[ntp:questions] stupid, simple question about precision
ssimon at domain.invalid
Mon Nov 20 18:53:30 UTC 2006
rgilbert88 at comcast.net wrote...
> 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.
I was thinking more of the general, ideal case -- where there might
be multiple Stratum 1 timeservers on a LAN, and a generic PC clock.
The machine I've been working with here has one ISP-provided NTP
server (stratum 4) within a few ms distance, but the other servers
are from the pool so...well, frankly, I'm happily surprised I've got
a 3.5 ms est error.
Dumb usage question on the est error: If I have a time of N.003555
and an estimated error of 0.003555, do that mean the actual range of
time is N.00355 +/- 0.003555, or +/- 0.001778 ?
Thank you again for your patience dealing with a newbie for what is
undoubtedly the 10,000th time you've explained this basic stuff.
> 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.
Thank you very much. Will do.
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