Re: Ada Annex E (Just curious :-)

[Dr Adrian Wrigley <amtw@linuxchip.demon.co.uk>]

Marin David Condic wrote:
...
> I would agree that the lack of predictable latency makes it of somewhat
> dubious value for realtime systems.
...

I was thinking about the viability of using Annex E in a system with
multiple microprocessors, sharing only part of their memory space.
Suppose there was a coherent cache or multi-port memory, the
Annex E implementation could work by referring to blocks of memory,
making response times predictable, and performance high.

Ideally, you could create a big data structure in shared memory, and
access it from many partitions at once.  If the memory pattern was
entirely read-only, once created, each partition could locally
cache the shared data, without serious time penalty for maintaining coherency.

> Or possibly if you had multiple processors in a system that controlled
> different hardware? Say, having a graphics processor to which you performed
> RPC's to get certain things rendered? (Of course, this would require either
> a very standardized processor/communications-interface or you'd never get
> the compiler modified to suit the environment. :-)

Funny you should say that... I was thinking of implementing rendering algorithms,
using Annex E over shared memory.  The performance I got from the little demo
(over 100Mbps network, 100kB transfers) was disappointing though.
I think hacking into the parameter marshaling code might sort that out.

> It would seem that it might be useful for either computationally intensive
> applications that lend themselves to distribution (I've seen it done with
> CFD calculations) or database systems such as, say, teller machines that
> want to verify a bank balance with a central computer before allowing a
> withdrawal.

I was thinking of the teller machines example as "controlling many remote
peripherals".  Each machine has its own local screen, keyboard, etc.
and Annex E could be used to put a partition in each machine to control them.
I don't think it would be considered a very practicable system though,
since people want to implement each machine independently, using an
agreed protocol, rather than unifying everything into a single 5000 partition
program.  How do you upgrade systems? Implement redundancy? etc.

> Maybe its just the application domain you are usually involved in? Remember,
> it is a "Special Needs Annex" and may just be the kind of thing that is more
> common for a different problem domain. (I'd think it would be used heavily
> in business applications where there was at most a soft-realtime
> requirement.)

I've been involved in several diverse domains.  Embedded systems with
hard realtime and soft realtime components and many peripherals
seems most plausible use today. Take a mobile 'phone, for example -
that might have hard realtime voiceband (maybe RF) DSP, as well as
keyboard, display, power control etc.  Currently, there would be
a "standard" DSP, and a "standard" microcontroller, but with
separate development groups, and an ad-hoc inter-processor communications
protocol.  Annex E could help here.  In theory.
--
Adrian Wrigley