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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