Re: [chrony-dev] Nanosecond timestamps

The offset is being taken at 15 second intervals via a script that calls "chronyc -c sourcestats” and feeds the data to rrd. My polling was not quite as aggressive, at 2 for the device shown in the graphs I sent.

I wouldn’t think I have much in the way of asymmetry, but anything’s possible. Both the monitoring system and the and the NTP servers under test are connected to the same switch. Here’s what my sources/sourcestats looked like:

210 Number of sources = 7
MS Name/IP address Stratum Poll Reach LastRx Last sample
===============================================================================
^+ 192.168.230.240 1 2 377 3 -218ns[ -240ns] +/- 23us
^x 192.168.230.241 1 2 377 0 -40us[ -40us] +/- 2930us
^x 192.168.230.242 1 2 377 0 -40us[ -40us] +/- 2930us
^* 192.168.230.244 1 2 377 0 -146ns[ -168ns] +/- 23us
=x 192.168.230.1 2 3 377 1 -7027ns[-7049ns] +/- 1530us
=x 192.168.230.2 2 2 377 4 +4821ns[+4799ns] +/- 86us
=x 192.168.225.10 2 2 377 5 -24us[ -24us] +/- 206us
210 Number of sources = 7
Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev
==============================================================================
192.168.230.240 47 28 202 -0.001 0.005 +53ns 558ns
192.168.230.241 25 14 97 -0.015 0.208 -41us 7733ns
192.168.230.242 16 6 60 -0.162 0.355 -48us 5623ns
192.168.230.244 26 13 105 +0.002 0.010 -52ns 410ns
192.168.230.1 39 17 355 -0.002 0.023 +361ns 3795ns
192.168.230.2 48 24 251 -0.008 0.020 +382ns 2505ns
192.168.225.10 64 38 606 +0.003 0.013 -17us 5355ns

23/24us for the error estimate isn’t bad considering that the minimum round trip transmission time through the switch is 21.44 us. If I direct connect the server (bypassing the switch), then this drops down to 21/22us (the switch is 3.44us).

Back on the switch, if I bring the polling down to 1 then things tighten up a bit more. But still nowhere near the 151ns standard deviation that you are getting.

210 Number of sources = 7
MS Name/IP address Stratum Poll Reach LastRx Last sample
===============================================================================
^+ 192.168.230.240 1 1 377 0 -212ns[ -212ns] +/- 24us
^x 192.168.230.241 1 2 377 0 -27us[ -27us] +/- 2930us
^x 192.168.230.242 1 2 377 1 -43us[ -43us] +/- 2930us
^* 192.168.230.244 1 1 377 1 +174ns[ +157ns] +/- 24us
=x 192.168.230.1 2 3 377 7 +2809ns[+2835ns] +/- 1552us
=x 192.168.230.2 2 1 377 3 -1394ns[-1411ns] +/- 82us
=x 192.168.225.10 2 1 335 0 -31us[ -31us] +/- 191us
210 Number of sources = 7
Name/IP Address NP NR Span Frequency Freq Skew Offset Std Dev
==============================================================================
192.168.230.240 16 6 30 +0.005 0.029 +4ns 264ns
192.168.230.241 64 28 255 +0.029 0.038 -25us 6948ns
192.168.230.242 39 23 154 +0.005 0.102 -46us 7070ns
192.168.230.244 45 27 92 -0.001 0.005 -23ns 237ns
192.168.230.1 23 13 210 +0.015 0.033 +1262ns 2521ns
192.168.230.2 64 29 215 +0.007 0.012 -2301ns 1733ns
192.168.225.10 63 33 474 +0.003 0.019 -19us 5579ns

Denny

On Nov 03, 2016, at 23:51, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:

On Thu, Nov 03, 2016 at 01:38:25PM -0700, Denny Page wrote:
I’m a little late to the party, but I thought it would be worth visually noting the positive impact that the nanosecond timestamp changes have had. The first graph is from a dedicated chrony monitoring system. The transition from 2.4 to the current version occurs midway though the graph. Nothing too dramatic, but still a noticeable improvement. The second graph is from a more active general server. The improvement here is much more significant.

Which offset do the graphs show? The one from statistics.log? The
improvement might be related also to the correction for asymmetric
jitter that was included recently.

Thank you for all the hard work. I’m really looking forward to hardware timestamps. :)

I'm busy with other things right now, but I hope I'll have something
ready for testing soon. It's a lot of new code and I need to put it in
a better shape before I can push it to git. Anyway, here is how my
chronyc sources and sourcestats look right now :).

210 Number of sources = 1
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
^* 192.168.100.1                 1   1   377     1   +259ns[ +267ns] +/-   23us
210 Number of sources = 1
Name/IP Address            NP NR Span Frequency Freq Skew Offset Std Dev
==============================================================================
192.168.100.1              24 11    46     +0.000      0.009     +0ns   151ns

--
Miroslav Lichvar