Re: [chrony-dev] [GIT] chrony/chrony.git branch, master, updated. 1.25-pre1-18-g20a4340

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Wed, Apr 13, 2011 at 01:30:28PM +0100, Ed W wrote:
> On 13/04/2011 12:10, Miroslav Lichvar wrote:
> > It seems the problem is that in the weights calculation is used
> > weighted variance, which can create the positive feedback. Using
> > unweighted variance instead should fix it nicely.
> > 
> 
> I guess I meant simply sanitise the range of values.
>
> Presumably there is a numerical speed advantage in using weighted
> values? Therefore use weighting, but only up to a constrained level.
> This will lead to suboptimal convergence in some edge cases, but faster
> convergence elsewhere?

In the case I've seen all weights except the last one would end up at
that limit. The limit would have to be high enough to allow giving
very small weight to samples on a wireless network where the delays
can vary by say 3 orders of magnitude, which in the problematic case
would mean that the regression line would go through the last sample.

Anyway, using the unweighted variance fixes the problem, but it
changes the clknetsim results. I'll see if I can get close to the
previous behavior by tweaking the SD_TO_DIST_RATIO constant.

-- 
Miroslav Lichvar

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