[ntp:questions] Packet timestamps when using Windows-7/Vista

David J Taylor david-taylor at blueyonder.delete-this-bit.and-this-part.co.uk.invalid
Sat Dec 12 08:31:23 UTC 2009

"Dave Hart" <davehart at gmail.com> wrote in message 
news:98c443b7-15b4-4472-b985-9c3e99788835 at j9g2000prh.googlegroups.com...
> On Dec 11, 6:31 pm, "David J Taylor" wrote:
>> I note that the line:  "using Windows clock directly"  appears in 
>> Gemini
>> and not Stamsund, and that "HZ 64.000 using 43 msec timer 23.256 Hz 64
>> deep" appears in Stamsund and not Gemini.  Stamsund also has
>> "NTP_USER_INTERP_DANGEROUS=1" which must have been a hangover from our
>> earlier experiments.
> So it seems.  You may be the only person to use that environment
> variable (though I'm pretty sure it's not spelled quite right there).
>> Perhaps this means I'm running Stamsund in a non-standard mode, without
>> having remembered I was, and what is the significance that it appears 
>> to
>> work well as a reference server (although nothing like as well as on
>> Windows XP)?
> It's actually very interesting to me, and I'm glad you reminded me of
> it.  It raises the question why is it interpolation is not horribly
> broken on this system with a 1ms resolution system clock, given that
> we know the scheduler resolution on all the known Windows versions is
> 1ms?  I thought the problem that broke interpolation on Win7 and Vista
> systems with the system clock precision driven to 0.5 or 1ms was
> caused by the sampling of clock and counter pairs occurring in phase
> with the clock updates, because the interpolation scheme wants it
> samples well-distributed so there is always at least one sample in the
> last second or two that happened to be taken soon after the clock
> ticked to a new value.
> The fact that is working despite the 1ms system clock means I don't
> understand the breakage as well as I thought, and hints of a
> possibility interpolation could be made to work on more or all Vista/7
> systems.
> Cheers,
> Dave Hart


It isn't spelt correctly - I was walking from one system to another and 
remembering the string in my head (a mistake!).  It's actually:


on a second check (give or take more walking errors).

The major difference between the two systems is that one has a ref-clock 
attached and the other doesn't.  Plus, as you noted, HAL and hardware 

I can test any 4.2.7 you want to pass me on both the LAN-synced Vista PC 
(which has these apparent "server received after transmitted timestamps") 
and the GPS-PPS-kernel-serial Windows-7 system, although I would prefer 
not to have to reboot if at all possible.  I could easily remove the 
ref-clock from the Windows-7 system (Stamsund), and the lead might then 
stretch to the Vista system (Gemini).


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