[Mpi-forum] Fortran change for discussion

Jeremiah Willcock jewillco at osl.iu.edu
Tue Sep 11 06:08:29 CDT 2012

Maybe the issue is that I'm not a Fortran programmer, but the copying 
language has always been confusing to me (its effect on what user-defined 
datatypes need to look like is another thing I asked about before).  Am I 
to understand the interface as something like this?

    For purposes of interpreting derived datatypes and other information
    in communication operations, array slices are treated as if they are
    stored contiguously in memory.  I.e., it is "as if" some kind of
    magic memory remapping occurs by the MPI implementation that maps the
    user's non-contiguous array into a contiguous one without copying the
    elements (in either direction).

-- Jeremiah Willcock

On Tue, 11 Sep 2012, Jeff Squyres (jsquyres) wrote:

> FWIW, we chose the copying language not because of the c language, but rather because we have to explain that the MPI data type is applied, and that seemed far easier to explain against a (virtual) contiguous buffer.
> Sent from my phone. No type good.
> On Sep 11, 2012, at 3:38 AM, "N.M. Maclaren" <nmm1 at cam.ac.uk> wrote:
>> On Sep 11 2012, Jeff Squyres wrote:
>>> You explain the situation in Fortran-eese, but the current text is an attempt to explain it in MPI-eese (i.e., in terms of copying). I think the MPI-reader-friendly text should be used.
>> Sort-of, I agree.  The problem with the current description is precisely
>> that it uses copying rather than aliasing as a basis, but I assume that
>> is for C programmers.  There is definitely a problem in trying to explain
>> something in C-friendly terms that is conceptually outside C's rudimentary
>> array model.
>>>> If the current text is kept, I think the clarification is important.
>>> Ok.
>> The other possibility is to stress that the copying can occur at any
>> time between the initial call and the Wait.  That is, after, all precisely
>> what asynchronous means in this context!
>> Regards,
>> Nick.
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