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

Miroslav Lichvar mlichvar at redhat.com
Tue Jun 18 07:41:17 UTC 2013

On Mon, Jun 17, 2013 at 07:02:09PM +0100, David Taylor wrote:
> Thanks, Steve.  My knowledge of the source tree is even more limited
> than my knowledge of "C"!  In refclock_shm.c, it does say that the
> "peek" routine is called every second, so if the type 28 driver has
> an internal poll of one second, does it matter what min/max poll is
> set in the ntp.conf file?  Does it even need to be set at all?

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.

> 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.

Miroslav Lichvar

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