Messages in this thread | | | Subject | RE: [patch 10/10] *Tests* Scheduler profiling - Use immediate values | Date | Fri, 6 Jul 2007 23:08:05 -0700 | From | "Li, Tong N" <> |
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Mathieu,
> cycles_per_iter = 0.0; > for (i=0; i<NR_TESTS; i++) { > time1 = get_cycles(); > for (j = 0; j < NR_ITER; j++) { > testval = &array[random() % ARRAY_SIZE]; > } > time2 = get_cycles(); > cycles_per_iter += (time2 - time1)/(double)NR_ITER; > } > cycles_per_iter /= (double)NR_TESTS; > printf("Just getting the pointer, doing noting with it, cycles per > iteration (mean) : %g\n", cycles_per_iter); >
Some comments on the code:
1. random() is counted in cycle_per_iter, which can skew the results. You could pre-compute the random addresses and store them in an array. Then, during the actual timing, walk the array:
index = 0; for (i = 0; i < ARRAY_SIZE; i++) index = *(int *)(array + index * CACHE_LINE_SIZE);
2. You may want to flush the cache before the timing starts.
3. You want to access memory at the cache-line granularity to avoid addresses falling into the same line (and thus unwanted hits).
If you do these, I expect you'll get a higher memory latency.
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