Joe Gwinn schrieb:
In article <84lv9b-i5v....@ubuntu-server-1.py.meinberg.de>, Martin
Burnicki <martin.burni...@meinberg.de> wrote:
Nick wrote:
Brian
thanks for your useful reply...
On Fri, 18 Jul 2014 02:03:48 +0000, Brian Inglis wrote:
Windows is using the MM timer as its high precision time source, not PM
timer, HPET or TSC.
Huh, I didn't know (and I doubt) that NTP uses the MM timer as time source.
Years ago there was a problem with NTP under Windows that the
interpolated system time used internally by ntpd got messed up when some
*other* program started to set the Windows MM timer to highest
resolution, e.g to play some multimedia stuff.
As a workaround I submitted a patch to ntpd which lets the NTP service
itself set the MM timer resolution high when it starts, so there are no
more other effects on the interpolated time when some other application
also does so.
The workaround comes in effect if the -M parameter is given on the
command line, and this is the default case for installations from the
Meinberg setup program.
Back in the day, video (and later audio) drivers were the culprit in
messing realtime up as well. The drivers performed a hardware bus lock
of some kind during transfers. This in addition to setting a very high
priority, and trumped anyone else setting high priority. I never knew
the exact details, but it was one of many reasons why one does not use
Windows for realtime, unless one's idea of realtime is *very* relaxed.
Although Windows has improved over the years, its latency still isn't
predictable enough for realtime of any stringency.
Reliability is also an issue - the rule is that if one cannot tolerate
a forced reboot every few months or so, don't use Windows. I know that
many Windows systems don't fall over nearly that often, but some do,
often for no known reason, and that's enough. Only worst-case behavior
matters.
A typical architecture in my world is to use Linux or a RTOS like
VxWorks for the RT core, talking via UDP exchange over ethernet to a
Windows box housing the GUI. Humans don't notice the timing burps, and
can reboot the GUI box as needed.
Joe Gwinn
I absolutely agree.
Martin
--
Martin Burnicki
Meinberg Funkuhren
Bad Pyrmont
Germany
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