Re: [chrony-users] Root delay and Root dispersion values in `chronyc ntpdata'

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Thu, Aug 03, 2017 at 11:09:09AM -0400, Chris Perl wrote:
> >> If you wanted to get a more accurate root distance on the client, you
> >> could set the delay of the SHM refclock on the server to 0 and add
> >> half of the delay to the precision instead. I think it might have a
> >> small effect on the timekeeping performance though.
> >
> > That is very helpful.  Thanks!
> 
> Are there any similar tricks to be able to get a more accurate root
> dispersion on clients?  In the setup I've been testing, chrony on the
> server generally reports a root dispersion of about 1us.  But, this is
> getting rounded up to 15us due to the aforementioned fixed-point
> format.

No, at least not with the current code. There might be a possibility
of moving the dispersion to the precision (which has an exponential
scale), which clients are supposed to include in their peer
dispersion, but I have not thought that through.

Do you need this to show compliance with the new MiFID directive?
I think one possibility here might be to reduce root delay on clients
by minimum latency of switches along the path to the server. If a
switch is guaranteed to add at least 5 microseconds in each direction,
you could substract 10 microseconds from the delay and get a more
accurate estimate of the maximum error.

-- 
Miroslav Lichvar

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