On Fri, 10 Sep 1999, Tomasz Motylewski wrote:
[...]
> I know this is no longer hard real time, but I also have a feeling that
> people writing about RT ethernet do not really need it (I may be wrong, but
> please show me a real life situation when 1 ms deadline on the net has to be
> met at more than 99.9999 % of cases).
Signal processing on clusters.
The $$$/power ratio of clusters beats that of single mainboard systems in some
cases, and allows more powerful systems to be built, so it's a very interesting
possibility for very demanding users. A latency of a few ms is ok in most cases,
but the bandwidth requirements are rather high, and drop-outs would simply
render the system useless.
> > > Recompiling the whole linux/net and linux/drivers/net subdirectories without
> > > -D__RT__ flag and testing it would make very interesting experiment :-)
> >
> > Don't bother. You've just turned your real-time system into a system that
> > only pretends to be real-time.
>
> I know. So we have returned back to the original point: no OS services can
> be used if the processing has to be done in hard real time. Calling these
> functions will work "most of the time" with all hard to predict consequences.
>
> But then what about testing first if the interrupts are software disabled at
> the moment, and if not, behaving like in normal interrupt context? Of course
> sometimes test will fail, then the action has to be delayed, and then the
> RT-BH mechanism (triggering soft interrupts) which I have demonstrated some
> weeks ago would be probably best.
This is soft real time. Useful, but not for the kind of stuff many of us are
dealing with. I'd implement a dedicated RT memory manager and that kind of
things instead. It's more work, and it means duplication (unless you can
replace the non-RT system with the RT capable one - which would likely degrade
non-RT system performance), but it's possible to keep everything hard real
time.
//David
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