[ntp:questions] stupid, simple question about precision

Richard B. Gilbert rgilbert88 at comcast.net
Mon Nov 20 14:20:49 UTC 2006


Simple Simon wrote:
> rgilbert88 at comcast.net wrote...
> 
>>Simple Simon wrote:
>>
>>
>>>If 'ntpq -c rl' shows the precision of my clock as -20 (roughly, 
>>>9ns), does that translate to timestamp confidence interval of +/- of 
>>>9ns, or of +/- of 4.5ns) ?
>>>
>>>Feel free to point to me to the section of the NTP FAQ I've missed, 
>>>or any other resource that I haven't seen in trying to answer this 
>>>question on my own.
>>>
>>>Any guidance greatly appreciated.
>>
>>I think you are mistaking precision for accuracy.  Precision is the 
>>measure of the shortest interval your clock can represent.  It has 
>>nothing (or very little) to do with accuracy.
>>
>>ntptime will show you the estimated error for your clock.  Unless you 
>>are using a hardware reference clock, the estimated error may be in 
>>milliseconds rather than microseconds.
> 
> 
> After some focused reading of newbie materials, you are right:  I am 
> mistaking precision for accuracy.  I appreciate your good manners.
> 
> I've seen the estimated error go from 18.3 ms to 3.5 ms since you 
> reply a few days back.  With a vanilla NTP configuration I suppose 
> I'm not going to see much additional improvement.
> 
> My understanding is the best accuracy I might get, on a generic PC 
> clock, is about 0.050 ms--and that's with SSP and the kernel clock.
> 
> Thanks for steering me towards better understanding.

The internet tends to introduce enough noise to render microsecond 
accuracy improbable!  If you have a hardware reference clock; e.g. a GPS 
receiver, WWV receiver, WWVB receiver, etc, you may get microsecond 
accuracy, or not, depending on the vagaries of radio propagation.

Selection of internet servers can have a strong influence on accuracy.
Look for low values of delay and jitter.  The servers, if operating 
properly, have the correct time; what you are really selecting is the 
quality of the usual network paths between your site and the server. 
Note the plural; the routers do their best to get the packet where it's 
going but the route used is by no means constant.

Have a look at the ~/scripts/stats directory for some tools that will 
allow you to study the quality of your servers and network paths. 
peer.awk is a real help; you say something like
gawk -f peer.awk /var/ntp/ntpstats/peerstats.20061119
and you get a statistical analysis of the stats in that file.




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