| Re: [chrony-dev] frequency fallback when running unsynchronised |
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On Fri, Apr 09, 2010 at 07:50:58AM -0700, Bill Unruh wrote:
> On Fri, 9 Apr 2010, Miroslav Lichvar wrote:
> >The attached patch implements a series of exponential moving averages
> >of absolute clock frequencies that are calculated over exponentially
> >increasing intervals. When chronyd is unsynchronised for some
> >specified interval, it will start switching the clock frequency to the
> >long-term averages.
>
> Interesting idea. The place that really really needs this most is the rtc.
> at present, the system saves the instantaneous rtc rate offset to use to
> correct the rtc next time it is turned on. But that rate is an "instantaneous
> rate", while typically the computer is left off for a long period.
There is a very similar problem with driftfile. Maybe we could store
in the file also the averages with timestamps and on start chrony
would choose the best value depending on for how long it wasn't
running.
Duplicating this for rtc shouldn't be very hard.
> (of course
> the bigger problem is that when the computer is off, the rtc is cold, and the
> cold rate is probably significantly different from the hot, but I have no idea
> how to get around that problem!)
Perhaps chrony on start could use temperature measured by onboard
sensor to estimate the cold drift, if it can be sure that it was
started soon after the machine was booted up and stopped just before
halt.
I have an unfinished patch for runtime temperature compensation, I'll
see how hard it would be to improve it for the cold drift compensation.
--
Miroslav Lichvar
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