linux-test-clocks.c

/*
 * linux-test-clocks.c
 * Test various clock sources available through clock_gettime().
 *
 * Author: Mikael Hirki <mikael.hirki@aalto.fi>
 */

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

#if __x86_64__ || __i386__
#define HAVE_RDTSC
#define RDTSC(v)							\
  do { unsigned lo, hi;							\
    __asm__ volatile("rdtsc" : "=a" (lo), "=d" (hi));			\
    (v) = ((uint64_t) lo) | ((uint64_t) hi << 32);			\
  } while (0)
#endif

clockid_t clk_ids[] = {
	CLOCK_REALTIME,
	CLOCK_REALTIME_COARSE,
	CLOCK_MONOTONIC,
	CLOCK_MONOTONIC_COARSE,
	CLOCK_MONOTONIC_RAW,
#ifdef CLOCK_BOOTTIME
	CLOCK_BOOTTIME,
#endif
	CLOCK_PROCESS_CPUTIME_ID,
	CLOCK_THREAD_CPUTIME_ID,
};

const char *clk_names[] = {
	"CLOCK_REALTIME",
	"CLOCK_REALTIME_COARSE",
	"CLOCK_MONOTONIC",
	"CLOCK_MONOTONIC_COARSE",
	"CLOCK_MONOTONIC_RAW",
#ifdef CLOCK_BOOTTIME
	"CLOCK_BOOTTIME",
#endif
	"CLOCK_PROCESS_CPUTIME_ID",
	"CLOCK_THREAD_CPUTIME_ID",
};

const int num_clocks = sizeof(clk_ids) / sizeof(*clk_ids);
const int num_iterations = 10000000;

static double gettimeofday_double() {
	struct timeval now;
	gettimeofday(&now, NULL);
	return now.tv_sec + now.tv_usec * 0.000001;
}

static void gettimeofday_getres(struct timespec *res) {
	struct timeval begin, now;
	gettimeofday(&begin, NULL);
	while (1) {
		gettimeofday(&now, NULL);
		if (now.tv_usec != begin.tv_usec) break;
	}
	if (now.tv_usec == 0) {
		now.tv_usec += 1000000;
	}
	int delta = now.tv_usec - begin.tv_usec;
	res->tv_sec = 0;
	res->tv_nsec = delta * 1000;
}

int main() {
	int i = 0, j = 0;
	struct timespec res = {0, 0};
	struct timeval now = {0, 0};
	printf("Clock time resolutions\n");
	printf("======================\n\n");
	
	for (i = 0; i < num_clocks; i++) {
		clock_getres(clk_ids[i], &res);
		printf("%s : %lld.%09lld\n", clk_names[i], (long long)res.tv_sec, (long long)res.tv_nsec);
	}
	
	// Special case: gettimeofday
	gettimeofday_getres(&res);
	printf("gettimeofday : %lld.%09lld\n", (long long)res.tv_sec, (long long)res.tv_nsec);
	
#ifdef HAVE_RDTSC
	printf("RDTSC : ?\n");
#endif
	
	printf("\n");
	printf("Current values\n");
	printf("==============\n\n");
	
	for (i = 0; i < num_clocks; i++) {
		clock_gettime(clk_ids[i], &res);
		printf("%s : %lld.%09lld\n", clk_names[i], (long long)res.tv_sec, (long long)res.tv_nsec);
	}
	
	// Special case: gettimeofday
	gettimeofday(&now, NULL);
	printf("gettimeofday : %lld.%09lld\n", (long long)now.tv_sec, ((long long)now.tv_usec) * 1000);
	
#ifdef HAVE_RDTSC
	uint64_t tsc = 0;
	RDTSC(tsc);
	printf("RDTSC : %llu\n", (long long unsigned)tsc);
#endif
	
	printf("\n");
	printf("Polling latencies\n");
	printf("=================\n\n");
	
	for (i = 0; i < num_clocks; i++) {
		double time_start = gettimeofday_double();
		for (j = 0; j < num_iterations; j++) {
			clock_gettime(clk_ids[i], &res);
		}
		double time_end = gettimeofday_double();
		printf("%s : %f nanoseconds\n", clk_names[i], (time_end - time_start) * 1000000000.0 / num_iterations);
	}
	
	// Special case: gettimeofday
	{
		double time_start = gettimeofday_double();
		for (j = 0; j < num_iterations; j++) {
			gettimeofday(&now, NULL);
		}
		double time_end = gettimeofday_double();
		printf("gettimeofday : %f nanoseconds\n", (time_end - time_start) * 1000000000.0 / num_iterations);
	}
	
#ifdef HAVE_RDTSC
	{
		double time_start = gettimeofday_double();
		for (j = 0; j < num_iterations; j++) {
			RDTSC(tsc);
		}
		double time_end = gettimeofday_double();
		printf("RDTSC : %f nanoseconds\n", (time_end - time_start) * 1000000000.0 / num_iterations);
	}
#endif
	return 0;
}