[ntp:questions] Slow convergence of NTP with GPS/PPS

Nicola Berndt nb at komeda-berlin.de
Tue Oct 21 19:12:41 UTC 2008

Unruh schrieb:

>> I see, that's too bad then.. :( I am already using minpoll 4 maxpoll 4
> OK, But that should have a convergence of minutes not hours. Mind you NTPs
> habit of throwing away 7 out of 8 queries of the clock does not help.
> (clock filter). Especially for a pps that is pretty extreme.

Today I moved the computer to a different location to work there and I 
found it to set its clock right after start and keep it within ms-range! 
I didn't change anything, just shut down, drove there and turned it on, 
so I am really confused about that. Both locations are normal rooms with 
normal room-temerature. Well, I duplicated the system (that was why I 
was there..) and came back home with mine and tomorrow I will see if it 
behaves different again. Very very weird! It looks as if all of a sudden 
the driftfile was used and before not! This is also very strange, since 
the driftfile was (re)written yesterday, so ntp knew about it yesterday. 
My ntp.conf also includes the driftfile location.

>> No, the Soekris will run linux an d ntpd and the oscillator will just be 
>> on an external little board. The computer is residing in an airport 
>> hangar for MONTH sometimes with no powersource at all! There is 
> Hard for it to be on all the time then. Or for it to have anthing like an
> accurate time. And that ovenized oscillator will also be pretty useless (
> much worse than the GPS) since it will have no power either and the crystal
> will not be oscillating nor the oven keeping the temp constant. 

Oh, so I got the word ovenized wrong: I understood it to be very immune 
against varying temperature. Ok, so if it needs an heater and all, it's 
useless in my case.

> So, what you have is a free standing computer which must come out of a cold
> shutdown (ie the oscillator frequency on startup will be way off its
> frequency in steady state because it is cold) so will be far from
> equilibrium. What is your time error requirement? Seconds, milliseconds,
> microseconds? In such a situation ntp would probably give you a few
> milliseconds. But it certainly is NOT designed to give you good accuracy in
> such a situation during startup.  What are you finding?

Well, one thing I can of course always do is to boot hte machine, let it 
run for a flittle while and reboot it, so it boots with a warmed up 
oscillator. This would give trouble with the driftfile, though..

We target for millisecond accuracy. As I understand, the oscillators on 
standard PCs are mostly cheapest crap and there are way better 
oscillators I could use to replace the original. Is that correct?

What I saw today was pretty much, what I was looking for: No running 
away and stable within minutes. We also took a fan and heated up the 
oscillator. We could watch a slow drift with ntpq -p, but nothing too 
bad. It went away for a few ms and came back within minutes again.

I'll look into that tomorrow..


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