On 24/11/2012 20:26, Dave Hart wrote:
On Sat, Nov 24, 2012 at 7:56 PM, David Taylor
<david-tay...@blueyonder.co.uk.invalid> wrote:
I've just started the statistics collection on my Linux/RPi NTP server, and
when plotting the jitter results I found that most of the values were zero.
I don't see that on my Windows stratum-1 systems. Is there a limit in the
calculation of the jitter value recorded (to 9 decimal places) in the
loopstats file.
I really can't imagine that such a low-powered server is /that/ good -
better than a nanosecond.
What version of ntpd is the embedded device using? ntpq -c "rv 0
version" <target name/IP>
ntpd's internal jitter calculation is done using C floating "double"
which is at least 64 bits. Is the value you see working down to zero
over time, or starting and staying at zero? How close are the
exceptions the "most of the values" to 0 nanoseconds?
I will try to keep an eye out for your response, but my email time
budget is severely limited through the end of year.
Cheers,
Dave Hart
Dave,
The version is:
ntpd 4.2.7p324@1.2483 Mon Nov 19 16:03:37 UTC 2012 (1).
There are a few non-zero values to start with, but once NTP is near
settled the values are zero. This is a stratum-1 device with a PPS
feed, running Linux. It starts at non-zero, works downwards, and then
drops suddenly to zero. 64-bit floating point seconds would have been
adequate, I believe. I wonder whether there's a 32-bit truncation
somewhere? Or if two large values are being subtracted where the
difference value may be of much lower precision?
This was, I believe, a start-up on the third line:
56255 64514.309 -0.000000340 -42.659 0.000000000 0.000128 4
56255 64530.309 -0.000000043 -42.659 0.000000000 0.000120 4
56255 64546.308 -0.000000393 -42.659 0.000000000 0.000117 4
56255 64596.175 0.000000000 -42.687 0.000001907 0.000000 5
56255 64603.148 0.000103553 -42.671 0.000036655 0.005731 5
56255 64607.148 0.000045082 -42.670 0.000000000 0.005366 5
56255 64613.148 0.000067766 -42.669 0.000000000 0.005049 5
56255 64615.160 0.000063706 -42.667 0.000000000 0.004773 4
56255 64631.148 0.000050000 -42.654 0.000000000 0.006210 4
The poll of 5 would have been the LAN servers, and the poll of 4 the
PPS. Could it be a limit in the Raspberry Pi's hardware?
For comparison, here is a Windows system (PC Alta) also running as a
stratum-1 server with version:
ntpd 4.2.7p326@1.2483-o Nov 21 15:15:30.79 (UTC-00:00) 2012 (1).
56255 161.055 -0.000012299 3.400 0.000019057 0.000191 4
56255 177.055 0.000010981 3.400 0.000019635 0.000188 4
56255 193.056 0.000002021 3.400 0.000018638 0.000176 4
56255 209.056 -0.000026469 3.400 0.000020135 0.000218 4
56255 225.055 -0.000012419 3.399 0.000019478 0.000215 4
56255 241.055 0.000024891 3.400 0.000022494 0.000242 4
56255 257.055 0.000017241 3.400 0.000021214 0.000244 4
56255 273.055 0.000005821 3.400 0.000020251 0.000231 4
--
Cheers,
David
Web: http://www.satsignal.eu
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