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- To: chrony-dev@xxxxxxxxxxxxxxxxxxxx
- Subject: Re: [chrony-dev] [PATCH] MacOS X - add drift removal
- From: Bryan Christianson <bryan@xxxxxxxxxxxxx>
- Date: Tue, 4 Aug 2015 04:52:22 +1200
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> On 4/08/2015, at 3:52 am, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:
>
> On Thu, Jul 30, 2015 at 09:50:26AM +1200, Bryan Christianson wrote:
>> The darwin kernel implementation of adjtime() does not require the adjustment to
>> be aligned to a tickadj boundary, and we can apply adjustments to the nearest microsecond.
>> Rounding is accounted for by adding any rounding errors back into the offset.
>
> Applied, thanks. What offset do you now see? I guess it doesn't solve
> the problem you were trying to fix in your previous patch.
New graph at http://whatroute.net/chrony/
It's an order of magnitude better than chrony-2.1.1 and is looking pretty good. I have maxpoll, minpoll both set to 4
My machine had just been rebooted just before I took the screenshot, and you can see the initial offset of about 1 second rapidly converge to the order of -10 usec
Graph legend
green: Offset sd
purple: frequency (ppm)
red: rms offset
blue: offset
brown: System time
orange: Weighted mean System Time (weighting 0.1)
>
> I was wondering if the error of the clock could be minimized by
> including in the requested adjustment a prediction of the offset in
> the middle of the update interval so the mean value is close to zero
> if the drift doesn't change much. E.g. with a drift of 10 ppm and
> current offset of 20 microseconds, request 25 microsecond adjustment
> instead of 20, so the clock overshoots a bit and reaches zero offset
> in about 0.5 seconds.
>
>
Yes, I think something like that could work, although I think some low pass filtering would be required in calculating the overshoot. It would be making the system act more like a PID controller and I think it should be approached on that basis.
>
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