1 files changed, 71 insertions, 0 deletions
diff --git a/util/jitter.c b/util/jitter.c
new file mode 100644
index 0000000..462fe12
--- /dev/null
+++ b/util/jitter.c
@@ -0,0 +1,71 @@
+/*
+ * 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 <stdio.h>
+#include <sys/time.h>
+
+#define NBUF 20002
+
+int
+main(
+	int argc,
+	char *argv[]
+	)
+{
+	struct timeval ts, tr;
+	struct timezone tzp;
+	long temp, j, i, gtod[NBUF];
+
+	gettimeofday(&ts, &tzp);
+
+	/*
+	 * Force pages into memory
+	 */
+	for (i = 0; i < NBUF; i ++)
+	    gtod[i] = 0;
+
+	/*
+	 * Construct gtod array
+	 */
+	for (i = 0; i < NBUF; i ++) {
+		gettimeofday(&tr, &tzp);
+		gtod[i] = (tr.tv_sec - ts.tv_sec) * 1000000 + tr.tv_usec;
+	}
+
+	/*
+	 * Write out gtod array for later processing with S
+	 */
+	for (i = 0; i < NBUF - 2; 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 - 2; 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 - 12; i < NBUF - 2; i++)
+	    fprintf(stderr, "%10ld%10ld\n", i, gtod[i]);
+	exit(0);
+}