74 lines
1.6 KiB
C
74 lines
1.6 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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#include <sys/time.h>
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#include <stdio.h>
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#define NBUF 10001
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main()
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{
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struct timeval tp, ts, tr;
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struct timezone tzp;
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long temp, j, i, gtod[NBUF];
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gettimeofday(&ts, &tzp);
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ts.tv_usec = 0;
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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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/*
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* Construct gtod array
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*/
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for (i = 0; i < NBUF; i ++) {
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gettimeofday(&tp, &tzp);
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tr = tp;
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tr.tv_sec -= ts.tv_sec;
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tr.tv_usec -= ts.tv_usec;
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if (tr.tv_usec < 0) {
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tr.tv_usec += 1000000;
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tr.tv_sec--;
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}
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gtod[i] = tr.tv_sec * 1000000 + tr.tv_usec;
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}
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/*
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* Write out gtod array for later processing with S
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*/
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for (i = 0; i < NBUF - 1; i++) {
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/*
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printf("%lu\n", gtod[i]);
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*/
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gtod[i] = gtod[i + 1] - gtod[i];
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printf("%lu\n", gtod[i]);
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}
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/*
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* Sort the gtod array and display deciles
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*/
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for (i = 0; i < NBUF - 1; i++) {
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for (j = 0; j <= i; j++) {
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if (gtod[j] > gtod[i]) {
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temp = gtod[j];
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gtod[j] = gtod[i];
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gtod[i] = temp;
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}
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}
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}
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fprintf(stderr, "First rank\n");
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for (i = 0; i < 10; i++)
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fprintf(stderr, "%10ld%10ld\n", i, gtod[i]);
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fprintf(stderr, "Last rank\n");
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for (i = NBUF - 11; i < NBUF - 1; i++)
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fprintf(stderr, "%10ld%10ld\n", i, gtod[i]);
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}
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