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perf stat: Use stddev_mean in stead of stddev 

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",