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 02:28:40PM -0700, Bill Unruh wrote:
> On Wed, 13 Apr 2011, Miroslav Lichvar wrote:
> >Why? A significant part of the distance is static delay corresponding
> >to the length of the path which the signal has travel through. I just
> >wanted to remove that from the weight calculation.
> 
> 
> But subtracting off the min_distance surely removes that already (the
> min_distance is calculated from the string of samples for that particular
> source).

I don't think it does.

Assume we have two sources with the same jitter, but different static
delay. Four samples for each source. Let's calculate the weights in
both ways with the same SD_TO_DIST_RATIO 1.0.

source1:
peer distances (ms):    11.0    13.0    12.0    14.0
min dist (ms):          11.0
last sd (ms):           3.0
weights1:               1.000   1.397   1.190   1.620
weights2:               1.000   2.778   1.778   4.000

source2:
peer distances (ms):    1001.0  1003.0  1002.0  1004.0
min dist (ms):          1001.0
last sd (ms):           2.0
weights1:               1.000   1.004   1.002   1.006
weights2:               1.000   2.778   1.778   4.000

weights2 are same for both sources, but weights1 are very different.

> >The selection algorithm takes care of that.
> 
> I was a bit confused. The weights are calculated separately for each source
> as far as I understand it. So the problem becomes what happens if that source
> suddenly has a large delay. Do you trust it? If all of the items from the
> source suffer the same delay, then subtracting off min_distance takes care of
> that. Surely you would trust a measurement whose delay suddenly became large
> less.

I think we still do that even by dividing by sd.

-- 
Miroslav Lichvar

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