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

[Hubert Ritzdorf]

I vote for #2.

Hubert
________________________________________
From: mpi3-rma-bounces at lists.mpi-forum.org [mpi3-rma-bounces at lists.mpi-forum.org] on behalf of Jim Dinan [dinan at mcs.anl.gov]
Sent: Wednesday, July 25, 2012 8:45 PM
To: mpi3-rma at lists.mpi-forum.org
Subject: Re: [Mpi3-rma] [EXTERNAL] Re: Disp_unit in allocate_shared should be returned by win_shared_query?

Hi All,

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().

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).

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.


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.

  * 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.

Thanks,
  ~Jim.
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