[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.
More information about the questions