Re: What is "real-time"?

[sdd.hp.com!hp-col!tbc@hplabs.hpl.hp.com (Tim Chambers)]

rlk@VisiCom.COM (Bob Kitzberger) writes:
> Can anyone from comp.realtime post a pithy summary definition of 
> comp.realtime's "conclusions" on what is real-time?

Hi, Bob,

First I'll explain the approach which is used in our organization.  Rather
than define "real-time" software, we break it into two categories:
"time-sensitive" and "time-critical."  Time-sensitive software is that for
which there is some kind of continuous function (typically linear) for how
"well" the software does its job based on how fast it runs.  A compiler is an
ideal example.  The faster it runs, the "better" it is -- everyone *wants*
faster compile times.

Time-critical software uses a step function to define its quality -- it either
runs fast enough to do the job or it doesn't do the job.  If the software
can't keep up with external stimulus it *ceases to function*.  And running
faster doesn't necessarily improve the quality of time-critical software,
either (what quality improvement could a 68040 add to a Hayes-compatible
1200-baud modem, anyway, other than allowing the modem to run UNIX at the same
time :-).

To contrast the two, consider the software in a GUI.  The software that moves
the sprite in response to mouse movements is more time-sensitive than
time-critical -- no one *wants* the sprite to lag behind mouse movements on a
window, but one can live with it.  The time-critical aspect is in the data
acquisition as the mouse ball sends data up the I/O port to the computer.

So to draw a distinction between "real-time" software and non-real-time
software, we only consider the "time-critical" software to be in the former
category.  "Real time systems" have a mix of time-sensitive and time-critical
software.

If you want other views, I've put a file on hpcsos.col.hp.com (15.255.240.16)
called pub/realtime.Z.  It contains about 40 articles from comp.realtime that
I've collected over the years on this subject.  Compressed it's 69Kb.
Uncompressed, it 3343 lines, 154Kb.  I haven't read it all thoroughly enough
to summarize.

I don't think I've seen a FAQ for this newsgroup -- if there's interest, I'm
willing to compile one, starting with an answer to this question.  Mail me
your votes and advice.


Tim Chambers                               ||  tbc@col.hp.com
Hewlett-Packard Company                    ||  
P.O. Box 2197                              ||  (719) 590-5570 (office)
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