Package: iputils-ping
Version: 3:20180629-2
Severity: normal
Tags: patch
Dear Maintainer,
As documented, ping fails to properly compute deviation where ping times
are small. A patch follows. A simple example, using variables used there:
Suppose we do 10 pings, and 5 take 10us, 5 take 11us. tsum2 is 1105,
tsum is 105. We divide them, getting 110 and 10 respectively. tmdev is
then llsqrt(110 - 10 * 10) = llsqrt(10) = 3us. But max-min is only 1us!
The exact solution is sqrt(110.5-110.25) which is sqrt(0.25)=0.5. The
new code will (more) correctly compute llsqrt(0) = 0 as the answer. It
takes care not to do any horrendous integer overflows if there's a lot
of long pings (a few thousand >1s pings would overflow without this).
Patch is against 3:20101006-1.
--
David Buckley
--- old/ping_common.c 2018-08-26 15:43:03.891105137 +0100
+++ new/ping_common.c 2018-08-26 16:01:39.046836333 +0100
@@ -855,13 +855,21 @@
if (nreceived && timing) {
+ long total = nreceived + nrepeats;
+ long tmavg = tsum / total;
+ long long tmvar;
long tmdev;
- tsum /= nreceived + nrepeats;
- tsum2 /= nreceived + nrepeats;
- tmdev = llsqrt(tsum2 - tsum * tsum);
+ if (nrepeats < INT_MAX) {
+ // This slightly clumsy computation order is important
to avoid
+ // integer rounding errors for small ping times.
+ tmvar = (tsum2 - tsum * tsum / total) / total;
+ } else {
+ tmvar = tsum2 / total - tmavg * tmavg;
+ }
+ tmdev = llsqrt(tmvar);
printf("rtt min/avg/max/mdev =
%ld.%03ld/%lu.%03ld/%ld.%03ld/%ld.%03ld ms",
(long)tmin/1000, (long)tmin%1000,
- (unsigned long)(tsum/1000), (long)(tsum%1000),
+ (unsigned long)(tmavg/1000), (long)(tmavg%1000),
(long)tmax/1000, (long)tmax%1000,
(long)tmdev/1000, (long)tmdev%1000
