[ET Trac] [Einstein Toolkit] #2112: move CCTK_MyHost and friends into flesh

[Einstein Toolkit]

#2112: move CCTK_MyHost and friends into flesh
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  Reporter:  rhaas        |       Owner:                     
      Type:  enhancement  |      Status:  new                
  Priority:  minor        |   Milestone:                     
 Component:  Cactus       |     Version:  development version
Resolution:               |    Keywords:  Carpet PUGH driver 
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Comment (by eschnett):

 Carpet's implementation is based on comparing host names as strings. This
 involves significant infrastructure to be able to exchange strings between
 MPI processes.

 On a Blue Gene, the "host name" includes not only what Unix calls host
 name, but also the core id on which the MPI process is running. This
 defeats the purpose, and thus one needs to perform string surgery to
 extract the actual host name from the topology.

 OpenMPI provides environment variables that contain the respective
 information. These are much easier to decode than exchanging host name
 strings. However, e.g. MPICH does not provide such a mechanism.

 Finally, the MPI 3 standard, which is supported by all interesting MPI
 implementation, but not necessarily by all supercomputers of interest to
 us, provides a function {{{MPI_Comm_split_type}}}, which accepts an
 argument {{{MPI_COMM_TYPE_SHARED}}}, which then defines new per-node
 communicators. With a bit of post-processing, one can extract a relevant
 grouping:
 - each per-node communicator chooses a leader (rank 0 within that
 communicator)
 - all processes exchange the global rank of their leaders
 - these ranks can be sorted in ascending order
 - this ranking then defines an integer numbering of the nodes
 - you probably want to collect additional statistics in case the number of
 processes per node isn't uniform

-- 
Ticket URL: <https://trac.einsteintoolkit.org/ticket/2112#comment:1>
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