Re: What's Real-Time? (was Re: Widespread C++ Competency Gap?)

[merlin@annwfn.com (Fred McCall)]

In <hbaker-3012941712040001@192.0.2.1> hbaker@netcom.com Henry Baker writes:

>In the RT community there are the terms '_hard_ real-time' and
>'_soft_ real-time'.  'Hard' RT means that the consequences of missing
>a deadline are catastrophic, while 'soft' RT means that the consequences
>are expensive, but not catastrophic.

This doesn't quite jibe with the way I learned it (although reflection
seems to show that we're talking about different aspects of the same
thing). Real-time (in general) means that there are time-driven
constraints on the system which *must* be met.

'Hard' real-time implies that the required timeline must be met
*exactly*; in other words, you need a deterministic machine where you
know the timing of all operations and combinations thereof in order to
do this correctly.  If you blow the timeline (either early or late,
within a very narrow window), the system will fail and cannot be
recovered. 

'Soft' real-time sets upper limits on how long things can take, but if
you get there early the system will run along fine.  Blown timelines can
generally be recovered if other later operations can be performed fast
enough to 'make up' the blown timeline.

>Unfortunately, nearly _all_ of the progress in HW performance in the last
>20 years has been in _mean_ performance, _not_ worst-case performance.
>In fact, worst-case performance has only improved modestly over this
>period.  The _ratio_ of mean to worst-case performance has increased
>dramatically, so that it may be 100:1 or more today.

Not to mention that most of the things that we do to make the hardware
faster also tend to make it less deterministic (super-scalar,
super-pipelined designs, etc.) and hence less suitable for 'hard'
real-time unless you design with extraordinary margins (so that you can
still make the timeline if the pipeline stalls and has to be flushed,
for example).

>

--
"Insisting on perfect safety is for people who don't have the balls to live
 in the real world."   -- Mary Shafer, NASA Ames Dryden
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merlin@annwfn.com -- I don't speak for others and they don't speak for me.