[ntp:questions] PSYCHO PC clock is advancing at 2 HR per second

Dave Hart hart at ntp.org
Tue Mar 20 18:50:09 UTC 2012

On Tue, Mar 20, 2012 at 17:28, unruh <unruh at invalid.ca> wrote:
> On 2012-03-20, Dave Hart <hart at ntp.org> wrote:
>> David Taylor is right that it is normal for Windows to keep running
>> the clock at whatever rate was last set after the program setting the
>> rate goes away.  It's also true that Windows does not have any rate
>> limits itself -- you can easily tell Windows to advance the clock 1
>> usec per tick (nominally 15.625 msec), one hour per tick.  Unless I
>> misread the nt_clockstuff.c code, it shouldn't be possible to get ntpd
>> to set the Windows clock rate more than 500 PPM from nominal.
> Presumably it could run away. It sets it at 500PPM on this cycle. The
> next reading then comes early and it again applies a 500PPM extra, etc.
> That would lead to an exponential runaway, and the hour per tick that he
> was seeing wouldn't it?

No, frequency adjustments aren't compounded like that.

Dave Hart

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