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

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

On Wed, 13 Apr 2011, Miroslav Lichvar wrote:

> On Wed, Apr 13, 2011 at 11:16:48AM -0700, Bill Unruh wrote:
> > Adding a constant to the
> > distance (assuming this did not change the min_distance) still changes the
> > weights in yours (and a lot if sd gets small) .
>
> I meant adding a constant to all distances.
>
> > If you wanted you
> > could put in sd+min_distance as the divisor. But this seems the
> > wrong thing to do anyway,
> > because really the importance of small differences in the distance should
> > always remain small.
>
> 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).

Ie, I still do not understand why you would divide by sd.

> > Also, if a server is further away, it IS less reliable. There could be a
> > constant offset-- outgoing times being a set amount more than incoming. Ie, if
> > a server has a round trip of 1 sec, and another has a roundtrip of 1ms, even
> > with the same jitter I would trust the second more.
>
> 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.

And I also do not understand why dividing by sd rather than min_distance would
help.

also, it is not clear to me why one would want to use sd, which is the
variance of the measurements from the best fit straight line, and not the
estimated variance in the slope of the curve which should also come in there.


> If there is a constant offset, no change in weight calculation will
> help. But if the paths are symmetric, we'll track both sources equally
> well, no matter how far it is.

--
William G. Unruh   |  Canadian Institute for|     Tel: +1(604)822-3273
Physics&Astronomy  |     Advanced Research  |     Fax: +1(604)822-5324
UBC, Vancouver,BC  |   Program in Cosmology |     unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1     |      and Gravity       |  www.theory.physics.ubc.ca/

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