[Mpi3-subsetting] MPI3: Proposal to remove PMPI-Requirement

[Rainer Keller]

Dear Alexander, dear all,
at the previous Chicago meeting, some of us (Rich Graham, Jeff Squyres, Hubert 
Ritzdorf) have been discussing about the MPI-Standard's requirement to 
provide PMPI-functions for each MPI-Call into the library (except MPI_Wtime, 
MPI_Wtick).

This is in my eyes a limitation: there could be some gains for the common case 
of large-scale applications:
 - Not using PMPI-based tools (dynamically loaded in)
 - Not using MPI_PROC_NULL
 - Not using MPI_STATUS_IGNORE

So, if this proposal would fit Your definition of MPI3-Subsets, I would like 
to discuss the removal of the PMPI-function requirement, allowing inlining of 
MPI-functions and, based on that, pre-processor hints, e.g. 
MPI_HINT_NO_THREADS or MPI_HINT_NO_ANY_SOURCE, so the compiler might be able 
to eliminate several if-statements in the fast-path, allowing inlining, 
allowing dead code elimination.
(of course, there is a whole lot of further issues involved, here -- please 
see the last section in the document).

I have included some timings with NetPipe on Open MPI using these hints 
(further information section 4 -- Ompi Patch availble upon request, as its 
work in progress). Please take the timings with a grain of salt. I would like 
to get better figures on a more well-defined cluster environment.

Any comments would be welcome.

With best regards,
Rainer

-- 
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Dipl.-Inf. Rainer Keller   http://www.hlrs.de/people/keller
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