Re: [Xenomai-help] task management, multi-thread, multi-core

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

Everett Wang wrote:
> Hi All,
> 
> I am working on a navigation program that has two realtime tasks. One
> is a high priority one executed in a higher frequency. Another task
> runs about 1/10th of the first one, but requires substantially more
> CPU computations. I am using an atom board which has dual threads
> capability. I am worried that the second task can block the execution
> of the first one until it finishs all the calculations. Can this
> happen in a dual thread and/or dual core CPU? If yes, should I break
> the task two into small chunks and let the task one to send a trigger
> to
> execute one chunk at a time? I know it is not xenomai specific
> question and I have tried to read manual or other documents trying to
> figure
> it out.

You did not tell us how was the priority of the second task, but since
you told us that the first task was high priority, I have to guess that
the second task is lower priority.

On an uni-processor machine, the priorities are used by the scheduler to
schedule your tasks, so, if a low priority task could prevent a high
priority task from running, the scheduler would need to be fixed.

On a multi-processor machine, you have to set the "affinity" of the
tasks to pin the tasks on a given cpu. On each cpu, tasks are scheduled
following their priorities.

> 
> I have read that tasks require more resources than threads in Linux.
> The inter-threads communication is also more efficient than that of
> tasks. Are these also true for xenomai realtime OS? Should I create
> threads instead of tasks?
> 
> 
> Any suggestions are welcome. Thanks for reading. :-)

task is an informal term which may mean process or thread depending on
the context. So, I guess what you mean is "process" instead of task. The
difference between processes and threads is that changing context
between processes requires changing the memory mapping, which is an
heavy operation. On the other hand, processes provide an isolation that
threads do not provide. So, choosing threads or processes is not only a
matter of performances (otherwise, we would all run Linux without MMU,
since in that case, everything is essentially a thread).

> 
> Everett
> 
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-- 
					    Gilles.