[ntp:questions] Re: Windows timekeeping - sudden degradation - why?

David J Taylor david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk.invalid
Wed Dec 7 18:55:25 UTC 2005

Martin Burnicki wrote:
> This points to the problems with Windows time if a multimedia
> application is started. I've mentioned this sometimes before here in
> the newsgroup, e.g. here:
> http://lists.ntp.isc.org/pipermail/questions/2005-September/006709.html
> This problem is NOT from the MM apps themselves but from the way
> Windows is keeping time, and AFAIK has been fixed by MS in SP2 for
> WinXP.
> So you won't fix the problem by using another version of ntpd. Either
> you must make sure no MM app runs at all (maybe even not in the
> background, if QT looks if updates are available), or you must have
> at least one MM app running continuously in order to have the MM
> timer set constantly set either to highest resolution or default
> resolution, but not switching between the 2 resolutions.
> If the MM timer is set constantly to high resolution then you should
> observe a little more jitter than with default resolution, but the
> large steps should go away. So as a test you just might to start
> Quicktime ant watch if the offset settles down.
> Martin

I've had a QuickTime video playing but paused now for the last two hours 
or so,
It's a bit early to say yet, but it looks as if a number of things have 

- the drift value is changing from -65 downwards, currently -59.385

- the offset has shot up to 120ms, but seems to be stable at that value (I 
guess if it goes up much more a step will happen at 128ms).

- the jitter figures seem much better, around 2 - 3ms.  Of course, that's 
with a high offset value so not strictly comparable.

However, it does perhaps suggest that:

- the poor performance before was from the time switching into and out of 
MM mode.

- having a permanently "on" MM mode may help this system, and may be a 
useful feature for NTP on some Windows systems.

I still have no idea what piece of software started enabling the MM-timer 
mode last Friday, though!  I should keep better records.  I'll report 
again tomorrow.


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