Re: [sv-ec] Strict FIFO ordering of mailboxes try_put/try_get

This is an interesting corner case that we didn't consider when this section
of the LRM was introduced.

I would not expect try_get() to have priority over a set of get() operations
that are currently queued up. Allowing a try_get() to swoop down and grab a
message before it can be retrieved by a previous get() that is now waiting
just doesn't seem to be in the spirit of the existing description of a
mailbox. As Dave mentioned, users can implement a priority scheme by using
a semaphore wrapped around a mailbox.


Neil


Rich, Dave wrote On 02/14/07 00:35,:
> OK, I see there are lots of misunderstandings.
> 
> First of all, to define a bounded mailbox, you need three queues.
> 
> There is the queue that holds the data, or the contents of the mailbox.
> The FIFO ordering of that queue guarantees that the first piece of data
> to be placed into the mailbox by a completed put/try_put will be the
> first to be received from the mailbox by a successful get/try_get.
> Completion means the blocking put/get returns (unblocks) or the
> try_put/try_get returns true.
> 
> When the mailbox if full, there is a queue of processes suspended on a
> blocking put. That queue guarantees that a put completes in the same
> order that they are suspended. That queue and the data queue give us the
> strict FIFO ordering for puts. Similarly, there is a third queue of
> processes suspended on a blocking get when the mailbox is empty.
> 
> The problem is not with multiple blocking puts/gets or multiple
> non-blocking try_puts/try_gets. The problem comes in when you start
> mixing blocking puts with non-blocking try_puts or mixing blocking gets
> with non-blocking try_gets. Look at this example which is a
> simplification of a more complex example:
> 
> mailbox #(int) mb = new(.bound(1));
> int t;
> initial 
> 	begin
> 		fork
> 			mb.get(t);
> 		join_none
> 		#1 // some delay
> 		mb.put(123);
> 		void'(mb.try_get(t));
> 		assert(mb.num()==0); //this should never fail
> 	end
> 
> 
> If you accept that try_get() succeeds if and only if the size of the
> mailbox is greater than 0, then you must also accept that try_get()
> intercepts a suspended blocking get.
> 
> 
> Dave
> 
>      
> 
> 

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Neil Korpusik                                     Tel: 408-720-4852
Senior Staff Engineer                             Fax: 408-720-4850
Frontend Technologies - ASICs & Processors (FTAP)
Sun Microsystems
email: neil.korpusik@sun.com
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