Hi, * Li, Tong N ([EMAIL PROTECTED]) wrote: > 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). >
This is true, my test code was not perfect. Thanks for the hints. The improvements you propose will clearly accelerate my test program quite a bit, but I doubt that it will cause even higher memory latencies. Although using a random() at each memory access is slow, it should give a good enough dispersion. And since do 3 cache trashing passes in my code, I make sure that each and every cache lines are trashed. In fact, since I do multiple accesses to each cache line (as you noted in point 3), it takes more time, but makes it more certain that I hit all of them at least once. > If you do these, I expect you'll get a higher memory latency. > I will use these comments in my next tests, thanks. :) However, I still feel confident that the numbers I got from my run still hold. Mathieu > tong > -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/