Tue, 05 Sep 2000 [EMAIL PROTECTED] wrote:
> > > Let's say I have N message buffers in a pool. What if
> > > there is no more free buffer?
> >
> > Then the design for your hard real
> > time system was wrong :-)
>
> Or system reached its limits.
Yep, and the only options are a) redesign and b) upgrading the hardware.
> It depends on your possibilities and intentions.
> What if amount of incoming data is unpredictable but you have
> to process at most as you can with a limited hardware?
That's a kind of *soft* real time system, which is not really what RTL is
intended for. (Although the input->"some kind of output" latency could have a
hard RT requirement - but the memory allocation would still be soft RT.) You
can't optimize a single API for both hard and soft RT, at least not without
making it a major PITA to use if you want hard RT, so IMHO, it's a very good
idea not to mix these two kinds of systems up when discussing APIs.
(The general problem of soft RT capable systems is that the dynamic allocation
and other stuff they provide makes it virtually impossible to write hard RT
applications, even if the kernel theoretically could provide the required
timing guarantees.)
> Then one can choice a stragegy of survival: throw away unprocessable
> data and do some clever. (Plan B: crash and reboot. ;-)
Well, with some systems, plan B and the "stragegy of survival" both actually
give the same result as plan C: miss a deadline and wreck some very expensive
machinery...
This basically applies to all control systems dealing with powerful and/or
sensitive machines, and particularly in situations where people may get hurt or
killed if the machines freak out. I'd say that a thoroughly tested RTL solution
could be appropriate at least for the less critical systems in this range, but
that doesn't go for *any* kind of soft RT system - and anything that shares
resources with non RT tasks in a similar way to a shared memory pool is
actually a soft RT system.
David Olofson
Programmer
Reologica Instruments AB
[EMAIL PROTECTED]
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