Re: [chrony-users] Isolated time domains

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Wed, Dec 04, 2013 at 12:46:10PM -0800, Bill Unruh wrote:
> On Wed, 4 Dec 2013, Miroslav Lichvar wrote:
> >With the cosine smear, the frequency offset changes slowly (small
> >wander), but can reach a large absolute value.
> >
> >If I calculate right, it can corrrect an offset of 10000 seconds over
> >1e6 seconds (11.6 days) with maximum wander of 0.05 ppm/s and maximum
> >frequency offset of 15915 ppm.
> >
> >Althought I'm not sure if the cosine function is the best choice here
> >as the wander is at the maximum at the start (2nd derivative) and the
> >clients are possibly using their longest polling interval and will
> >react slowly.
> 
> I guess the above was what I was worried about. The clients take a long time
> both the poll times, and the reaction time-- the time to realise that the
> offsets imply that their own frequency has to change, which is many poll
> intervals. Ie, even at the minimum of poll 6 (a minute) it will take many
> minutes to react to a change in frequency in the client, and an attempt to
> close the offset gap. And the huge change in frequency makes (15000PPM)
> makes it even harder since chrony tends to resist large changes. Ie,
> I would expect the clients to end up all over hell's half acre in
> their
> offsets.

But the frequency doesn't change instantly by 15000 ppm, it takes half
of the 11.6 day period to reach that value and the other half to get
back to the original frequency. At the beginning it changes by 0.05
ppm per second, that's about 50 ppm in 1024 seconds.

In this example I think clients running chronyd would stay together
within 1 second during whole operation even with 1024s polling
interval.

The initial response could be improved if the wander was a sine
function or triangle instead of cosine. The frequency would start
changing slowly and the clients would have time to shorten their
polling intervals and follow closely faster frequency changes later.

-- 
Miroslav Lichvar

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