[ntp:questions] Speed of ntp convergence

Unruh unruh-spam at physics.ubc.ca
Sun Nov 2 01:47:47 UTC 2008

"David J Taylor" <david-taylor at blueyonder.neither-this-part.nor-this-bit.co.uk> writes:

>Unruh wrote:
>> Nope, it is not my "ssytem" if by that you mean my computer. The
>> convergence is a beautiful exponential convergence with a time scale
>> of 1
>> hour almost exactly. That is not hardware. That is the software ntp
>> protocol.

>Just along the lines that if my system converges in 10 minutes, I am at a 
>loss to see why yours takes ten hours.  It seems to me that there is 
>nothing inherently wrong with the NTP version I have.

>> Try switching it off, changing the value int he drift file by say
>> 50PPM and
>> then switching it on again, and see how long it takes to recover from
>> that.

>Why would I do that?  The drift values rarely change by more than five, 

To test to see how long your system takes to converge. It is clear on my
system and somehow it is getting a very bad idea of the frequency. For
example, the time drifts to offset larger than 125ms, and resets to 0 ( because of
the -g) but almost immediately it drifts at about 140PPM.  It then takes 10
hours to get the offset back to 0.

Why is the drift off so badly? I do not know, but that is irrelevant. ntp
should NOT take 10 hours to correct a badly drifting clock. 

So how long does it take your system to correct a bad drift file? 

>certainly not by 50.  If you are seeing a change of 50, then perhaps that 
>it part of your problem?

It may be. But ntp's problem is that it is taking far far far too long to
correct a bad drift.

>> Note, if you are running gps, why have a poll level 6? The
>> recommendation
>> for ref- clocks is poll level 4?

>Probably because the system is also polling Internet servers, and I didn't 
>want to hammer them at 16 second intervals.  It seems that the Internet 
>poll interval /must/ be the same as the ref-clock poll interval, and that 
>it doesn't automatically adjust upwards (which would be nice).

No, each source has its own poll interval. They do NOT need to be the same. 

>Where did you get your information from, by the way?  I found this:

>  http://sunsite.ualberta.ca/Documentation/Misc/ntp-4.0.99a/clockopt.htm

>  "For most directly connected reference clocks, both minpoll and maxpoll 
>default to 6 (64 s)."

I recall from somewhere that that it was 4, but I could be wrong. I have it
as 4 for my refclock. (4.0.99 us a but old)

>from a quick Google search, but I don't know if that's in the official 

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