[ntp:questions] Loopstats jitter field mostly zero?

David Taylor david-taylor at blueyonder.co.uk.invalid
Sun Nov 25 06:25:46 UTC 2012


On 24/11/2012 20:26, Dave Hart wrote:
> On Sat, Nov 24, 2012 at 7:56 PM, David Taylor
> <david-taylor at 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 at 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 at 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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