[ntp:questions] Re: ntpd PLL and clock overshoot

David Woolley david at djwhome.demon.co.uk
Mon Oct 16 21:10:57 UTC 2006


In article <eh0a2t$7eh$1 at scrotar.nss.udel.edu>,
"user at domain.invalid"  (David Mills???????) wrote:

> Look more carefully at the clock state machine. If the initial offset is 
> greater than the step threshold, the clock is set, which results in an 

In the case in question, it was just under 90ms, so this condition
is not true.

> initial offset of zero. After the stepout interval the frequency is 
> measured and corrected, but the phase at that time could be quite large, 

There's no frequency measurement because the offset was less than the
time limit, and, I presume, this wasn't the very first time that ntpd
was started.

> threshold, ordinary linear mode results. This is NOT overshoot, just an 
> initial phase error.

But it's not applicable here because there has been no use of the 900 
second stepout timer.

> If the initial offset is less than the step threshold, the freqjuency 
> measurement is made, but the phase is disciplined normally during the 

Only if it starts in state NSET, i.e no ntp.drift.  Although the
person making the original 10% overshoot report really ought to confirm
this, I'd be very surprised if there weren't a valid ntp.drift file,
in which case it starts in state FSET and, because the initial offset
is less than 128ms, almost immediately goes into the FLL/PLL phase.

> stepout interval. Again, this could result in a phase offset after the 
> stepout interval, but this is NOT overshoot.

But, again, its a scenario that doesn't apply here.




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