[ntp:questions] NTP Bug 2328 - Vista/Win7 time keeping inaccurate and erratic

David Taylor david-taylor at blueyonder.co.uk.invalid
Wed Nov 27 13:20:31 UTC 2013

On 26/11/2013 16:20, Martin Burnicki wrote:
> I know all this is pretty tricky.
> The workaround submitted in bug 2328 is to yield more accuracy under
> Windows Vista and newer only, and ntpd checks at runtime if it is
> running under a Windows version requiring the workaround, or not.
> For example, if running under Win XP the workaround is not enabled
> anyway since the underlying Windows problem doesn't exist, and thus the
> time discipline works in the same way as without the workaround.
> I've seen installations with Windows 7 where the Windows problem didn't
> seem to happen, and even ntpd 4.2.6p5 (without the workaround) was able
> to discipline the Windows time as expected.
> On the other hand, in all installations of Win Vista and newer where the
> time adjustment loop didn't settle properly the workaround has
> significantly increased the resulting accuracy.
> I've put an example graph here:
> http://support.ntp.org/people/burnicki/windows/bug2328_workaround.pdf
> This has been recorded on a Win 7 system, where the system time is
> disciplined by NTP from a single upstream server running Linux. The time
> quality has been measured against a GPS PCI card built into the windows
> machine.
> In the first part of the graph ntpd 4.2.6p5 has been running. The tick
> adjustment values only change in small steps, but the disciplined
> Windows time has large offset and jitter.
> Then the ntpd binary has been replaced by a version which includes the
> workaround. As can be seen, adjustments are now applied in larger steps,
> but the disciplined system time is very much smoother than before.
> The guy who has submitted the code for the workaround said his changes
> don't work as expected if the polling interval is smaller than 6. Since
> everything else works like he described I also believe this is true, but
> I haven't tested smaller poll intervals with the patch.
> Martin

Thanks, Martin, that's most interesting.  I can get nothing like that 
performance from my Win-7 systems, although they are all connected via 
Wi-Fi which doesn't help.  I've just changed the order in my MRTG 
presentation to show XP, versus  Vista, Win-7, and Win-8.  As you'll 
see, there's nothing like the smooth graph you got from Win-7, although 
these are offsets reported by NTP rather than measurements against a PCI 
reference like yours. However, the offset scale is different, as I have 
a range of +/- 3 milliseconds, and your is +/- 30 milliseconds, so 
perhaps your noise would be the same as mine once the graph was scaled.

Maybe my Win-7 systems are the unaffected ones?  Did the one you tested 
have any NTP environment variables set, to enable/disable interpolation 
etc. etc.?

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