[ntp:questions] NTPD can take 10 hours to achieve stability

unruh unruh at wormhole.physics.ubc.ca
Mon Apr 18 21:13:28 UTC 2011

On 2011-04-18, Harlan Stenn <stenn at ntp.org> wrote:
> Bill wrote:
>> However, do the following test. While ntpd is running, use adjtimex to
>> change the system clock rate by say 20PPM. Watch the errors and see
>> how long it takes to settle down to the original accuracy.
> I could be wrong but I think you are describing a documented "don't do
> it - this is a bad idea" case.

It is a test. It is not something to be done by anyone during normal
operation, I certainly agree. However,
temperature fluctuation do it to you with no effort on your part.
Usually not 20PPM, but 1PPM. The 20PPM is so you can see the effect
clearly and easily. 
> Please note I am freshly awake, and almost functional.  I have not dug
> thru any documents on this point.
> What are the documented operational parameters of ntp's model?  Is a
> spontaneous frequency change of at least 20ppm one of them?

One of the key features of ntpd is its stabilty under weird conditions.
So yes, a spontaneous change of 20PPM is within the parameters. Not to
be encouraged, but also not something it cannot handle. It does. fine.
Just slowly.

> If so, what is the documented "recovery" time for that?  Does ntpd meet
> that specification?
> If a spontaneous 20ppm frequency change is not within the stated
> operational parameters, why should it be?  As an aside, in reality (ie,
> the "natural" reality, not the one where humans mess with it just to see
> what happens) what sort of spontaneous frequency changes are actually
> observed?
> What happens if you *stop* ntpd, then shift the freq by 20ppm, and then
> restart ntpd?

The same thing. 

> H

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