[mpi3-coll] Nonblocking collectives standard draft

[Christian Siebert]

Hi Torsten et al.,

> the new version is in the wiki [1] and at [2]. Please check!
Much better, thanks!

I did a second reading and found several but only minor issues with the 
text. If you send me the TeX sources, I can send you my proposed 
corrections/improvements directly as a patch file (diff).

Some more things for discussion:

- p. 49 l. 41: MPI has the concept of error handlers. I don't see a 
reason why we should forbid ("return an error code") its usage. My 
suggestion is to simply delete the two sentences (from "If the call ..." 
to "... cases.").

- p. 61 l 33-38: Should we really keep this Advice for implementors 
(same result for MPI_IREDUCE when called with same arguments)? For the 
blocking variant, there stands this argument of reproducibility vs. 
performance. However, the non-blocking variant could be optimized much 
better (e.g., might handle bad process skew) and we should consider to 
weaken this semantic restriction to allow this better performance.

- another argument _for_ MPI_REQUEST_FREE is the existence of 
intercommunicators where all processes (i.e., in group A and in group B) 
have to call the collective but only part of them might be involved 
(e.g., for MPI_IBCAST all but the root process in group A don't really 
participate in the operation - they pass MPI_PROC_NULL as root 
argument). So having a lightweight MPI_Wait alternative might not be too 
bad ...

The NBC examples are quite buggy (missing MPI_STATUS in MPI_Recv, 
missing semicolons, duplicate usage of same req-argument -> bad Waitall, 
...) - but as with the text, I can send you corrected versions in the diff.

Best regards,
    Christian

-- 
Christian Siebert, Dipl.-Inf.               Research Associate

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