[ntp:questions] Min & max poll no longer needed for SHM/GPSD driver?

Miroslav Lichvar mlichvar at redhat.com
Fri Sep 12 10:11:42 UTC 2014


On Thu, Sep 11, 2014 at 07:37:10PM +0100, David Taylor wrote:
> It has been pointed out to me that this page:
> 
>   http://www.eecis.udel.edu/~mills/ntp/html/drivers/driver28.html
> 
> says: "The gpsd man page suggests setting minpoll and maxpoll to 4. That was
> an attempt to reduce jitter. The SHM driver was fixed (ntp-4.2.5p138) to
> collect data each second rather than once per polling interval so that
> suggestion is no longer reasonable"
> 
> So what should minpoll and maxpoll be set to for the GPSD shared memory
> driver?  Or should they be omitted?  I'm confused....

Hm, that paragraph doesn't make much sense to me either as the default
refclock poll is 6.

When collecting samples each second and processing them in the median
filter, the output has a lower jitter, so it's better to use a shorter
poll if the goal is to get the best accuracy, not longer.

On Linux when the GPS PPS signal has 1us jitter, poll 3 or 4 usually
works best for me.

FWIW, in clknetsim simulations with 1us jitter and 1ppb/s wander I get
these results:

poll	RMS time error (s)	RMS freq error (1)
3	6.0e-07			1.7e-08
4	1.5e-06			9.5e-09
5	4.3e-06			1.0e-08
6	1.1e-05			1.3e-08

With 10us jitter:

poll	RMS time error (s)	RMS freq error (1)
3	2.4e-06			1.6e-07
4	2.2e-06			6.7e-08
5	4.5e-06			2.7e-08
6	1.3e-05			1.7e-08

On other systems (using the standard PLL time constant shift) the best
poll would be even shorter.

-- 
Miroslav Lichvar


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