mpi: Add basic2 mode

[georgebisbas]

Preallocated buffers using MPIMsg.
An MPIMsgEnriched version for send/recv, will follow

[codecov]

Codecov Report

Attention: Patch coverage is 98.77301% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.30%. Comparing base (34dba05) to head (68631a5).

Files with missing lines	Patch %	Lines
devito/mpi/routines.py	98.69%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #2307    +/-   ##
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  Coverage   87.29%   87.30%            
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  Files         238      238            
  Lines       46063    46191   +128     
  Branches     4080     4098    +18     
========================================
+ Hits        40211    40325   +114     
- Misses       5162     5175    +13     
- Partials      690      691     +1     

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[FabioLuporini]

why do you need a haloid here ? never had to use it. What makes this mode require it?

[georgebisbas]

Similarly to the same method in OverlapHalo. Each call to sendrecv has its own message indexedpointer

[FabioLuporini]

why not :

for i, halo in enumerate(self.halos):

like we have in the other message types?

[georgebisbas]

I followed the basic style, which does not have yet a method to cleanup the redundant halos as in diag2 for example:
e.g.:

        # Only retain the halos required by the Diag scheme
        halos = sorted(i for i in hse.halos if isinstance(i.dim, tuple))

I tried this but did not manage to get it working nicely.

[FabioLuporini]

I would drop basic here and below... or overlap perhaps. These tests can be quite expensive

[EdCaunt]

This could use some whitespace to make it more readable

[georgebisbas]

agree, added a docstring as well

[EdCaunt]

Is documentation required somewhere to explain what the new Basic2 mode does?

[georgebisbas]

Is documentation required somewhere to explain what the new Basic2 mode does?

you are right, will add!

[georgebisbas]

@EdCaunt I just remember that I have some enhanced docs on another branch, so this can be fine for the moment!
THe branch is this:

https://github.com/devitocodes/devito/tree/bench-refresh_II

[georgebisbas]

Superseded by #2995, which brings this branch up to date with main and adds a basic3 mode on top (a further-optimized basic2 that reads ranks/offsets off the message struct inside a single loop instead of unrolling one sendrecv() call per direction). Closing this one in favor of #2995.