Re: [Xenomai-help] Which process synchronization primitive?

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

Daniele Nicolodi wrote:
> Gilles Chanteperdrix wrote:
>> Daniele Nicolodi wrote:
>>> Gilles Chanteperdrix wrote:
>>>>> I think message queues can work well but I'm not sure they are the best
>>>>> choice when I do not have to communicate any message. Would be
>>>>> semaphores best suited to my needs?
>>>> You can use process-shared mutexes or semaphores.
>>> There is somewhere a comparison of the overhead of the different
>>> solutions? Do I need to do my own benchmarks? Is the API the only
>>> difference?
>> Mutexes have the lowest overhead, since they can be handled in
>> user-space starting with the 2.5 branch.
>>
>> Differences betwenn mutexes and semaphores are the same with Xenomai
>> than with any other known API: mutexes are binary semaphores with a
>> notion of ownership.
> 
> Thanks for the clarification. I have been reviewing the documentation
> and I think that what I need is a mutex with a condition variable shared
> between processes, or a semaphore.
> 
> The reason is that in my application I have processes that consume data
> produced by other processes, and I need to suspend the execution of the
> consumer until the producer has new data to handle.
> 
> Are condition variables also implemented in user space, so that their
> overhead is smaller compared to semaphores?

No. Condition variables imply scheduler decisions and context switches,
which can only occur in kernel-space, so, it would not really make sense
to implement them in user-space. The only case for which it would be
interesting is when you are signaling a condition while no thread is
waiting for it, but this case, is not really interesting in practice.

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-- 
Gilles Chanteperdrix, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com