MVAPICH2 RDMA-based communication without explicit PUT/GET use?

Question

My cluster utilizes MVAPICH2 over Infiniband FDR and and I am considering the use of RDMA for my simulations. I am aware of the MPI_Put and MPI_Get calls for explicitly invoking RDMA operations, however I would like to know if this is the only way to use RDMA within MPI.

My current implementation involves channel semantics (send/receive) for communication, along with MPI_Reduce and MPI_Gatherv. I know that MVAPICH2 has configuration paramaters that can be used to enable RDMA. If a program using MPI has send/receive calls and RDMA is enabled, does MPI automatically convert from channel semantics over to memory semantics (put/get) or is the explicit use of MPI_Put and MPI_Get the only method for implementing RDMA in MVAPICH2?

MPI_Send requires a corresponding MPI_Receive, whether they are blocking or non-blocking doesnt matter as a send must be met with a receive. RDMA does not have this requirement and instead only implements either MPI_Put (write to remote memory) or MPI_Get (read from remote memory). I am trying to find out if enabling rdma while still using send and receives, allows MVAPICH2 to somehow automatically convert the send/receives into the appropriate rdma call.

Answer 1

If MVAPICH2 has been built with the correct options, it will use RDMA for all MPI operations including MPI_Send and MPI_Recv on supported hardware, which includes InfiniBand. So, you do not need to use MPI_Put/Get to take advantage of RDMA-capable hardware. In fact, using MPI_Send/Recv might be faster because they are often better optimized.

MPI libraries use various designs to translate MPI_Send/Recv operations to RDMA semantics. The details can be found in the literature.