[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)

Unruh unruh-spam at physics.ubc.ca
Sat Jan 26 21:48:52 UTC 2008


David Woolley <forums at david-woolley.me.uk> writes:


>What I was expecting was for it to unconditionally do both frequency and 
>phase calibration, in the absence of the drift file.  I presume that 
>chrony does a correction on the first couple of samples and then refines it.

Yes. Actually it does a recalibration using the last n samples ( where n is
dynamic and grows with stabiltiy and shrinks if the linear fit is not a
very good one-- good defined by looking at how often the errors in the
linear fit cross zero) It then uses the adjtimex OFFSET single shot
adjustment to get rid of the ofset and uses the slope to set the freuency,
adjusting the old samples to account for the change in offset and
frequency, and keeping track of the offset ajustment in case it was
interrupted or did not completely conpensate. 



>Incidentally, the "else FREQ" doesn't seem to match the code and looks 
>like it would prevent it ever getting out of the calibration under some 
>conditions.

>It looks like I need to fetch the latest source, although it looks, from 
>your observations, as though it is still far from what I would consider 
>right.




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