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From: frazier@oahu.cs.ucla.edu (Greg Frazier)
Newsgroups: comp.realtime
Subject: Re: Software primitives for real-time programming languages
Message-ID: <39294@shemp.CS.UCLA.EDU>
Date: 25 Sep 90 15:31:51 GMT
References: <1844@tuvie> <224@srchtec.UUCP> <1853@tuvie>
Sender: news@CS.UCLA.EDU
Organization: UCLA Computer Science Department
Lines: 56

In article <1853@tuvie> alex@vmars.tuwien.ac.at (Alexander Vrchoticky) writes:
+johnb@srchtec.UUCP (John T. Baldwin) writes:
+>To broaden the horizon of this particular discussion thread: stochastic
+>behavior of some tasks in the system can really be a pain, and cause
+>all sorts of heartache (and heartburn! :-)) for the real-time practitioner,
+>but it is also a fact of life.  Some algorithms simply don't have a *known*
+>alternative having deterministic execution time.  We *sure* don't want to
+>pad out the nondeterministic ones out to their worst case!  :) :) :)
+
+But then you have to live with the fact that your system will fail from time
+to time. I am sure that there are lots of applications where you can live with
+this, but there *are* applications where you don't want to ....
+
+`Hey, ground control, the A* algorithm in the active control system 
+ for the control surfaces is just digging through the 537th level 
+ of the search tree. What are we ... (silence)'
+
+There is a tradeoff between the power and the predictability of algorithms.
+For hard real-time systems, where a timing failure can lead to a catastrophe
+and endanger human lives I will always argue towards the deterministic
+predictable approaches.

Always arguing for deterministic approaches I think is very
limitting, and not necessary.  No system is 100% reliable.
When one is designing the flight control for an airplane, there
is no point in making the computer 100% reliable when the airplane
itself is less.  For example, if an airplane "fails" once every
10^5 hours, then there is really no reason to install a computer
that will fail less than 1/10^7 hours.  Furthermore, if you allow
yourself to use stochastic realtime, it allows you to utilize
significantly more complex hardware and algorithms, which in turn
allows you to do more with your system.  Mind you, the stochastic
approach is less than simple, and I am still searching for literature
on the subject.
>After all the heartburn of the programmer is less of a problem than the 
>burns and deaths of people in an airplane.
>
>>John T. Baldwin                      |  johnb%srchtec.uucp@mathcs.emory.edu
>>Search Technology, Inc.              | 
>>                                     | "... I had an infinite loop,
>>My opinions; not my employers'.      |  but it was only for a little while..."
>					
>				 	 So *this* is the algorithm
>					 you talk about ? ;-)
>
>--
>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'!)


--
"They thought to use and shame me but I win out by nature, because a true
freak cannot be made.  A true freak must be born."  K. Dunn, _Geek_Love_

Greg Frazier	frazier@CS.UCLA.EDU	!{ucbvax,rutgers}!ucla-cs!frazier