[ntp:questions] Allan deviation survey

unruh unruh at wormhole.physics.ubc.ca
Mon Sep 13 02:34:46 UTC 2010

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
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

> 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!
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