[Mpi-forum] [Mpi3-rma] [EXTERNAL] Re: Disp_unit in allocate_shared should be returned by win_shared_query?

Torsten Hoefler htor at illinois.edu
Sun Jul 22 13:31:41 CDT 2012

On Sun, Jul 22, 2012 at 08:30:26PM +0200, Rolf Rabenseifner wrote:
> Torsten and Bill,
> Torsten:
> > I would agree to Bill. We should have an info argument allowing the user
> > to state that all displ_units are the same like we have for count :-).
> > A topic for MPI-3.1!
> Yes, it is not needed for MPI-3.0, because with two Allreduces with 
> MIN and MAX of disp_unit, all processes know in O(log(p)) whether 
> it is the same value in all processes.
Sure, this is trivial but I still like an O(1) (zero communication) info
argument better :-).

> Bill, yes, here a draft:
>   Advice to users. If an application should scale to a large 
>   number of processes, then it is recommended for scalability reasons 
>   that all processes provide the same disp_unit argument
>   on all processes. End of advice to users.
I think this does not belong into the standard. If we start those
things then we need to add the same for the size and dozens of other
"collective" arguments (think compressibility of graphs etc.).

I also think that we're digressing :-). We still didn't decide if the
WG likes the displ in the query function or not. 


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Torsten Hoefler         | Performance Modeling and Simulation Lead
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