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

[Horia Muntean <horia.muntean@xxxxxxxxx>]

On Wed, Aug 9, 2017 at 12:38 PM, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:
> 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

The root dispersion just after a clock update contains what the server
reports right ?
In my opinion the values (root delay end dispersion) taken just after
the clock update
along with a short update interval (less than few seconds) would give
a good max. error estimate.
In a short amount of time between the updates the root dispersion
should not increase too much.
And for a good synchronization I don't think one can afford large
update intervals.

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