[MPI3 Fortran] MPI non-blocking transfers

[N.M. Maclaren]

There has been a lot of cross-purpose discussion on MPI non-blocking
transfers, the ASYNCHRONOUS attribute and related topics.  This is an
attempt to find out the technical background to people's views, in the
hope of reaching a solution.

I should particularly like response from implementors, as I believe the
main objections are from them.  Obviously, I am hoping for detailed
technical responses, as this area is a hard one to tie down.

Note that this is asking about MPI non-blocking transfers ONLY, and not
one-sided ones (which are very different, semantically).


Questions:

    1) Most people seem to agree that the semantics of the buffers used
for MPI non-blocking transfers and pending input/output storage
affectors are essentially identical, with READ, WRITE and WAIT
corresponding to MPI_Isend, MPI_IRecv and MPI_Wait (and variations).

Do you agree with this and, if not, why not?


    2) Most people seem to agree that a data attribute is essential, and
a purely procedure-based solution will not work.

Do you agree with this and, if not, why not?


    3) It would be very easy to extend the wording of the ASYNCHRONOUS
attribute etc. to allow for asynchronous I/O started and completed by a
companion processor (including MPI, here).  It would also be very easy
to add a new one (say, ASYNCH_EXTERNAL), with the same properties, but
applying only to companion processor I/O.

Do you think that adding a new attribute is desirable and, if so, why?


    4) For Fortran asynchronous I/O, the processor obviously knows when
an input/output storage affector becomes and ceases to be pending.  From
the point of view of program correctness, this information is not
needed, but it might be useful for implementors.  I proposed such a
mechanism, but it seemed to confuse some people.

Do you believe that there needs to be a standardised mechanism for the
companion processor to inform the Fortran processor of such state
changes and, in either case, why or why not?



Regards,
Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
Email:  nmm1 at cam.ac.uk
Tel.:  +44 1223 334761    Fax:  +44 1223 334679