[ntp:questions] Allan deviation survey

Maarten Wiltink maarten at kittensandcats.net
Sat Sep 11 15:47:45 UTC 2010


"David L. Mills" <mills at udel.edu> wrote in message
news:4C8B9AB7.2050807 at udel.edu...
> David Woolley wrote:
>> David L. Mills wrote:

>>> 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.
>>
>> 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!
>
> 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.

Timer interrupts are very much affected by virtual machines. Timer
interrupts are among the things virtualised and it turns out they show
much larger jitter in a virtual than in a physical machine.

The oscillator is influenced by the ambient temperature in the casing
and this varies, indirectly and with a certain delay, with CPU load.

Running SETI at home may actually be a good idea after all.

Groetjes,
Maarten Wiltink





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