[chrony-dev] Improvements with asymmetric jitter

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

I've just pushed to git an implementation of the correction for
asymmetric jitter that was discussed here couple weeks ago. It should
improve accuracy and stability on asymmetric links like ADSL or cable.
Currently, the setting is fairly conservative. The correction is
activated only when the estimated asymmetry is larger than 0.45 for at
least 10 updates, so it should't have a negative effect on normal
symmetric network links.

It my tests it worked nicely. In one instance the std dev reported in
sourcestats improved by a factor of 10. Surprisingly, I saw a
significant improvement with some servers in local network too, e.g.
stddev going down from ~10 us to ~2 us.

Please test and report any problems. The statistics log has a new
column for the asymmetry, so you can see when the correction is
active. I'd like to extend the chronyc sourcestats output too, but I'm
not sure how to do that yet as I'd rather avoid breaking compatibility
and fit the output in the standard terminal width of 80 columns.

There is definitely a room for further improvements. One problem is
accuracy of the estimated minimum delay. There is a possibility to
keep a long-term minimum delay (e.g. day or so), but I'm worried about
computers moving between different networks, changes in network
routing, etc. There is also an interesting interaction with the
maxdelaydevratio filter. As the correction for asymmetry reduces the
variance, samples with larger delay that would otherwise pass the test
are dropped. I've tried to modify the filter to keep samples that fit
well with the estimated asymmetry, but this made the estimate of the
minimum delay less stable as the low-delay samples were pushed out and
the accuracy of the clock was actually worse. So it seems the first
problem needs to be solved first.

-- 
Miroslav Lichvar

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