jitter.c revision 82499 
1/*
2 * This program can be used to calibrate the clock reading jitter of a
3 * particular CPU and operating system. It first tickles every element
4 * of an array, in order to force pages into memory, then repeatedly calls
5 * gettimeofday() and, finally, writes out the time values for later
6 * analysis. From this you can determine the jitter and if the clock ever
7 * runs backwards.
8 */
9
10#include <stdio.h>
11#include <sys/time.h>
12
13#define NBUF 20002
14
15int
16main(
17	int argc,
18	char *argv[]
19	)
20{
21	struct timeval ts, tr;
22	struct timezone tzp;
23	long temp, j, i, gtod[NBUF];
24
25	gettimeofday(&ts, &tzp);
26
27	/*
28	 * Force pages into memory
29	 */
30	for (i = 0; i < NBUF; i ++)
31	    gtod[i] = 0;
32
33	/*
34	 * Construct gtod array
35	 */
36	for (i = 0; i < NBUF; i ++) {
37		gettimeofday(&tr, &tzp);
38		gtod[i] = (tr.tv_sec - ts.tv_sec) * 1000000 + tr.tv_usec;
39	}
40
41	/*
42	 * Write out gtod array for later processing with S
43	 */
44	for (i = 0; i < NBUF - 2; i++) {
45		/*
46		  printf("%lu\n", gtod[i]);
47		*/
48		gtod[i] = gtod[i + 1] - gtod[i];
49		printf("%lu\n", gtod[i]);
50	}
51
52	/*
53	 * Sort the gtod array and display deciles
54	 */
55	for (i = 0; i < NBUF - 2; i++) {
56		for (j = 0; j <= i; j++) {
57			if (gtod[j] > gtod[i]) {
58				temp = gtod[j];
59				gtod[j] = gtod[i];
60				gtod[i] = temp;
61			}
62		}
63	}
64	fprintf(stderr, "First rank\n");
65	for (i = 0; i < 10; i++)
66	    fprintf(stderr, "%10ld%10ld\n", i, gtod[i]);
67	fprintf(stderr, "Last rank\n");
68	for (i = NBUF - 12; i < NBUF - 2; i++)
69	    fprintf(stderr, "%10ld%10ld\n", i, gtod[i]);
70	exit(0);
71}
72