit("fork");

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

        if (sched_setaffinity(getpid(), sizeof(set), &set) == −1)
            errExit("sched_setaffinity");

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

        exit(EXIT_SUCCESS);

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

        if (sched_setaffinity(getpid(), sizeof(set), &set) == −1)
            errExit("sched_setaffinity");

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

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

SEE ALSO

lscpu(1), nproc(1), 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), sched(7)