[ntp:questions] Building FreeBSD V8.0 kernel for PPS

David J Taylor david-taylor at blueyonder.co.uk.invalid
Wed Apr 14 16:08:10 UTC 2010

> I am still confused as to why people recompile their kernel to put in
> the kernel PPS. Just use the regular ntp devices like shm and a program
> like gpsd to run pps from userland. I have no idea what advantages the
> kernel PPS provides, certainly considering the pain of recompiling the
> kernel. In fact I would argue that userland is the place where you
> should be doing the PPS shaping-- throwing out popcorn spikes, making
> sure that the PPS is producing sensible output (eg each pulse coming one
> second after the previous on, not 3ms, or something like that, which can
> happen).
> Ie, you do not need kernel pps to make use of pps.

I think you are correct in your final statement, but my understanding 
(albeit limited) is that kernel PPS provides better performance, and is 
used for a very limited set of operations (possibly just timestamping the 
PPS signal).  All the filtering /is/ done in  user-land.  It isn't a pain 
to do, once you have found the appropriate two or three command-lines for 
your system.


More information about the questions mailing list