How do I tune large message behavior in the Open MPI v1.3 series?
The Open MPI v1.3 series generally uses the same protocols for sending long messages as described for the v1.2 series, but the MCA parameters for the RDMA Pipeline protocol were both moved and renamed (all sizes are in units of bytes): The change to move the “intermediate” fragments to the end of the message was made to better support applications that call fork(). Specifically, there is a problem in Linux when a process with registered memory calls fork(): the registered memory will physically not be available to the child process (touching memory in the child that is registered in the parent will cause a segfault or other error). Because memory is registered in units of pages, the end of a long message is likely to share the same page as other heap memory in use by the application. If this last page of the large message is registered, then all the memory in that page — to include other buffers that are not part of the long message — will not be available to the child. By moving the
