clock_gettime_overhead.c

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

#define NUM_ITERATIONS 1000000

uint64_t timespec_to_ns(struct timespec ts) {
    return (uint64_t)ts.tv_sec * 1000000000 + ts.tv_nsec;
}

uint64_t measure_overhead(clockid_t clock_type) {
    struct timespec start, x, end;
    uint64_t start_ns, end_ns, total_ns = 0;

    clock_gettime(CLOCK_MONOTONIC_COARSE, &start);
    for (int i = 0; i < NUM_ITERATIONS; i++) {
        clock_gettime(clock_type, &x);
    }
    clock_gettime(CLOCK_MONOTONIC_COARSE, &end);

    start_ns = timespec_to_ns(start);
    end_ns = timespec_to_ns(end);
    total_ns += (end_ns - start_ns);

    return total_ns / NUM_ITERATIONS;
}

int main() {
    uint64_t thread_cputime_id_overhead = measure_overhead(CLOCK_THREAD_CPUTIME_ID);
    uint64_t monotonic_overhead = measure_overhead(CLOCK_MONOTONIC);
    uint64_t monotonic_coarse_overhead = measure_overhead(CLOCK_MONOTONIC_COARSE);

    printf("CLOCK_THREAD_CPUTIME_ID overhead: %lu ns\n", thread_cputime_id_overhead);
    printf("CLOCK_MONOTONIC overhead: %lu ns\n", monotonic_overhead);
    printf("CLOCK_MONOTONIC_COARSE overhead: %lu ns\n", monotonic_coarse_overhead);

    return 0;
}

In my Linux machine (NUC 10, Ubuntu 22.04), a typical output is something like this:

CLOCK_THREAD_CPUTIME_ID overhead: 192 ns
CLOCK_MONOTONIC overhead: 16 ns
CLOCK_MONOTONIC_COARSE overhead: 4 ns