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>
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 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 @@
}
/*
- * 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 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",