[ntp:questions] stupid, simple question about precision

Simple Simon ssimon at domain.invalid
Mon Nov 20 06:20:46 UTC 2006

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.

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