[ntp:questions] Allan deviation survey

David L. Mills mills at udel.edu
Mon Sep 13 05:02:36 UTC 2010


Bill,

Please reread the definition of Allan deviation. It is a measure of 
frequency differences, not time errors. I principle, it could be applied 
to a virtual machine with virtual timer interrupts, but nobody familiar 
with the principles would do that and it would serve no useful purpose. 
The phase noise would be huge due to processor scheduling, etc. This 
would push the Allan intercept to very large values. The NTP clock 
discipline does that automatically. So, it makes no sense at all to use 
Allan analysis in such cases.

I'm getting really tired of this discussion, and it servers no useful 
purpose. And, by the way, mail sent to your alleged mail address is 
returned to sender as undeliverable.

Dave

unruh wrote:

>On 2010-09-11, David L. Mills <mills at udel.edu> wrote:
>  
>
>>David,
>>
>>With due respect, your comment has nothing to do with the issue. Allan 
>>deviation is between a quartz crystal oscillator,  timer interrupt, 
>>interpolation mechanism and a kerel syscall to read. the clock. It has 
>>nothing whatsoever to do with virtual machines.
>>    
>>
>
>?? Allan deviation is a measure of the error of a clock as a function of
>lag. It does NOT specify the error soruce. It is not simply defined for
>only certain machines used in certain ways. Now it may be simple for
>some systems (like your lightly loaded systems) but that is largely
>irrelevant.
>The purpose of the doing things like measureing Alan deviation is to
>understand the noise sources affecting a clock. If those happen to be
>diurnal temperature variations, then that is what needs to be handeled.
>If it is Virtual Machines and their clock reading then that is what you
>need to look at. 
>
>Errors are errors and understanding them is crucial to designing a
>decent error mitigation procedure. Closing one's eyes to a dominant
>error source will ssimply mean that the error mitigation procedure will
>suck.
>
>  
>
>>Dave
>>
>>David Woolley wrote:
>>
>>    
>>
>>>David L. Mills wrote:
>>>
>>>      
>>>
>>>>Bill,
>>>>
>>>>Running a precision time server on a busy public machine with a 
>>>>widely varying load is not a good idea and I have no interest in 
>>>>that. Running 
>>>>        
>>>>
>>>As indicated by the sort of questions the group is getting recently, 
>>>it is becoming the norm to run time servers on virtual machines, 
>>>because that is how businesses now run all their servers.  The whole 
>>>point of virtual machines is that the host is busy and running a 
>>>varied load!
>>>
>>>_______________________________________________
>>>questions mailing list
>>>questions at lists.ntp.org
>>>http://lists.ntp.org/listinfo/questions
>>>      
>>>
>
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>  
>




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