Rendezvous or Eager Protocol?

Is your program sending many long, unexpected messages? Sun MPI offers message rendezvous, which requires a receiver to echo a ready signal to the sender before data transmission can begin. This can improve performance for the case of a pair of processes that communicate with a different order for their sends as for their receives, since receive-side buffering would be reduced. To allow rendezvous behavior for long messages, set the MPI_EAGERONLY environment variable:

% setenv MPI_EAGERONLY 0 

The threshold message size for rendezvous behavior can be tuned independently for each protocol with MPI_SHM_RENDVSIZE, MPI_TCP_RENDVSIZE, and MPI_RSM_RENDVSIZE.

Rendezvous will often degrade performance by coupling senders to receivers. Also, for some "unsafe" codes, it can produce deadlock.