https://stackoverflow.com/questions/48741921/why-is-the-first-run-always-much-slower

timer.c

#include <sched.h>
#include <stdio.h>
#include <time.h>

static long nanotime() {
    struct timespec tp;
    clock_gettime(CLOCK_MONOTONIC, &tp);
    return tp.tv_sec * 1000000000L + tp.tv_nsec;
}

static void measure_yield() {
    long times[10];
    int i;

    for (i = 0; i < 10; i++) {
        long start_time = nanotime();
        sched_yield();
        times[i] = nanotime() - start_time;
    }

    printf("[%ld %ld %ld %ld %ld %ld %ld %ld %ld %ld]\n",
           times[0], times[1], times[2], times[3], times[4],
           times[5], times[6], times[7], times[8], times[9]);
}

int main() {
    int i;
    for (i = 0; i < 10; i++) {
        measure_yield();
    }
    return 0;
}

timer.txt

[6075 778 591 594 588 685 584 591 589 588]
[966 598 598 581 598 588 595 588 591 641]
[725 558 591 558 592 545 592 558 585 558]
[672 578 598 565 594 558 652 561 591 558]
[665 552 591 554 591 571 599 558 594 561]
[668 552 598 565 601 562 588 559 594 554]
[695 558 598 554 585 554 588 558 591 555]
[688 554 591 564 595 565 605 562 588 552]
[685 565 595 558 595 558 598 555 595 558]
[662 558 588 565 595 558 621 551 599 558]