measure rumprun sched_yield() time in infinite loop

yield_time_nanos.c

#include <err.h>
#include <stdio.h>
#include <limits.h>
#include <sys/time.h>
#include <sched.h>


#define ITERATIONS 10000000

/*
 * Steps:
 * 1) Measure time for clock_gettime()
 * 2) Measure time for sched_yield()
 * 3) Present final number for sched_yield(), subtracting time for clock_gettime()
 */
int main(int argc, char *argv[])
{
    struct timespec first_ts, second_ts;
    long max_clk_t = 0;
    long min_clk_t = LONG_MAX;
    long long avg_clk_t = 0LL;

    long max_yield_t = 0;
    long min_yield_t = LONG_MAX;
    long long avg_yield_t = 0LL;

    long time_sysc;
    int  iter;

    /* Measure time to execute clock_gettime() */
    for (;;)
    {
        if (clock_gettime(CLOCK_REALTIME, &first_ts))
            err(1, "clock_gettime");

        if (clock_gettime(CLOCK_REALTIME, &second_ts))
            err(1, "clock_gettime");

        /* Just ignore this situation, simplifies the math */
        if (first_ts.tv_sec != second_ts.tv_sec)
            continue;

        time_sysc = (long)second_ts.tv_nsec - (long)first_ts.tv_nsec;

        if (time_sysc > max_clk_t)
            max_clk_t = time_sysc;

        if (time_sysc < min_clk_t)
            min_clk_t = time_sysc;

        avg_clk_t += time_sysc;

        if (++iter >= ITERATIONS)
            break;
    }

    avg_clk_t /= ITERATIONS;

    printf("------------------ clock_gettime() stats ------------------\n");
    printf("avg = %lldns; max = %ldns; min = %ldns\n", avg_clk_t, max_clk_t, min_clk_t);
    printf("==========================================\n");
    /****************************************/

    /* Measure time to execute sched_yield() */
    iter = 0;
    for (;;)
    {
        if (clock_gettime(CLOCK_REALTIME, &first_ts))
            err(1, "clock_gettime");

        sched_yield();

        if (clock_gettime(CLOCK_REALTIME, &second_ts))
            err(1, "clock_gettime");

        /* Just ignore this situation, simplifies the math */
        if (first_ts.tv_sec != second_ts.tv_sec)
            continue;

        time_sysc = (long)second_ts.tv_nsec - (long)first_ts.tv_nsec;

        if (time_sysc > max_yield_t)
            max_yield_t = time_sysc;

        if (time_sysc < min_yield_t)
            min_yield_t = time_sysc;

        avg_yield_t += time_sysc;

        if (++iter >= ITERATIONS)
            break;
    }

    avg_yield_t /= ITERATIONS;

    printf("------------------ sched_yield() stats ------------------\n");
    printf("avg = %lldns; max = %ldns; min = %ldns\n", avg_yield_t, max_yield_t, min_yield_t);
    printf("sched_yield() overhead : %ldns\n", min_yield_t - min_clk_t);
    printf("==========================================\n");
    /****************************************/

    return 0;
}