HardenedBSD/usr.sbin/xntpd/util/jitter.c

/*
 * This program can be used to calibrate the clock reading jitter of a
 * particular CPU and operating system. It first tickles every element
 * of an array, in order to force pages into memory, then repeatedly calls
 * gettimeofday() and, finally, writes out the time values for later
 * analysis. From this you can determine the jitter and if the clock ever
 * runs backwards.
 */
#include <sys/time.h>
#include <stdio.h>

#define NBUF 10001

main()
{
	struct timeval tp, ts, tr;
	struct timezone tzp;
	long temp, j, i, gtod[NBUF];

	gettimeofday(&ts, &tzp);
	ts.tv_usec = 0;

	/*
	 * Force pages into memory
	 */
	for (i = 0; i < NBUF; i ++)
		gtod[i] = 0;

	/*
	 * Construct gtod array
	 */
	for (i = 0; i < NBUF; i ++) {
		gettimeofday(&tp, &tzp);
		tr = tp;
		tr.tv_sec -= ts.tv_sec;
		tr.tv_usec -= ts.tv_usec;
		if (tr.tv_usec < 0) {
			tr.tv_usec += 1000000;
			tr.tv_sec--;
		}
		gtod[i] = tr.tv_sec * 1000000 + tr.tv_usec;
	}

	/*
	 * Write out gtod array for later processing with S
	 */
        for (i = 0; i < NBUF - 1; i++) {
/*
                printf("%lu\n", gtod[i]);
*/
		gtod[i] = gtod[i + 1] - gtod[i];
                printf("%lu\n", gtod[i]);
	}

	/*
	 * Sort the gtod array and display deciles
	 */
	for (i = 0; i < NBUF - 1; i++) {
		for (j = 0; j <= i; j++) {
			if (gtod[j] > gtod[i]) {
				temp = gtod[j];
				gtod[j] = gtod[i];
				gtod[i] = temp;
			}
		}
	}
	fprintf(stderr, "First rank\n");
	for (i = 0; i < 10; i++)
		fprintf(stderr, "%10ld%10ld\n", i, gtod[i]);
	fprintf(stderr, "Last rank\n");
        for (i = NBUF - 11; i < NBUF - 1; i++)
                fprintf(stderr, "%10ld%10ld\n", i, gtod[i]);
}