[ntp:questions] What should the poll be for the shared memory driver (type 28)?

David Taylor david-taylor at blueyonder.co.uk.invalid
Tue Jun 18 09:07:47 UTC 2013

On 18/06/2013 08:41, Miroslav Lichvar wrote:
> IIRC, the one second interval is used only to collect the SHM samples
> and store them in a buffer. In the minpoll/maxpoll interval the
> collected samples are processed in a median filter and one final
> sample is used to update the clock. This improves the jitter.

Yes, that makes sense.

>> I'll try setting it to 4, as a test, and see whether anything changes.
> The difference with NTP sources is that changing the poll interval
> affects also the jitter as there will be a different number of samples
> in the filter.

I've put a couple of graphs here:


which show a drop of RMS offset from around 5 microseconds to 4 
microseconds and a drop in averaged jitter from just below 7 to just 
above 5 microseconds, when the minpoll was changed from 3 to 4.
Web: http://www.satsignal.eu

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