Re: [chrony-dev] [GIT] chrony/chrony.git branch, master, updated. 1.26-26-g9a01ccc

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

On Wed, 16 Nov 2011, Miroslav Lichvar wrote:

> On Tue, Nov 15, 2011 at 02:19:16PM -0800, Bill Unruh wrote:
> > One of the issues with ntpd that started me being interested in this whole
> > timing stuff, and chrony was the presence of what seemed like oscillations in
> > the data. In this latest  one, I again see oscillations, with about a 3000s
> > period. Do you have any idea what those could be?
>
> No idea. It looks random to me. I think it should be just output of a
> high-pass filter of changes in the clock frequency.

Low pass filter? but the frequency looks too regular. I suspect a fourier
transform would reveal a spike in the data. Of course I do not know if it is
just an accident of the particular realisation of the random process.


> > How long is the typical time scale over which chrony keeps the data to fit the
> > linear regression to? What is the polling period?
>
> Polling interval was fixed to 64 seconds, the number of samples is
> around 30-40. With higher jitter or more stable clock (longer Allan

So that is about 1800 sec. which looks to be something like that period

I suppose not surprizingly, since the linear least squares fitting would eliminate
any higher frequencies.

What is interesting is that I do not see it in the other runs, which might
suggest that there is something a bit strange in the filtering which is taking
place. I guess you could run the simulations with the jitter zero, and then
the frequency wander zero to see if if one of those were driving it.

What do you do, every second add a small random amount to the frequency and a
random amount to the offset with a roughly gaussian distribution?


> intercept) the number of samples would be higher.

--
William G. Unruh   |  Canadian Institute for|     Tel: +1(604)822-3273
Physics&Astronomy  |     Advanced Research  |     Fax: +1(604)822-5324
UBC, Vancouver,BC  |   Program in Cosmology |     unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1     |      and Gravity       |  www.theory.physics.ubc.ca/

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