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

Dave Hart davehart at gmail.com
Sat Dec 12 06:32:07 UTC 2009

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

Dave Hart

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