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

David Lord snews at lordynet.org
Mon Jun 17 16:12:25 UTC 2013

David Taylor wrote:
> What should the poll be for the shared memory driver (type 28)?  This 
> Web page:
>   http://www.eecis.udel.edu/~mills/ntp/html/drivers/driver28.html
> suggests that the driver internally has a poll interval of 1 second, 
> another document I've seen suggests setting:
>   server minpoll 1 prefer
> and yet in the ntpq -p billboard the poll is shown as 8 seconds. 
> Loopstats are reported at 8 second intervals, and the poll interval is 
> shown as 3 (i.e. 8 seconds).
> I'm confused!  What should be the correct setting in the ntp.conf file?

I've used type-28 + radioclkd2 for many years with MSF and for a
while with DCF. I've never set minpoll/maxpoll but it has always
defaulted to 6, ie 64 sec. DCF has a phase shift second marker
but is too distant for that to useful. When DCF reception was
good the timekeeping was better than from msf (+/- 0.1 ms vs 1ms).

As you mentioned above, type 28 refclock pages indicate the driver
is polled by ntp at 1 second intervals (as from ntpd-4.2.5p138).


> BTW: this comes about from Folkert van Heusden who is using a user-mode 
> program to make NTP on a Raspberry Pi more precise, i.e. providing PPS 
> support without having to modify the kernel.  See:
>   http://www.febo.com/pipermail/time-nuts/2013-June/077484.html

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