[PATCH] Implement a better mean and standard deviation calculation
Based on Knuth's, volume 2.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/stat.c b/stat.c
index 6e7c235..a665ec8 100644
--- a/stat.c
+++ b/stat.c
@@ -283,7 +283,7 @@
static int calc_lat(struct io_stat *is, unsigned long *min, unsigned long *max,
double *mean, double *dev)
{
- double n, o;
+ double n = is->samples;
if (is->samples == 0)
return 0;
@@ -292,17 +292,12 @@
*max = is->max_val;
n = (double) is->samples;
- *mean = (double) is->val / n;
- *dev = 0.01;
+ *mean = is->mean;
- if (n <= 1.0)
- return 1;
-
- o = ((double) is->val_sq - (*mean * is->val)) / n;
- if (o < 0.0)
- *dev = -1.0;
+ if (n > 1.0)
+ *dev = sqrt(is->S / (n - 1.0));
else
- *dev = sqrt(o);
+ *dev = -1.0;
return 1;
}
@@ -550,15 +545,21 @@
free(runstats);
}
-static inline void add_stat_sample(struct io_stat *is, unsigned long val)
+static inline void add_stat_sample(struct io_stat *is, unsigned long data)
{
- if (val > is->max_val)
- is->max_val = val;
- if (val < is->min_val)
- is->min_val = val;
+ double val = data;
+ double delta, n;
- is->val += val;
- is->val_sq += val * val;
+ if (data > is->max_val)
+ is->max_val = data;
+ if (data < is->min_val)
+ is->min_val = data;
+
+ delta = val - is->mean;
+ n = is->samples + 1.0;
+ is->mean += delta / n;
+ is->S += delta * (val - is->mean);
+
is->samples++;
}