[ntp:questions] Ignore one server except in extreme conditions?

A C agcarver+ntp at acarver.net
Thu Nov 17 06:35:09 UTC 2011

On 11/16/2011 16:09, Dave Hart wrote:
> On Wed, Nov 16, 2011 at 23:09, A C<agcarver+ntp at acarver.net>  wrote:
>> Yes, if I disable the NMEA source with a "noselect" and leave the Internet
>> servers and the PPS clock (22) running, my overall system offset drops and
>> holds at a few tens of microseconds.  If I leave it in, the system offset
>> wanders around.  The magnitude of the wander appears to correlate with the
>> magnitude of the NMEA offset.  For very large NMEA offsets (sometimes
>> exceeding +/- 50ms) the system itself starts to drift away to large ms
>> offsets.  Overall I seem to get better performance without the NMEA driver
>> contributing than with it included hence the desire to make it accessible
>> only when all the other Internet sources fail (but GPS is still working).
> Which version of ntpd are you seeing this with?  Based on
> http://www.eecis.udel.edu/~mills/ntp/html/prefer.html
> I would expect the PPS to control the clock on its own, when it's
> active.  The combine algorithm is run, but the averaged offset is then
> replaced with the PPS offset which actually drives the clock.  I
> suspect you're running 4.2.6 or older and it may not be following the
> current mitigation rules.
>> Both PPS and NMEA are coming from the same physical GPS just using two
>> serial ports, one for the PPS and one for the serial data (this split is
>> required due to serial port driver limitations).
> You don't have to use two separate drivers for NMEA and PPS, even with
> the signals coming in on different serial ports.  At least in 4.2.7,
> the NMEA driver tries /dev/gpsppsX for PPSAPI before falling back to
> using the same port as for NMEA, /dev/gpsX.  In this configuration,
> you don't need to mark any peers as prefer, which I much prefer
> compared to using ATOM and marking all your network peers and NMEA
> prefer, because prefer has profound effects on mitigation which are
> tricky to wrap your head around.
> I encourage you to try a recent 4.2.7 ntpd with only the NMEA driver
> with its PPS handling enabled

I compiled the latest development version 4.2.7p234.  The NMEA driver 
does not pick up /dev/gpspps for whatever reason even if I have the 
symlink defined.  PPS is only working through the PPS driver, it does 
not function with the NMEA driver and I suspect it's related to the 
serial drivers.  I haven't marked any peers as prefer in any way 
including the NMEA source.  This behavior is seen using all the sources 
without the keyword.

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