1 files changed, 19 insertions, 6 deletions

diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c
index 9c6377f7152f..e9424fa72420 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",