[ntp:questions] 500ppm - is it too small?

Hal Murray hal-usenet at ip-64-139-1-69.sjc.megapath.net
Fri May 8 08:29:38 UTC 2009


In article <2MDMl.25650$OO7.18534 at text.news.virginmedia.com>,
 "David J Taylor" <david-taylor at blueyonder.not-this-part.nor-this.co.uk.invalid> writes:
>I've recent been suggesting the Windows port of NTP as a program suitable 
>for an application where the timekeeping needed to be within a second or 
>two.  Yes, NTP is overkill, but it has the advantages of multiple servers, 
>best server selection, adaptive poll rate, and memory of the clock drift 
>etc.  However, on quite a few installations - at a guess between 1% and 
>5% - NTP has failed because the click frequency error appears to be too 
>great for NTP to correct.
>
>Is there any feeling for changing the 500ppm limit, perhaps to 1000ppm or 
>even as much as 5000ppm (to pull a figure out of the hat)?  Or is 500ppm 
>generally believed to be the worst error which should be compensated?
>
>One possibility is that some of the problem PCs are portables, with some 
>sort of power-saving or even hibernation scheme.  I don't have direct 
>visibility of the type of PC.

I'm pretty sure the 500 ppm limit is there to ensure that the PLL
activities are stable.

The real question I would ask is: How stable is the clock when ntp
isn't running?  (I'd probably setup ntpd to run with a local refclock
and monitor it from another ntpd.)  By stable I mean drifts at a
constant rate even if it is more than 500 ppm.  If you plot offset
as a function of time, a stable clock will be a straight line with
a slope.  (A perfect clock will have a slope of 0.)

If the clock isn't stable, ntp is going to have problems.  If the
problem is dute to temperature, ntp might be able to track it, but
that doesn't explain why the clock would be off more than 500 ppm.

If the clock is stable but off by, say, 600 ppm, you might find
a software adjustment to fix it.  The idea is to put the 600
correction in the back door while ntpd puts its correction
in the front door and the system adds them together.

The power saving stuff is definitely a good place to cause problems.


-- 
These are my opinions, not necessarily my employer's.  I hate spam.




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