[ntp:questions] drift value very large and very unstable

Unruh unruh-spam at physics.ubc.ca
Mon Mar 10 16:30:24 UTC 2008


andy.helten at dot21rts.com (Andy Helten) writes:


>Fran Horan wrote:
>> <snip lots of detail>
>>
>>   
>>> So, the summary is that drift goes to 500ppm when stepping is disabled
>>> but runs normally when stepping is enabled and both situations never
>>> require a time step.  This makes no sense to me.  By the way, as
>>> mentioned previously, we require that time does not step backward due to
>>> a problem in some commercial software that cannot currently tolerate
>>> time moving backwards.
>>>
>>> Quite frankly, I don't think it's unreasonable that a system require
>>> time to monotonically increase.
>>>     
>>
>> Forgive me if this answer misses a point in the earlier details, or shows my
>> ignorance of NTP, but a few ideas/thoughts.
>>
>> Oscillators and drift can go in either direction, fast or slow, its a
>> physics-based situation. You can't write code around that and provide a
>> software solution that is monotonic at all times. However, a single negative
>> step just at the start may be required before going monotic after that
>> event. (Not an expert, but that is my understanding).

drift is the difference between the actual rate of the oscillator and 1.
You need a drift rate of -1000000PPM for it to no longer be monotonic. 
If you have such a drift rate plug the stupid machine in!


>>
>> With this ref clock and a GPS-drive IRIG source, you may only see a single
>> negative step when NTP first begins running on a new system with no drift
>> file, or a system that has been powered off a long time with a
>> battery-driven clock drifting over that long time. Once NTP is humming along
>> after the initial step and some updates, you shouldn't see a step again.
>> This makes me think that you should insert a delay in launching your
>> sensitive application, or block the application at some point, so it does
>> not see the (possible) first time step.
>>
>> Fran Horan
>> JHU/APL
>>
>>   
>Hey Fran,

>Yes, exactly, we do perform an initial time sync with stepping enabled. 
>This is done prior to initializing the commercial software and so it
>does not cause problems if time moves backwards.  And, yes, if we are

Time should never move backward. "Steps" should be fast drifts. (10000PPM
if necessary but not 1000000PPM.




>below the step threshold after the initial sync (which should always be
>the case), then we should stay below that threshold until the end of
>time.  Following this logic, we should allow time steps and be comforted
>knowing they will never occur in a normally functioning system.  I agree
>this is reasonable and does not conflict with my own rant that "if we
>have an offset of more than 10ms in this system, then something isn't
>working correctly".

>This approach is definitely worth considering and I'll bring it up with
>the decision makers.  However, there is always concern that months or
>years from now someone will say -- "Hey, some dumbass left time stepping
>enabled, let's disable it on all systems immediately".  Surely this
>wouldn't be done without some regression testing, but then again such a
>mundane change shouldn't need exhaustive testing, right?  Riiiiight.

>I guess was just hoping someone will say, "Oh, right, that's a known
>problem.  You need to do 'X' to fix it."

>Andy




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