Re: [ECOS] Hard-Realtime behaviour

[Luis Friedrich <fernando@inf.ufsc.br>]

Wolfgang Köbler wrote:

Hi,

Most important of all in hard real-time systems is predictability which 
means you are able to
calculate the execution time (WCET) of your system and so you can 
predict it.

Also important is to be able to describe your application to the system, 
like period, deadline, priority and all the parameters that describe the 
temporal behavior of tasks (or threads).

Finally, the scheduler or the scheduler mechanisms that are available 
are also important to have flexibility enought to use different 
scheduling policies.

How you define for example the temporal paremeters of a thread on eCos?
 - the function /cyg_thread_create/   has no arguments about the 
temporal behavior of the thread

RTAI provides an API with functions like /rt_task_make_periodic/ which 
allow the manipulation of temporal information for the tasks.
This is important for the scheduler to be able to schedule hard 
real-time threads or tasks.


Friedrich
 


>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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