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perf stat: Use stddev_mean in stead of stddev

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When we're computing the mean by sampling the distribution,
then the std dev of the mean is related to the std dev of the
sample set by:

  stddev_mean = std_dev / sqrt(N)

Which is exactly what we want.

This results in the error on the mean decreasing with
increasing number of samples.

Also fix the scaled == -1, aka not counted case.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Peter Zijlstra 2009-09-04 17:03:13 +02:00 committed by Ingo Molnar
parent 9e9772c458
commit 63d40deb2e
1 changed files with 19 additions and 6 deletions
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25
tools/perf/builtin-stat.c
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@ -75,7 +75,7 @@ static int null_run = 0;
static int fd[MAX_NR_CPUS][MAX_COUNTERS];
static u64 event_res[MAX_COUNTERS][3];
static u64 event_scaled[MAX_COUNTERS];
static int event_scaled[MAX_COUNTERS];
struct stats
{
@ -97,17 +97,31 @@ static double avg_stats(struct stats *stats)
}
/*
* stddev = sqrt(1/N (\Sum n_i^2) - avg(n)^2)
* http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance
*
* (\Sum n_i^2) - ((\Sum n_i)^2)/n
* s^2 -------------------------------
* n - 1
*
* http://en.wikipedia.org/wiki/Stddev
*
* The std dev of the mean is related to the std dev by:
*
* s
* s_mean = -------
* sqrt(n)
*
*/
static double stddev_stats(struct stats *stats)
{
double avg = stats->sum / run_count;
double variance = (stats->sum_sq - stats->sum*avg)/(run_count - 1);
double variance_mean = variance / run_count;
return sqrt(stats->sum_sq/run_count - avg*avg);
return sqrt(variance_mean);
}
struct stats event_res_stats[MAX_COUNTERS][3];
struct stats event_scaled_stats[MAX_COUNTERS];
struct stats runtime_nsecs_stats;
struct stats walltime_nsecs_stats;
struct stats runtime_cycles_stats;
@ -343,11 +357,10 @@ static void abs_printout(int counter, double avg, double stddev)
static void print_counter(int counter)
{
double avg, stddev;
int scaled;
int scaled = event_scaled[counter];
avg = avg_stats(&event_res_stats[counter][0]);
stddev = stddev_stats(&event_res_stats[counter][0]);
scaled = avg_stats(&event_scaled_stats[counter]);
if (scaled == -1) {
fprintf(stderr, " %14s %-24s\n",