MPI_Send/Recv in Open MPI implements both protocols and chooses based on the message size which one to use.
You can use the mca parameter "btl_sm_eager_limit" to modify the behaviour.
Here the corresponding info obtained from the ompi_info tool:
"btl_sm_eager_limit" (current value: <4096>, data source: default value)
Maximum size (in bytes) of "short" messages (must be >= 1)
High Performance Computing Center Stuttgart (HLRS)
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Von: "Siddhartha Jana" <siddharthajana24_at_[hidden]>
An: "OpenMPI users mailing list" <users_at_[hidden]>
Gesendet: Samstag, 14. Dezember 2013 13:44:12
Betreff: [OMPI users] Configuration for rendezvous and eager protocols: two-sided comm
In OpenMPI, are MPI_Send, MPI_Recv (and friends) implemented using rendezvous protocol or eager protocol?
If both, is there a way to choose one or the other during runtime or while building the library?
If there is a threshold of the message size that dictates the protocol to be used, is there a way I can alter that threshold value?
If different protocols were used for different versions of the library in the past, could someone please direct me to the exact version numbers of the implementations that used one or the other protocol?
Thanks a lot,
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