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

[Horia Muntean <horia.muntean@xxxxxxxxx>]

On Thu, Aug 10, 2017 at 4:49 PM, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:
> On Thu, Aug 10, 2017 at 01:22:25PM +0300, Horia Muntean wrote:
>> The root dispersion just after a clock update contains what the server
>> reports right ?
>
> It includes dispersion that accumulated since the best measurement was
> made (it doesn't have to be the last one) and also peer dispersion of
> the measurement.
>
>> 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.
>
> If the network is heavily loaded, chronyd may drop a large number of
> consecutive measurements.
>
> 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 ?

> 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 ?
Or that was a rhetorical question ?

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