comments in line: On Oct 3, 2011, at 1:56 PM, Rolf Rabenseifner wrote:

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- Be able to use all three versions of the Fortran bindings within a single translation unit. The goal being to make it possible to migrate incrementally to the new proposed bindings, within the applications current file structure.

This is only one aspect of the proposed Fortran interface. The major aspect is to solve the inconsistency problems between Fortran and MPI. And here mainly the problems with nonblocking routines.

[rich] As I understand this, the interface and the Fortran/MPI interoperability are two distinct issues. That is, the API and the issues that arise from compiler optimizations are two distinct items.

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- Take advantage of the ability to have Fortran and C binding be exactly the same, bypassing a translation layer.

This is not a choice because it does not solve the major problem: The correct use of nonblocking routines.

[rich] Again, if I understood this correctly, the form of the interface is a different issue than what to do to prevent the compiler optimizations that cause issues with nonblocking routines.

Using the C interface from Fortran means that the user has to - translate all the handles from Fortran to C and vice versa, - pass the C_LOC(buffer) instead of buffer, - use the TARGET attribute for the buffer (otherwise the use of C_LOC with buffer is incorrect).

[rich] Can the arguments in fortran be declares as c types, avoiding all the translation you mention ?

The TARGET attribute does not solve the nonblocking problem because it protects only code movement optimization around the MPI_Wait calls, but it does not solve problems based on automatic off-loading of data to a GPU memory while an internal MPI progress thread is communicating parts of the array. Such "using only C from Fortran" may be only portable when there is no progress thread as, e.g., in mpich2.

The MPI Forum clearly decided ***not*** to mandate one of these approaches (with or without a progress thread), and therefore, a pure TARGET based solution and to forbid progress threads is not an option compared to the ASYNCHRONOUS buffers in the new mpi_f08 module.

[rich] The forum has not decided anything at this stage, which is why I ask these questions before we go ahead and actually make decisions.

If users want to use the C interface from Fortran, they can do it, but they should know that they write non-portable applications. There are real user who do this, as Torsten reported.

[rich] The question was strictly about the interface. I think that what Craig said, is that the current proposal is what Fortran users expect is what answers my question. Rich

Best regards Rolf

----- Original Message -----

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From: "Richard L. Graham" <[email protected]> To: "Main MPI Forum mailing list" <[email protected]> Sent: Monday, October 3, 2011 6:30:43 PM Subject: [Mpi-forum] Fortran bindings At the risk of upsetting the apple cart, I want to ask a question about the new proposed Fortran bindings for MPI 3.0. I rather sure that the question I am about to ask has been addressed in the past, but to be honest, I did not understand the details of the proposal enough to note the difference. It was not until the rather heated (and fruitful) discussions that occurred at the last meeting in Santorini, and a follow on discussion that this came up.

It seems to me that two options were considered with respect to these bindings: - Be able to use all three versions of the Fortran bindings within a single translation unit. The goal being to make it possible to migrate incrementally to the new proposed bindings, within the applications current file structure. - Take advantage of the ability to have Fortran and C binding be exactly the same, bypassing a translation layer.

First, did I describe the two possibilities correctly ? If so, why opt for the first over the second ?

Rich _______________________________________________ mpi-forum mailing list [email protected] hxxp://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum

-- Dr. Rolf Rabenseifner . . . . . . . . . .. email [email protected] High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530 University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832 Head of Dpmt Parallel Computing . . . www . hlrs . de/people/rabenseifner Nobelstr. 19, D-70550 Stuttgart, Germany . (Office: Allmandring 30) _______________________________________________ mpi-forum mailing list [email protected] hxxp://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum