Re: How to: communication between multiple tasks using protected objects - with no polling?

[Esa Riihonen <esa.riihonen.er@gmail.com>]

keskiviikko 21. tammikuuta 2015 22.02.38 UTC+2 Dmitry A. Kazakov kirjoitti:
> On Wed, 21 Jan 2015 09:34:51 -0800 (PST), Esa Riihonen wrote:
> 
> > keskiviikko 21. tammikuuta 2015 10.28.08 UTC+2 Dmitry A. Kazakov kirjoitti:
> >> On Tue, 20 Jan 2015 14:36:20 -0800 (PST), Esa Riihonen wrote:
> >> 
> >>> Here is the problem. My typical C-process 'listens' to several FIFOs
> >>> connected to separate processes. This is realized using a ppoll-type
> >>> 'reactor' - so in essence the process is sleeping until there is some
> >>> activity in any of the monitored file descriptors (I guess this is in
> >>> essence quite basic arrangement in the 'C-world').
> >>> 
> >>> I first thought I can implement this by the 'select' - something like this:
> >>> 
> >>> PO1 and PO2 are instantiations of a protected type with entry 'Get'.
> >>> 
> >>> ...
> >>> loop
> >>>   ...
> >>>   select
> >>>     PO1.Get(...);
> >>>   or
> >>>     PO2.Get(...);
> >>>   or 
> >>>     ...
> >>>   end select;
> >>>   ...
> >>> end loop;
> >> 
> >> The typical design of 1-n is reverse: the publisher queues request to the
> >> consumer rather than lets the consumer to come and pick it.
> >> 
> >> The publisher may call the consumer's entry or else queue the request into
> >> a queue. If publishers shall not be blocked (e.g. in order to prevent
> >> priority inversion or due to time constraints), also when it is n-m, use
> >> the blackboard instead of a queue.
> > 
> > Thansk for the answer - and yes, the last one is my situation: the
> > publishers just report their state on certain points and then proceed
> > immediately in their more important stuff and should not be blocked.
> > I effect there is no queue inside my POs - just the most recent status
> > information from the associated task - I guess this is at least some kind
> > of a 'blackboard'.
> > 
> > So - in my mind I have a small number of students each with a separete
> > blackboard. And instead of my teacher walking around checking whether the
> > answer of any of the sudents is ready - I wan't to let the teacher doze in
> > his/her chair and only be woken up when any of the students have their
> > answer ready. And after the student briefly nudges the teacher he/she must
> > be free to immediately continue with his/her other exams.
> > 
> > I guess I just don't know how this 'blackboard' model should be implemented?
> 
> You can use one blackboard for all. The item would contain the student
> identity, reference etc.
> 
> For an implementation of lock-free blackboard see:
> 
> http://www.dmitry-kazakov.de/ada/components.htm#10.2
> 
> Since you have many publishers you must lock the blackboard when writing
> it. Readers are still non-blocking. OK, you have just one.
> 
> Now you want have a non-busy wait for an event when somebody writes
> something into the blackboard. That is a pulse event, see example here:
> 
> http://www.dmitry-kazakov.de/ada/components.htm#11.2.2
> 
> The reader reads the blackboard until there is no new items (the reader's
> cursor is behind the cursor to the last item on the blackboard). Then the
> reader waits for an event pulsation.
> 
> Note that this is a race condition. So you cannot directly use the pulse
> event described above (and also for reasons described below). You should
> modify it in a way that it would test if the reader's cursor indeed points
> after the blackboard end and only then enter waiting. Since this is a
> protected action, the event will not be signaled during the test and the
> race condition is eliminated.
> 
> Naturally you would use a single PO for the pulse event and the write-mutex
> in order to minimize the number of protected actions. Protected actions are
> expensive. The action releasing the write-mutex will as well pulse the
> event [and also release the publisher's object, if reference counting used
> for dynamically allocated publishers.]

Thanks for the explanation. And the refenced web page looks really good resource for my aim to learn to 'think in Ada'. Added to bookmarks.

> I know it may look complicated, but it only looks so.

I hope that will eventually prove to be so even in my case ;)
 
> -- 
> Regards,
> Dmitry A. Kazakov
> http://www.dmitry-kazakov.de