[Info-vax] Real time performance, was: Re: VSI: "Official 8.4-1H1 Launch"

[Simon Clubley]

On 2015-06-07, JF Mezei <jfmezei.spamnot at vaxination.ca> wrote:
> On 15-06-07 08:09, Simon Clubley wrote:
>
>> You have made this mistake before JF, so once again:
>> 
>> Real time does not mean fast; it means a guaranteed bounded response time.
>
>
> The context of this was making primitive VMS boot as a guest of a VM to
> help VSI engineers get it to boot to the DIR command.
>

That was indeed what you were interested in. I was interested in the
implications of a VM on real time applications which is why I talked
about real time applications and didn't talk about the startup code.
Looking back at my posting I believe I made that very clear.

Trying to run real time stuff under a VM isn't something I have any
experience with; I believe that when you are doing real time applications
you need to be as close to the metal as possible. (IE: with as few layers
of software as possible between your application and the hardware.)

>
> In fact, when you debug primitive code, you probably want it to run very
> slowly and (maybe) have the ability to stop it midway so debugging tools
> can be used to look at memory etc,
>

I don't know if you have ever done any embedded programming JF but
you want the code to run at normal speed when possible. What you
can do (given the appropriate remote debugger and target support) is
to single step a section of code and set a breakpoint at certain
locations.

Of course, it's equally possible VSI are planning to use a low level
debugger running on the target machine itself instead of using remote
debugging capabilities while debugging the initial startup code.

BTW, if you have to debug code without any of these capabilities and
if you have GPIO lines available on the target, a LED makes a great
debugging tool in some circumstances, especially in an interrupt
handler. :-)

Simon.

-- 
Simon Clubley, clubley at remove_me.eisner.decus.org-Earth.UFP
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