[ntp:questions] Clock PPM steps

A C agcarver+ntp at acarver.net
Fri Feb 24 03:53:39 UTC 2012

I'd like to try and understand how the PPM of the clock is stepped when 
using the ATOM driver and flag3 is set or unset.  If you look at the 
loopstats file http:/acarver.net/ntpd/loops.20120223 and observe the PPM 
column, you'll see that it will sit at one value for a while and then 
step to another value and hold again (say from -77.058 to -77.112). 
This is with flag3 set (kernel discipline enabled).  It never quite 
settles, it's always adjusting the number.

Now, if I clear flag3, the ATOM driver continues to adjust the clock PPM 
but it does so in very fine increments of 0.001 and then manages to hold 
that for a very long time without jumping around.  It may drift up and 
down by +/- 0.005 PPM or so depending on the room temperature but it 
never really strays from some center value.  It also makes the 
adjustments sooner than with flag3 set.  If a temperature change starts 
to occur while flag3 is clear, the PPM shifts within one or two polling 
cycles.  If flag3 is set, the PPM is held steady for many polling cycles 
(typically at least 8-10) and then makes the sudden jump instead of a 
graceful slew.

I don't really understand why flag3 is making a difference in the step 
size of the adjustment.  It seems that flag3 causes a much more coarse 
correction to the clock which isn't very stable compared to no flag3 
which stabilizes the clock fairly well (stable in terms of a very mildly 
changing PPM offset).  I would have expected the adjustments to be the 
same regardless of mode and try to fine tune the clock.  I can't imagine 
the large steps are keeping things as stable as they could be.

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