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From: alex@vmars.tuwien.ac.at (Alexander Vrchoticky)
Newsgroups: comp.realtime
Subject: Re: Software primitives for real-time programming languages
Message-ID: <1889@tuvie>
Date: 27 Sep 90 10:30:35 GMT
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johnb@srchtec.UUCP (John Baldwin) writes:

>When you refer to the "system" meeting its timing constraints, are you
>talking about the system as a whole, or a particular task (subroutine, or
>what-have-you) completing execution in <= 'n' units time?

I'm talking about the set of all tasks that have hard deadlines specified. 


>I thought that one of the "charter" items of hard real-time was, in fact,
>to be able to ensure that *when* your system begins to fail, it does so
>in a rational, and predictable way, incrementally.

Hmmm ... `graceful degradation' is a matter that is not necessarily
directly related to the criticality of the system. There are hard
real-time application where there is an `emergency exit' into a safe
system state.
Consider a railway safety system. If the system is failing (for whatever
reason) the reasonable course of action is to switch all the signals to 
`red' and that's it. In this particular case this is to be 
preferred over `graceful degradation' where you might miss deadlines 
and have trains crash.

Of course this is not an option in aviation: `hold it right there ... :-)' 
The point is that there are systems in which missed deadlines are permissible
and systems in which they are not; and there are systems which have a safe
exit and systems which don't. These two concepts are largely orthogonal
and should be treated as such.

>As I have stated before, the project I'm involved in *involves* aviation.
>It also involves AI, which (inherently) means that the algorithms used
>will tend towards more power and less predictability.  To get rid of
>these concerns is to get rid of the project.  Instead of getting rid
>of it, I want to get it on a sound RT footing.

Best of luck. I really don't know much about AI systems, but I would 
be interested in your approaches into the power/predictability tradeoff. 

>BTW, the system doesn't directly drive the control surfaces.   :-}
>(I thought you'd like to be able to sleep at night.)

I sleep perfectly at night ... maybe not so in airplanes :-)

--
Alexander Vrchoticky  Technical University Vienna, Dept. for Real-Time Systems
Voice:  +43/222/58801-8168   Fax: +43/222/569149
e-mail: alex@vmars.tuwien.ac.at or vmars!alex@relay.eu.net  (Don't use 'r'!)