111 lines
1.9 KiB
C
111 lines
1.9 KiB
C
/*
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* This program can be used to calibrate the clock reading jitter of a
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* particular CPU and operating system. It first tickles every element
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* of an array, in order to force pages into memory, then repeatedly calls
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* gettimeofday() and, finally, writes out the time values for later
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* analysis. From this you can determine the jitter and if the clock ever
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* runs backwards.
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*/
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#ifdef HAVE_CONFIG_H
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# include <config.h>
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#endif
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#include "ntp_types.h"
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#include <stdio.h>
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#include <stdlib.h>
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#define NBUF 100001 /* size of basic histogram */
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#define NSRT 20000 /* size of overflow histogram */
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#define NCNT (600 * 1000000) /* sample interval (us) */
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int col (const void *, const void *);
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int
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main(
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int argc,
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char *argv[]
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)
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{
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struct timeval ts, tr, tp;
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struct timezone tzp;
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int i, j, n;
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long t, u, v, w, gtod[NBUF], ovfl[NSRT];
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/*
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* Force pages into memory
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*/
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for (i = 0; i < NBUF; i++)
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gtod[i] = 0;
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for (i = 0; i < NSRT; i++)
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ovfl[i] = 0;
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/*
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* Construct histogram
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*/
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n = 0;
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gettimeofday(&ts, &tzp);
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t = ts.tv_sec * 1000000 + ts.tv_usec;
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v = t;
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while (1) {
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gettimeofday(&tr, &tzp);
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u = tr.tv_sec * 1000000 + tr.tv_usec;
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if (u - v > NCNT)
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break;
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w = u - t;
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if (w <= 0) {
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/*
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printf("error <= 0 %ld %d %d, %d %d\n", w, ts.tv_sec,
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ts.tv_usec, tr.tv_sec, tr.tv_usec);
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*/
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} else if (w > NBUF - 1) {
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ovfl[n] = w;
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if (n < NSRT - 1)
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n++;
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} else {
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gtod[w]++;
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}
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ts = tr;
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t = u;
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}
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/*
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* Write out histogram
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*/
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for (i = 0; i < NBUF - 1; i++) {
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if (gtod[i] > 0)
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printf("%ld %ld\n", i, gtod[i]);
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}
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if (n == 0)
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return;
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qsort(&ovfl, (size_t)n, sizeof(ovfl[0]), col);
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w = 0;
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j = 0;
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for (i = 0; i < n; i++) {
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if (ovfl[i] != w) {
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if (j > 0)
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printf("%ld %ld\n", w, j);
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w = ovfl[i];
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j = 1;
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} else
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j++;
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}
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if (j > 0)
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printf("%ld %ld\n", w, j);
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exit(0);
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}
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int
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col(
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const void *vx,
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const void *vy
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)
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{
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const long *x = vx;
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const long *y = vy;
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return (*x - *y);
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}
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