[tip:perfcounters/core] perf stat: Use stddev_mean in stead of stddev

tip-bot for Peter Zijlstra
2009-09-04T15:44:06Z

Commit-ID: 63d40deb2e7c64ed55943d49f078e09fc4b64b54
Gitweb: http://git.kernel.org/tip/63d40deb2e7c64ed55943d49f078e09fc4b64b54
Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
AuthorDate: Fri, 4 Sep 2009 17:03:13 +0200
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Fri, 4 Sep 2009 17:38:14 +0200

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>

---
tools/perf/builtin-stat.c | 25 +++++++++++++++++++------
1 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index 9c6377f..e9424fa 100644
--- a/tools/perf/builtin-stat.c
+++ b/tools/perf/builtin-stat.c
@@ -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",