jitter.c revision 280849 
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
5 * reads the system clock and, finally, writes out the time values for
6 * later analysis. From this you can determine the jitter and if the
7 * clock ever runs backwards.
8 */
9
10#ifdef HAVE_CONFIG_H
11# include <config.h>
12#endif
13
14#include <stdio.h>
15#include <sys/time.h>
16#include <stdlib.h>
17#include "jitter.h"
18
19#define NBUF	800002
20#define FRAC	4294967296.		/* a bbbbillion */
21#define JAN_1970 2208988800UL		/* Unix base epoch */
22#define CLOCK_GETTIME			/* Solaris hires clock */
23
24char progname[10];
25double sys_residual;
26double average;
27void sys_gettime(l_fp *);
28
29int
30main(
31	int argc,
32	char *argv[]
33	)
34{
35	l_fp tr;
36	int i, j;
37	double dtemp, gtod[NBUF];
38
39	/*
40	 * Force pages into memory
41	 */
42	for (i = 0; i < NBUF; i ++)
43	    gtod[i] = 0;
44
45	/*
46	 * Construct gtod array
47	 */
48	for (i = 0; i < NBUF; i ++) {
49		get_systime(&tr);
50		LFPTOD(&tr, gtod[i]);
51	}
52
53	/*
54	 * Write out gtod array for later processing with Matlab
55	 */
56	average = 0;
57	for (i = 0; i < NBUF - 2; i++) {
58		gtod[i] = gtod[i + 1] - gtod[i];
59		printf("%13.9f\n", gtod[i]);
60		average += gtod[i];
61	}
62
63	/*
64	 * Sort the gtod array and display deciles
65	 */
66	for (i = 0; i < NBUF - 2; i++) {
67		for (j = 0; j <= i; j++) {
68			if (gtod[j] > gtod[i]) {
69				dtemp = gtod[j];
70				gtod[j] = gtod[i];
71				gtod[i] = dtemp;
72			}
73		}
74	}
75	average = average / (NBUF - 2);
76	fprintf(stderr, "Average %13.9f\n", average);
77	fprintf(stderr, "First rank\n");
78	for (i = 0; i < 10; i++)
79		fprintf(stderr, "%2d %13.9f\n", i, gtod[i]);
80	fprintf(stderr, "Last rank\n");
81	for (i = NBUF - 12; i < NBUF - 2; i++)
82		fprintf(stderr, "%2d %13.9f\n", i, gtod[i]);
83	exit(0);
84}
85
86
87/*
88 * get_systime - return system time in NTP timestamp format.
89 */
90void
91get_systime(
92	l_fp *now		/* system time */
93	)
94{
95	double dtemp;
96
97#if defined(HAVE_CLOCK_GETTIME) || defined(HAVE_GETCLOCK)
98	struct timespec ts;	/* seconds and nanoseconds */
99
100	/*
101	 * Convert Unix clock from seconds and nanoseconds to seconds.
102	 */
103# ifdef HAVE_CLOCK_GETTIME
104	clock_gettime(CLOCK_REALTIME, &ts);
105# else
106	getclock(TIMEOFDAY, &ts);
107# endif
108	now->l_i = ts.tv_sec + JAN_1970;
109	dtemp = ts.tv_nsec / 1e9;
110
111#else /* HAVE_CLOCK_GETTIME || HAVE_GETCLOCK */
112	struct timeval tv;	/* seconds and microseconds */
113
114	/*
115	 * Convert Unix clock from seconds and microseconds to seconds.
116	 */
117	gettimeofday(&tv, NULL);
118	now->l_i = tv.tv_sec + JAN_1970;
119	dtemp = tv.tv_usec / 1e6;
120
121#endif /* HAVE_CLOCK_GETTIME || HAVE_GETCLOCK */
122
123	/*
124	 * Renormalize to seconds past 1900 and fraction.
125	 */
126	dtemp += sys_residual;
127	if (dtemp >= 1) {
128		dtemp -= 1;
129		now->l_i++;
130	} else if (dtemp < -1) {
131		dtemp += 1;
132		now->l_i--;
133	}
134	dtemp *= FRAC;
135	now->l_uf = (u_int32)dtemp;
136}
137