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

unruh unruh at invalid.ca
Tue Mar 20 20:24:10 UTC 2012

On 2012-03-20, Dave Hart <hart at ntp.org> wrote:
> 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.

Lets hope not, but on his system, something DID trigger a runaway. So
while frequency adjustments shouldn't be compounded like that, perhaps
they were due to a bug in the ntpd software. 

> Cheers,
> Dave Hart

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