The current SHM driver looks for a sample each polling interval.
The documentation for gpsd suggests using minpoll and maxpoll of 4.

I've fixed the SHM driver to look every second. The refclock
infrastructure already had a FIFO and code to process things
like this. This reduces the jitter significantly. (At least
in my test setup.)

I also added some clockstats printout if flag4 is set.

I'm testing with gpsd. Does anybody have a list of other
packages that feed the SHM driver?

I'm looking for people to help test this, just in case there
is something strange that I broke. If you are interested,
please contact me by email.

These are my opinions, not necessarily my employer's. I hate spam.