ISO/IEC JTC1/SC22/WG5 N1755

                Request for new features from MPI Forum

                         Craig Rasmussen     

References: 08-185r1, 08-255

The MPI Forum has been restarted in order to update the Message Passing
Interface (MPI) standard.  As part of this standardization effort, a new
Fortran binding will be created for version 3.0 of the MPI standard.  The
MPI Forum has requested that J3 consider modifying the Fortran standard
(or provide guidance) to accomplish the following:

  1. A mechanism to suppress copy-in/copy-out semantics for MPI
     asynchronous operations.

  2. Suppress argument checking for MPI choice buffers (C void * formal
     parameters).

A series of straw votes were taken at J3 meeting 184 to determine how to
address the request from the MPI Forum.  The results were:

  1. The VOLATILE attribute should be given to both the actual and dummy
     arguments to suppress copy-in/copy-out.

  2. J3 voted to consider three different candidates for suppressing
     argument checking of C void * formal parameters.  The results of the
     vote were mixed.  At a later date, the HPC subgroup decided on one
     of the options:

       a. Create new syntax, TYPE(*) and DIMENSION(:), to allow any type,
          kind, and rank of actual argument.  A TYPE(*) dummy can be
          argument associated with any type and kind of actual argument.
          A dummy argument with the DIMENSION(:) attribute can be argument
          associated with any rank actual argument, including a scalar.

WG5 is asked to consider TYPE(*) and DIMENSION(:) as a work item for the C
Interop TR.

Regarding straw vote 1: 

A problem with using VOLATILE attribute is that it
can have performance implications.  Therefore Aleksandar Donev in paper
08-255 proposed the following:

We should explicitly allow a variable with the ASYNCHRONOUS attribute to be
modified or examined by means external to the processor, similarly to
VOLATILE variables. If such a variable is modified or examined externally
during a segment, that variable must not be referenced or define during
that segment.

This simple modification solves an existing problem with MPI non-blocking
transfer, namely, the need to prevent movement of code across calls to
MPI_Wait. The programmer can use SYNC MEMORY to indicate to the compiler
that ASYNCHRONOUS variables may be affected and therefore old copies in
registers should be discarded and new values written to memory. This is
exactly as for coarrays and TARGETs (which may be modified by other images)
and also just like SYNC MEMORY needs to be put around external
synchronization routines such as MPI_Barrier (see Note 8.39).

This idea was subsequently tabled in subgroup as being too substantial a
change for the 2008 standard.


MPI 3.0 Interface
-----------------

An example of a possible interface for MPI 3.0 is shown below for the
asynchronous recv function:

interface
   subroutine MPI_Irecv(buf, count, datatype, source, tag, comm, request, err)
      import :: MPI_Comm, MPI_Datatype, MPI_Request
      TYPE(*), DIMENSION(*), VOLATILE, intent(inout) :: buf
      integer, intent(in)               :: count
      type(MPI_Datatype), intent(in)    :: datatype
      integer, intent(in)               :: source
      integer, intent(in)               :: tag
      type(MPI_Comm), intent(in)        :: comm
      type(MPI_Request), intent(out)    :: request
      integer, optional, intent(out)    :: err
   end subroutine MPI_Irecv
end interface