Re: [chrony-users] chronyc tracking question

[Chris Perl <cperl@xxxxxxxxxxxxxx>]

On Thu, Apr 20, 2017 at 5:32 AM, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:
> Yes. That's a good description. If you would like to improve the
> manual pages, I'll gladly accept patches :).

I'll take another read through what is already there and see if I
think I can add anything useful.

> The root distance (root delay / 2 + root dispersion) is the
> uncertainty of the "true" time, which is increasing when no updates of
> the clock are made. You can graph "system time" +/- "root distance" to
> show the maximum assumed error of the clock at given time.
>
> Graphing "last offset" can be useful to show the stability of the
> synchronization and estimate the minimum error of the clock.

Great, thanks, that was very helpful.

In my setup it seems the largest potential error comes from the root
delay piece (in my case, usually something like 50us).

I believe that taking one half that value is essentially saying that
its possible that the delay is asymmetric with one direction being
instantaneous and the other taking the full amount of the measured
round trip.

That would obviously not generally be the case in a reasonable
network.  Although I guess "reasonable" is the important part of that
sentence.

But, I think what you're saying is I could watch the measured offsets
to get a "feel" for how stable things seem to be.

Maybe something like the following set of statements:

1.  If there is asymmetry, its unlikely it is constant for the entire
life of the chrony process, assuming you're running chrony for a
reasonable period and have a reasonably designed network and your time
sources are located reasonably close ("reasonable" can obviously be
different for different people).

2.  If the asymmetry is totally constant, there isn't much you can do
to detect it.

3.  If there is asymmetry, it would seem unlikely (although not
impossible) that it would change in such a way that the TX delay and
the RX delay both changed by the same amount with opposite signs
(meaning you wouldn't actually see any change to the offset
measurements).

4.  If asymmetry is introduced then its possible we could detect that
via the offsets.  Either seeing some step in the offsets or just an
increase in the variability of the offset measurements.

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