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

[Jim Dinan]

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.