On Thu, Jun 25, 2015 at 04:21:40PM +0100, Damien Lespiau wrote:
> The master plan would be to get a bit more stats in it, at least the
> standard deviation and confidence interval. Just need the average for
> now.
Definitely, I wouldn't even print the average without the std.dev and
how normal the distribution is. (Otherwise the information can be very
misleading.) Without them, I'd rather have the median than the mean.
> +void igt_stats_push(igt_stats_t *stats, uint64_t value)
> +{
> + igt_assert(stats->n_values < stats->capacity);
Might as well make this a realloced array from the start.
> + stats->values[stats->n_values++] = value;
> +}
> +
> +double igt_stats_get_average(igt_stats_t *stats)
> +{
> + unsigned int i;
> + double a = 0.0;
> +
> + for (i = 0; i < stats->n_values; i++)
> + a += (double)stats->values[i] / stats->n_values;
For fun, we could write numerically more stable versions. Welcome to the
bikeshed.
static double kahan_summation(igt_stats_t *stats)
{
double sum = 0;
double error = 0;
unsigned i;
for (i = 0; i < stats->n_values; i++) {
double x = stats->values[i] - error;
double tmp = sum + x;
error = (tmp - sum) - x;
sum = tmp;
}
return sum;
}
double igt_stats_get_average(igt_stats_t *stats)
{
return kahan_summation(stats) / stats->n_values;
}
or
static knuth_avergae(igt_stats_t *stats)
{
double mean = 0;
unsigned i;
for (i = 0; i < stats->n_values; i++)
mean += (stats->values[i] - mean) / (i + 1);
return mean;
}
double igt_stats_get_average(igt_stats_t *stats)
{
return knuth_average(stats);
}
or if we are sorting to get the median anyway, doing a pairwise
summation would also reduce the numerical errors...
-Chris
--
Chris Wilson, Intel Open Source Technology Centre
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