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

[Simon Clubley]

On 2015-06-07, David Froble <davef at tsoft-inc.com> wrote:
> JF Mezei wrote:
>>>> I wonder what kind of jitter/latency this will cause for real time
>>>> applications ?
>> 
>> 
>> 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.
>> 

You have made this mistake before JF, so once again:

Real time does not mean fast; it means a guaranteed bounded response time.

In soft real time systems you can miss some of the timing constraints
and still have your system function. In hard real time systems you cannot
miss any timing constraints.

The number of times you can miss your timing constraints in a soft real
time system is application dependant.

Also, I was talking about real time performance during normal operation
not during booting.

>> 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.
>> 

Totally irrelevant. :-)

A few years ago, (before the free version was locked down), I did some
experiments with QNX on the same hardware as a Linux installation.

QNX was slower than Linux at doing some things but that didn't change
the fact that the QNX installation was a real time system whereas the
Linux installation was not. This is because QNX offers guarantees that
Linux does not.

>
> Sticking my big fat mouth into a discussion that I know nothing about ..
>

You know more about this stuff than JF apparently does. :-)

> There is performance, and, there is response time.  They re not always 
> the same thing.  "Overall" doesn't really matter, if the specific you 
> need doesn't perform well enough.

Exactly.

Simon.

-- 
Simon Clubley, clubley at remove_me.eisner.decus.org-Earth.UFP
Microsoft: Bringing you 1980s technology to a 21st century world