8269 dtrace stddev aggregation is normalized incorrectly

illumos/illumos-gate@79809f9cf402f130667349b2d4007ecd65d63c6f
https://github.com/illumos/illumos-gate/commit/79809f9cf402f130667349b2d4007ecd65d63c6f

https://www.illumos.org/issues/8269
  It seems that currently normalization of stddev aggregation is done
  incorrectly.
  We divide both the sum of values and the sum of their squares by the
  normalization factor. But we should divide the sum of squares by the
  normalization factor squared to scale the original values properly.

Reviewed by: Bryan Cantrill <bryan@joyent.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Andriy Gapon <avg@FreeBSD.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/lib/libdtrace/common/dt_consume.c b/lib/libdtrace/common/dt_consume.c
index 7557e98ae700..fff0616e23dd 100644
--- a/lib/libdtrace/common/dt_consume.c
+++ b/lib/libdtrace/common/dt_consume.c
@@ -381,8 +381,10 @@ dt_stddev(uint64_t *data, uint64_t normal)
 	 * The standard approximation for standard deviation is
 	 * sqrt(average(x**2) - average(x)**2), i.e. the square root
 	 * of the average of the squares minus the square of the average.
+	 * When normalizing, we should divide the sum of x**2 by normal**2.
 	 */
 	dt_divide_128(data + 2, normal, avg_of_squares);
+	dt_divide_128(avg_of_squares, normal, avg_of_squares);
 	dt_divide_128(avg_of_squares, data[0], avg_of_squares);
 
 	norm_avg = (int64_t)data[1] / (int64_t)normal / (int64_t)data[0];