RE: [patch 10/10] *Tests* Scheduler profiling - Use immediate values

Li, Tong N
2007-07-07T06:08:32Z

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.

tong