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

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Thu, Aug 10, 2017 at 05:47:26PM +0300, Horia Muntean wrote:
> On Thu, Aug 10, 2017 at 4:49 PM, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:
> > I think what people are interested in is the maximum possible error of
> > the clock at any time, not just the points when the clock was updated.
> 
> I agree. Maybe a whole different log with this info would be more appropriate
> or one should should just put 'chronyc -n tracking' in a loop and
> compute system_time + root_delay / 2 + root_dispersion ?

I think the log should provide all necessary information to avoid
having to run chronyc in a loop.

> > If we know at time t the error was 10 +/- 50 microseconds and at time
> > t+100 (seconds) it was -20 +/- 70 microseconds, what can we say about
> > t+1, t+50 and t+99, if we assume the frequency of clock is accurate to
> > 1 ppm?
> 
> In 100 seconds you would have a maximum drift of 100 ppm = 100 us
> and so in 100 seconds your error could grow to 60 + 100 = 160 us.
> But because at the second measurement you have only a 90 us error it means
> that during 100 seconds period the error was not higher then 90 us ?

I'm not sure if that is safe to assume. I think the error could go up
and then down in the interval. The question is what should be reported
in the log in order to give an estimate of maximum error.

- root dispersion right after the update
- root dispersion right before the update
- a combination of the two

-- 
Miroslav Lichvar

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