= Name and subject of the action

Last edition: '''[[Wikinfo(changed_ts)]]''' by '''[[Wikinfo(changed_by)]]'''

The PI is responsible to closely follow the progress of the action, 
and especially to contact NEMO project manager if 
the delay on preview (or review) are longer than the 2 weeks expected.

[[PageOutline(2, , inline)]]

== Summary

||=Action       || MPI3 collective neighbours communications instead of point to point communications                        ||
||=PI(S)        || Silvia Mocavero and Italo Epicoco                                                 ||
||=Digest       || MPI-3 provides new neighbourhood collective operations that allow to perform halo exchange with a single MPI communication call.     ||
||=Dependencies || If any                                                ||
||=Branch       || source:/NEMO/branches/{YEAR}/dev_r{REV}_{ACTION_NAME} ||
||=Previewer(s) || Mirek Andrejczuk                                                 ||
||=Reviewer(s)  || Mirek Andrejczuk                                                 ||
||=Ticket       || #XXXX                                                 ||

=== Description


This is the continuation of the work started in 2019 (HPC-12_Mocavero_mpi3). \\

MPI-3 provides new neighbourhood collective operations (i.e. MPI_Neighbor_allgather and MPI_Neighbor_alltoall) that allow to perform halo exchange with a single MPI communication call.\\

These collective communications have been integrated and tested on the NEMO code during 2019 in order to evaluate the code performance compared with the traditional point-to-point halo exchange currently implemented in NEMO. The first version of the implementation uses a cartesian topology, so it does not support 9-point stencil neither land domain exclusion and the north fold is handled as usual. The use of new collective communications has been tested on a representative kernel implementing the FCT advection scheme.\\

Preliminary tests show an improvement within a range of 18%-32% on the GYRE_PISCES configuration (with nn_GYRE=200), depending on the allocated number of cores. The output accuracy is preserved. \\

During 2020 we intend to integrate the graph topology to support the routines that use a 9-point stencil, the land domain exclusion and the north fold exchanges through MPI3 neighbourhood collective communications. \\


=== Implementation


Step 1: alignment of the dev_r11470_HPC_12_mpi3 branch with the new trunk\\

Step 2: integration of graph topology to allow each process to exchange halo with diagonal processes (when 9-point stencil is needed) or with non-neighbours processes (when land domain exclusion is activated or north fold has to be handled)\\

Step 3: replacement of point-to-point communications with collective ones within the NEMO code\\




=== Documentation updates

{{{#!box width=55em help
Using previous parts, define the main changes to be done in the NEMO literature 
(manuals, guide, web pages, …).
}}}

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== Preview 

{{{#!box width=50em info
[[Include(wiki:Developers/DevProcess#preview_)]]
}}}

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== Tests

{{{#!box width=50em info
[[Include(wiki:Developers/DevProcess#tests)]]
}}}

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== Review

{{{#!box width=50em info
[[Include(wiki:Developers/DevProcess#review)]]
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