Re: [ECOS] Hard-Realtime behaviour

["Wolfgang Köbler" <wk-list@koebler.com>]

Hello,

I am also interested in using eCos for hard realtime applications.

So just some thoughts about hard realtime and eCos:
I think that simple hard realtime applications are already possible with
eCos, if you are carefull enough.

With eCos I know all the software that is running on my system. I can strip
it down to the absolute minimum and review the code I use (when necessary).

When creating a hard realtime application I need to make sure I always meet
my deadlines. This means
1. I have to write my time-critical code so that it has deterministic
behaviour (Very few, well known, things should affect its
worst-case execution-time).
2. I have to make sure that no other code disturbs my time-critical
code.

With RTAI and Linux there is usually a whole lot of code that may not
disturb realtime code and is unknown. With eCos usually the whole system is
known and so the problem is much smaller.

So what do we need (or want) for a hard realtime system:

1. Anything that is called by realtime code needs deterministic behaviour
itself. This is probably what you are talking about here:
> I am currently working on making eCos hard-real time. As the developers of 
> RTAI claim that RTAI is hard-realtime, I have been comparing the code of 
> RTAI and eCos w.r.t the interrupt handling, kernel primitives, system 
> calls etc.

2. Something to control DMA-transfers, as they may unexpectedly reduce memory
bandwidth and so slow down realtime code execution. But usually this is
ignored.

3. Something to control interrupts, including special interrupts such as NMI
and SMI on PCs. They may not unexpectedly interrupt realtime code.
If you only want hard realtime in some IRQs, propper interrupt priorities
might be enough. If you want realtime in tasks you need to 
- know the high priority interrupts and
- bound the time needed by low pritority interrupts

RTAI does some interrupt virtualization and thus makes it possible to run
realtime-threads that have a higher priority than the non-realtime-interrupts.
This would certainly be a nice feature for eCos, too.

4. One important point is to make sure that no "normal" code can disable
interrupts, as this could add unpredictable additional latencies to interrups
(and the scheduler). Therefore RTAI replaces all cli-commands in the Linux
kernel with its own code.

5. If you want hard realtime in tasks, you need a scheduler that does what
you want. Usually you want a scheduler that only executes the task with the
currently highest priority instead of giving cpu-time to all tasks. (Or you
might even want an EDF scheduler)

6. It often helps if you have inter-task-communication mechanisms that
support things like priority inheritance and priority ceiling. 

7. A realtime TCP/IP stack (or rather UDP/IP) would be nice.

8. ...

I have not checked which of these wishes are already fulfilled by eCos.
And I do not claim that the above list is complete.

Bye,
Wolfgang



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