Re: Definition of "realtime"

kwrohrer@ce.mediaone.net
Fri, 2 Oct 1998 14:42:27 -0500 (CDT)

And lo, Albert D. Cahalan saith unto me:
> Olaf Titz writes:
>
> > A problem is "hard realtime" if time constraints are involved in the
> > specification of the problem such that, when the time constraint is
> > not met, the behaviour of the program is seen as _not correct_.
> ...
> > Note the last two words. The definition doesn't deal with possible
> > consequences from wrong behaviour, it just states that the behaviour
>
> This distinction is not useful then, since the success of a hard
> real-time process may be far less important than the on-time behavior
> of a soft real-time process.
Not useful to you, perhaps. However, it is at least a design decision,
and in fact has implementation consequences: hard realtime avoids all
possibility of missing deadlines, whereas soft realtime avoids missing
deadlines where possible and attempts to handle failures when deadlines
are missed.


> It is only the consequences that matter, and the OS can not determine
> how expensive the consequences are. Humans let the OS know, and the
> OS must avoid missing deadlines.
Humans must *make* the OS avoid missing deadlines, if they are designing
the OS or the system.

> (so it was bad to bother Richard Gooch with such a pointless question)
The original error I pointed out (whose, I don't remeber, but it was
consistent) was a completely different confusion: shortness of deadline
and/or very low slack time/permissible latency was being confused with
the hardness of the real time system. You may have pooh-poohed a
clarification of a different error, but nonetheless you seem to be
scoffing without understanding what you're scoffing at. If I knew of
a good RTOS book, I'd be recommending it to you right now.

Keith 

-- 
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