Re: [chrony-dev] [GIT] chrony/chrony.git branch, master, updated. 1.26-26-g9a01ccc

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Wed, Nov 16, 2011 at 11:06:27AM -0800, Bill Unruh wrote:
> On Wed, 16 Nov 2011, Miroslav Lichvar wrote:
> >Polling interval was fixed to 64 seconds, the number of samples is
> >around 30-40. With higher jitter or more stable clock (longer Allan
> 
> So that is about 1800 sec. which looks to be something like that period
> 
> I suppose not surprizingly, since the linear least squares fitting would eliminate
> any higher frequencies.

I think there is not much chrony can do here to eliminate higher
frequencies besides using a shorter polling interval and increasing
the maximum number of samples.

> What is interesting is that I do not see it in the other runs, which might
> suggest that there is something a bit strange in the filtering which is taking
> place. I guess you could run the simulations with the jitter zero, and then
> the frequency wander zero to see if if one of those were driving it.

With this wander/jitter ratio the jitter is the one driving it.

Here are another simulations, jitter with wander, only jitter and only
wander.

http://mlichvar.fedorapeople.org/tmp/chrony_corr4.png
http://mlichvar.fedorapeople.org/tmp/chrony_corr4_nowander.png
http://mlichvar.fedorapeople.org/tmp/chrony_corr4_nojitter.png

With no wander the buffer is mostly full - 64 samples (except the
spike in the middle where the number of samples is 8), with no jitter
the buffer has mostly fewer than 10 samples.

> What do you do, every second add a small random amount to the frequency and a
> random amount to the offset with a roughly gaussian distribution?

Yes, but only to the frequency. The offset is updated by the
difference of the clock and kernel frequency.

-- 
Miroslav Lichvar

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