I was originally using MPI_Send paired with MPI_Irecv, but I recently found out that MPI_Send may block until the message is received. So, I'm changing to MPI_Isend and I need to send the same message to N different processors. Assuming the buffer will get destroyed later, should I have a for loop with MPI_Isend and MPI_Wait in the loop, or should I make an array of requests and have only MPI_Isend in the loop with MPI_Waitall after the loop?
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2 Answers
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For distributing the same buffer to "n" remote ranks, MPI_Bcast is the "obvious" choice. Unless you have some "overwhelming" reason to avoid MPI_Bcast, it would be advisable to use it. In general, MPI_Bcast is very well optimized by all the major MPI implementations.
If blocking is an issue, MPI 3.0 Standard introduced MPI_IBcast along with other non-blocking collectives. The initial implementation of non-blocking collectives appears to be "naive" and built as wrappers to non-blocking point-to-point routines (e.g. MPI_IBcast is implemented as a wrapper around calls to MPI_ISend and MPI_IRecv). The implementations are likely to improve in quality over the next year or two - depending partly on the speed of adoption by the MPI application developer community.
MPI_Send will "block" until the send buffer can be safely re-used by the calling application. Nothing is guaranteed about the state of the corresponding MPI_[I]Recv's.
If you need non-blocking, then the best advice would be to call MPI_ISend in a loop. Alternatively, persistent requests could be used with MPI_Start or MPI_Startall if this is a message pattern that will be repeated over the course of the program. Persistent communication requests.
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Since its the same message, you should be able to use MPI_Bcast. You'll just have to create a new communicator to define a subgroup of processes.
