Fix the computation of standard deviation for a group of jobs. Please see the below link for the approximation formula used. <http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance The formula was originally presented here: <ftp://reports.stanford.edu/pub/cstr/reports/cs/tr/79/773/ CS-TR-79-773.pdf> Signed-off-by: Yu-ju Hong <yjhong@xxxxxxxxxx> --- stat.c | 19 +++++++++++++------ 1 files changed, 13 insertions(+), 6 deletions(-) diff --git a/stat.c b/stat.c index 9f22c6e..8be4be5 100644 --- a/stat.c +++ b/stat.c @@ -474,21 +474,28 @@ static void sum_stat(struct io_stat *dst, struct io_stat *src, int nr) dst->min_val = min(dst->min_val, src->min_val); dst->max_val = max(dst->max_val, src->max_val); - dst->samples += src->samples; /* - * Needs a new method for calculating stddev, we cannot just - * average them we do below for nr > 1 + * Compute new mean and S after the merge + * <http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance + * #Parallel_algorithm> */ if (nr == 1) { mean = src->mean; S = src->S; } else { - mean = ((src->mean * (double) (nr - 1)) - + dst->mean) / ((double) nr); - S = ((src->S * (double) (nr - 1)) + dst->S) / ((double) nr); + double delta = src->mean - dst->mean; + + mean = ((src->mean * src->samples) + + (dst->mean * dst->samples)) / + (dst->samples + src->samples); + + S = src->S + dst->S + pow(delta, 2.0) * + (dst->samples * src->samples) / + (dst->samples + src->samples); } + dst->samples += src->samples; dst->mean = mean; dst->S = S; } -- 1.7.3.1 -- To unsubscribe from this list: send the line "unsubscribe fio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
