Chuck Forsberg WA7KGX wrote:
I did a quick comparison between Lady Heather under Wine+Linux,
Lady Heather under Win7, and WWV.
The NTP time on my office machine agrees with WWV on 5 MHz
as closely as my eyes and ears can tell. Linux is running its default
NTP, Win7 is running Meinberg (as I recall).
Windows is a lousy timekeeper, and the Windows port of the NTP software
includes a number of workarounds for deficiencies in the Windows kernel.
If the offset reported by "ntpq -p" on the Windows machine is more than
just a few milliseconds then the following hints may be helpful.
The latest Windows bug which came to our attention is that some Windows
versions don't apply small time adjustments at all. For example, if NTP
applies an adjustment less than 16 ticks to the Windows time this is
simply ignored by Windows. However, NTP expects the adjustment to have
some effect, but if there is no effect then the next time comparison
yields a much larger difference than expected, and thus causes another
adjustment which is probably larger than necessary. As a summary this
can cause large swings in the time adjustment values.
Newer developer version of the NTP package contain a workaround for this
Windows bug. The report and fix are discussed here:
NTP Bug 2328 - Vista/Win7 time keeping inaccurate and erratic
https://bugs.ntp.org/show_bug.cgi?id=2328
The problem is also explained on the Microsoft support page:
SetSystemTimeAdjustment May Lose Adjustments Less than 16
http://support.microsoft.com/kb/2537623
Even though the MS report only mentions Windows 7, the Windows Server
2008 kernel is similar to Windows 7 and has probably the same bug. So if
you want to give it a try you can download a NTP developer version here
which includes a workaround:
http://support.ntp.org/people/burnicki/windows/
You should try the release version first. Just unzip the ZIP archive,
stop the NTP service, copy all extracted files over the files in your
NTP installation directory (e.g. C:\Program Files (x86)\NTP\bin\), and
restart the NTP service.
We have found that this version has greatly improved the resulting
accuracy on Windows 7 and Windows Server 2008 installations.
Please note under Windows you should configure all upstream servers with
a line reading
server aa.bb.cc.dd iburst minpoll 6 maxpoll 6
where aa.bb.cc.dd has to be replaced with the host name or IP address of
your NTP server.
Generally you should use a polling interval as short as possible under
windows to let let ntpd apply adjustments quickly.
However, please don't use polling intervals below 6 with the developer
version since this prevents the workaround from working correctly as
discussed in the bug report.
Also, higher polling intervals can cause problems under Windows. See:
NTP Bug 2341 - ntpd fails to keep up with clock drift at poll > 7
http://bugs.ntp.org/show_bug.cgi?id=2341
So our advice is to use "minpoll 6 maxpoll 6" as indicated in the
example above.
The patched ntpd has caused no drawbacks on any Windows machines, but
has improved accuracy on a number of installations.
Martin
--
Martin Burnicki
Senior Software Engineer
MEINBERG Funkuhren GmbH & Co. KG
Email: [email protected]
Phone: +49 (0)5281 9309-14
Fax: +49 (0)5281 9309-30
Lange Wand 9, 31812 Bad Pyrmont, Germany
Amtsgericht Hannover 17HRA 100322
Geschäftsführer/Managing Directors: Günter Meinberg, Werner Meinberg,
Andre Hartmann, Heiko Gerstung
Web: http://www.meinberg.de
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