How to use sched_getaffinity and sched_setaffinity in Linux from C?

To use sched_setaffinity to make the current process run on core 7 you do this:

cpu_set_t my_set;        /* Define your cpu_set bit mask. */CPU_ZERO(&my_set);       /* Initialize it all to 0, i.e. no CPUs selected. */CPU_SET(7, &my_set);     /* set the bit that represents core 7. */sched_setaffinity(0, sizeof(cpu_set_t), &my_set); /* Set affinity of tihs process to */                                                  /* the defined mask, i.e. only 7. */

See http://linux.die.net/man/2/sched_setaffinity & http://www.gnu.org/software/libc/manual/html_node/CPU-Affinity.html for more info.

Minimal runnable example

In this example, we get the affinity, modify it, and check if it has taken effect with sched_getcpu().

main.c

#define _GNU_SOURCE#include <assert.h>#include <sched.h>#include <stdbool.h>#include <stdio.h>#include <stdlib.h>#include <unistd.h>void print_affinity() {    cpu_set_t mask;    long nproc, i;    if (sched_getaffinity(0, sizeof(cpu_set_t), &mask) == -1) {        perror("sched_getaffinity");        assert(false);    }    nproc = sysconf(_SC_NPROCESSORS_ONLN);    printf("sched_getaffinity = ");    for (i = 0; i < nproc; i++) {        printf("%d ", CPU_ISSET(i, &mask));    }    printf("\n");}int main(void) {    cpu_set_t mask;    print_affinity();    printf("sched_getcpu = %d\n", sched_getcpu());    CPU_ZERO(&mask);    CPU_SET(0, &mask);    if (sched_setaffinity(0, sizeof(cpu_set_t), &mask) == -1) {        perror("sched_setaffinity");        assert(false);    }    print_affinity();    /* TODO is it guaranteed to have taken effect already? Always worked on my tests. */    printf("sched_getcpu = %d\n", sched_getcpu());    return EXIT_SUCCESS;}

GitHub upstream.

Compile and run:

gcc -ggdb3 -O0 -std=c99 -Wall -Wextra -pedantic -o main.out main.c./main.out

Sample output:

sched_getaffinity = 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 sched_getcpu = 9sched_getaffinity = 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 sched_getcpu = 0

Which means that:

• initially, all of my 16 cores were enabled, and the process was randomly running on core 9 (the 10th one)
• after we set the affinity to only the first core, the process was moved necessarily to core 0 (the first one)

It is also fun to run this program through taskset:

taskset -c 1,3 ./a.out

Which gives output of form:

sched_getaffinity = 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 sched_getcpu = 2sched_getaffinity = 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 sched_getcpu = 0

and so we see that it limited the affinity from the start.

This works because the affinity is inherited by child processes, which taskset is forking: How to prevent inheriting CPU affinity by child forked process?

nproc respects sched_getaffinity by default as shown at: How to find out the number of CPUs using python

Python: os.sched_getaffinity and os.sched_setaffinity

See: How to find out the number of CPUs using python

Tested in Ubuntu 16.04.

Don't use CPU_SETSIZE as cpusetsize parameter for sched_[set|get]affinity. The names are misleading but this is wrong.The macro CPU_SETSIZE is (quoting man 3 cpu_set) "a value one greater than the maximum CPU number that can be stored in cpu_set_t."You have to use

sched_setaffinity(0, sizeof(cpu_set_t), &my_set);

instead.