[Mpi3-rma] Next RMA Telecon

[Torsten Hoefler]

On Mon, Dec 26, 2011 at 10:36:28PM -0600, Pavan Balaji wrote:
>
> On 12/26/2011 10:31 PM, Torsten Hoefler wrote:
>>> Also, the case I had in mind was using lock/unlock.  If it is valid for
>>> the user to do a for loop of Win_lock(exclusive), I'm not sure how we
>>> are taking away the MPI implementation burden by not allowing
>>> Win_lock_all(exclusive).
>> A for loop is a silly user program, having a slow P^2 lock_all
>> exclusive (in the extreme case) is a problem in the MPI lib.
>
> True.  Hence the need for a lock_all(EXCLUSIVE).  But if that's not  
> provided, there should at least be a fall back option, and the for loop  
> is that.
>
> Besides, silly or not, it's a valid MPI program and the MPI  
> implementation has to deal with it.
No, the MPI library does not have to deal with silly use programs. If a
user decides to write a non-scalable program, then the MPI library
cannot do much.

>>> More importantly -- even if we want to not allow exclusive locks in
>>> WIN_LOCK_ALL, I would suggest retaining the lock_type parameter and say
>>> that the behavior of EXCLUSIVE locks is undefined, so that it's future
>>> proof in case we want to add it later.  Also, implementations that want
>>> to provide it can do so without random hacks or new API functions.
>> Yes, I thought about this, however, our original #270 didn't have this
>> feature and I decided to revert to the original. We can do a straw vote
>> here and fix it quickly. I have no strong opinion.
>
> I'd prefer a straw vote on this.  I don't see any additional burden on  
> implementors who don't want to allow it.  But it's much cleaner for  
> implementations that decide to allow it.
>
>>> The idea is that a process gets a lock on a shared memory region and
>>> uses it for remote RMA communication.
>> Yes, that should work but the semantics of overlapping windows are
>> hairy.
>
> I think they are quite well defined, and fairly intuitive once you think  
> through it.  But that's just personal opinion.
I agree, but I fear that 98% of our user-base (the people who did not
spent dozens of hours discussing the semantics :-) ) may think
differently (remember that there was a large and credible group in the
Forum that wanted to completely abandon those semantics just two years
ago :-) ).

All the Best,
  Torsten

-- 
 bash$ :(){ :|:&};: --------------------- http://www.unixer.de/ -----
"All that we need to make us really happy is something to be enthusiastic
about." -- Charles Kingsley