I encountered an error with send/recv pairs on AMD64. The first send
completes successfully, but subsequent send/recv pairs hang. I also
believe that MPI_Bcast also hangs under simmilar conditions, but have
not created an explicit testcase for this (it happened in the code I
was working on). Both machines are running AMD64 processors.
The problem only manifests when the tasks are on seperate machines.
The problem does not occur on x86 (32-bit) machines.