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

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Tue, Aug 08, 2017 at 06:40:29PM +0300, Horia Muntean wrote:
> How would one compute the estimated error of the system time from UTC
> at any point in time from information given by 'chronyc tracking' ?
> Is 'System Time' + 'Root Delay' / 2  + 'Root Dispersion' the correct formula ?

Yes, that would be a good estimate of the maximum error. When the
clock is synchronized and the system time offset is just noise which
is already included in the dispersion, I guess the first term can be
ignored.

> If yes then then all these 3 values would be helpful to be put in
> tracking.log (or maybe other log) regardless
> they are before or after an update as long as they are consistent
> (they refer to the state of time from the same moment).

The value of the "System time" offset right after the update can be
calculated by adding (or substracting?) "Offset" and "Rem. corr.". It
decreases over time at a rate which can be calculated from "Offset sd"
and corrtimeratio in chrony.conf.

> For consistency maybe they should be the values 'chronyc tracking'
> would display after the last update.

The trouble is that the dispersion is not constant. It's increasing at
a rate of MAX(skew, maxclockerr) and the formula could change in a
future version of chrony.

If the log had root dispersion right before clock update, one could
get a (slightly more pessimistic) estimate of the maximum error simply
by adding values from the log (root delay and dispersion from one
line, and offset and remaining correction from the previous line).

-- 
Miroslav Lichvar

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