[ntp:questions] Re: Drift handling....

David J Taylor david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk.invalid
Tue Jan 3 07:58:16 UTC 2006

Harlan Stenn wrote:
> OK, listen to me now and believe me later.
> Dave said it better when he said the drift value is updated at every
> clock update.  We only write the value once an hour.
> If you really want to play with this stuff, code it up in C in the
> actual codebase and run it thru the simulator (which is part of the
> distribution).
> A surprising number of folks have thought they had a good idea on how
> to "make it better" and then got a rude awakening from the simulator.
> H


You seem to come across aggressively, although I'm sure that's not your 

With respect, the /last/ stage would be coding up something.  First, I 
would rather discuss the principle of what might be done, and learn from 
those with a better understanding than I.  Dave has said there might be an 
obscure error, and others have observed that the drift value gained over a 
period of time is too easily discarded, so I'd like to wait until Dave has 
had a chance to study the many reports he has of what happened over the 
leap second before jumping into coding.

My own small contribution has been updated a little:


I would still like to understand whether having both a steady value plus a 
diurnal component for the drift is required for best performance, or 
whether a 24-hour average value might be enough.  It does not seem to be 
the optimum behaviour to discard a value gained over many days within an 
hour when writing to the file.  This does mess up ntpd when it restarts 
itself because of the offset being too great.


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