Re: Re: Replacement for Xenomai's Message queues?

["M. Koehrer" <mathias_koehrer@arcor.de>]

Hi Darren,

thanks for the response.
You are right, I should clarify the needs I have...
I have two real time applications (two processes), each running a few real time threads.
For inter process communication I want to exchange data between the two applications.
For this I want to use two queues - one for direction A->B and one for B->A.
Now I have the situation that one thread in B that handles the A->B queue is waiting for data from A.
Whenever A has the data available, it writes the data into the A->B queue.
Immediately after receiving data from A, B unblocks and computes some answer data that is written
into the B->A queue. 
The time between sending the data in A and getting the data back from B should be as low as possible 
(real time requirements: at max tens of microseconds...).

The functionality of mq_send, mq_receive looks suitable.
However I am not sure if this can fulfil my performance needs...
Perhaps I still have the "Xenomai" thinking which means that only some special API functions are real time capable
all others are not... With the RT_PREEMPT patch this seems to be much easier as I can use all suitable functionality
(or did I miss some important thing here?!?).

Regards

Mathias


> > Hi all,
> > 
> > I am about to switch an realtime application from Xenomai to the
> RT_PREEMPTION patch.
> > Everything works really smooth, however I have one question.
> > With my Xenomai application I use the message queue which combine transfer
> (including queuing)
> > of data with activation of the thread.
> > In my Xenomai application I use this feature quite frequently. Now, I am
> not sure how to do an equivalent
> > with the RT_PREEMPT patch.
> > (see
> http://www.xenomai.org/documentation/xenomai-2.4/html/api/group__native__que
> ue.html)
> > Of course I could use condition variables and a struct or array to hold
> the data I want to pass with the activation
> > of a thread. However, using this approach I do not have the queuing (which
> is some extra effort I could spent...).
> > Another idea I have: I could use sockets which realize the queuing but the
> overhead is too high.
> > 
> > Is there any suitable way to implement a message queue like approach using
> RT_PREEMPT patch?
> > I am running x86 (Dual/Quad Cores).
> 
> I don't if we have many xenomai experts on this list. It would help to 
> understand which aspects of the Xenomai message queue are important to 
> you (other than the ability to block on a queue and wake when a message 
> is available). For a basic message queueing system with the ability to 
> prioritize messages, you can look to the POSIX.4 Message Queues. A 
> google search will provide plenty of howto's, and the API appears at 
> least similar to what you were using with Xenomai.
> 
> I haven't looked, but I don't think Linux supports message driven 
> Priority Inheritance 
> (http://www.qnx.com/developers/docs/6.3.2/neutrino/sys_arch/kernel.html#Prio
> rity_inheritance_messages).
> 
> -- 


-- 
Mathias Koehrer
mathias_koehrer@arcor.de


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