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

[Torsten Hoefler]

Hi Jim,

Bill's argument convinced me, I think we should have the displ_unit in
the query function because this would support future systems and comes
at essentially no cost. If the unit is the same on all processes, it can
be stored. This can be determined in O(log(P)) or O(1) time with an info
argument. Compression is trivial.

> We should try to reach a decision on this issue before the end of the  
> week.  I think it would be helpful to hear some other opinions.  So far,  
> we have three options to choose from:
>
> 1) Require that the displacement unit be the same at all processes for  
> MPI_Win_allocate_shared().
This would never allow us to run across heterogeneous systems. I don't
know any heterogeneous system that supports the unified model but GPUs
are coming :-). I agree to Jim that coherency would be tricky, so this
hardware is unlikely. We could also always lift this restriction in
future MPI versions, so I'm not opposed.

> 2) Allow different displacement units at each process in shared memory  
> windows and allow processes to determine the displacement unit at remote  
> processes via MPI_Win_shared_query().  This is in addition to the MPI-2  
> self-query already possible with MPI_Win_get_attr(MPI_WIN_DISP_UNIT).
Yes, this is what we have at this point.

> 3) Allow different displacement units at each process in shared memory  
> windows and provide only the MPI-2 self-query already possible with  
> MPI_Win_get_attr(MPI_WIN_DISP_UNIT).  Manual communication would be  
> needed to exchange disp_unit information.
This would require a wording change.

> A (very) rough summary of the discussion on this issue:
>
>  * Given that non-uniform disp_unit values are currently allowed,  
> processes may need to use disp_unit in address calculations.
They may check if all processes' disp_unit is the same, but that's
another complication in the interface.

>  * Requiring programmers to use the disp_unit at each process in address 
> calculations diminishes the usefulness of the shared memory programming 
> model.  It's also not clear if there is a use case to support the 
> non-uniform disp_unit model.
Agreed.

I vote for #2.

Best,
  Torsten

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Torsten Hoefler         | Performance Modeling and Simulation Lead
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