[Info-vax] VSI: "Official 8.4-1H1 Launch"

[David Froble]

Chris Scheers wrote:
> Bob Koehler wrote:
>> In article <55734365$0$32453$c3e8da3$cc4fe22d at news.astraweb.com>, JF 
>> Mezei <jfmezei.spamnot at vaxination.ca> writes:
>>> When working on the early boot sequence/stages, I doubt that performance
>>> really matters.  And if most operating systems can run just fin on a VM,
>>> it means that the VM gives it adequate CPY time slots.
>>>
>>> Also, remember that VMS ran on the All Mighty Microvax II, where
>>> performance wasn't measure in instructions per second, but seconds per
>>> instruction :-)  Pretty sure a VM can offer better performance overall
>>> than an MV II.
>>
>>    Yes, but a VM can intererfe with hard real-time to the extent that
>>    your processor needs to be 1000 times faster.  You can probably get
>>    processors 1000 times faster than an MV II now, but you can also
>>    get VM interference tht's worse.
> 
> Well, yes and no.
> 
> Theoretically, in a VM, you actually run the native instruction set at 
> full speed.  You only have issues if you need to enter/exit the hypervisor.
> 
> Depending on how your real-time code is written, you may be able to stay 
> out of the hypervisor, especially if you have enough cores available to 
> dedicate one or more to your task.
> 
> With an emulator, every instruction may be emulated and the factor of 
> 1000 comes a lot closer.
> 
> I do hard real-time conversions of Data General systems to an emulator, 
> sometimes running under a VM, so I've seen these issues.
> 
> It seems that each real-time system is unique and quite often requires 
> special tuning, but it can be done.
> 
> Interestingly, its not uncommon for the problem to be an emulation that 
> runs too fast.
> 

Strictly curiosity.  Would that problem be rather easy to solve?  You 
could say "do this, but don't give me the result for at least this 
amount of time".  I know I'm being massively simplistic.

Not sure how much timer ASTs would interfere with real time.