[ET Trac] [Einstein Toolkit] #1884: provide support for multi-model runs in thorn MPI

[Einstein Toolkit]

#1884: provide support for multi-model runs in thorn MPI
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 Reporter:  rhaas                  |       Owner:                     
     Type:  enhancement            |      Status:  new                
 Priority:  minor                  |   Milestone:                     
Component:  EinsteinToolkit thorn  |     Version:  development version
 Keywords:  MPI                    |  
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 The attached patch adds a very thin layer of code to ExternalLibraries/MPI
 to provide access to two communicators {{{MPI_Comm_World}}} and
 {{{MPI_Comm_Universe}}} which can be used for multi-model runs (or runs
 started via {{{MPI_Comm_Spawn}}}).

 The code does not itself split a communicator, it only provides routines
 to get and set the communicators.

 It supports C and F90 calls (and C++ calls through external C and call
 from F77 files since Cactus compiles those with at a F90 compiler).

 Documentation is provided in the tex file and in the grdoc comments.

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Ticket URL: <https://trac.einsteintoolkit.org/ticket/1884>
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