Re: [OMPI users] Performance of MPI_Isend() worse than MPI_Send() and even MPI_Ssend()

[George Bosilca]

Your conclusion is not necessarily/always true. The MPI_Isend is just the non blocking version of the send operation. As one can imagine, a MPI_Isend + MPI_Wait increase the execution path [inside the MPI library] compared with any blocking point-to-point communication, leading to worst performances. The main interest of the MPI_Isend operation is the possible overlap of computation with communications, or the possible overlap between multiple communications.

  george.

On Oct 14, 2007, at 2:38 PM, Jeremias Spiegel wrote:

However, depending on the size of the message this might not be true. For large messages, in order to keep the memory usage on the receiver at a reasonable level, a rendezvous protocol is used. The sender [after sending a small packet] wait until the receiver confirm the message exchange (i.e. the corresponding receive operation has been posted) to send the large data. Using MPI_Isend can lead to longer execution times, as the real transfer will be delayed until the program enter in the next MPI call. In general, using non-blocking operations can improve the performance of the application, if and only if the application is carefully crafted.

Hi,

obvious I'm missing?

Regards,
Jeremias
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I'm working with Open-Mpi on an infiniband-cluster and have some strange effect when using MPI_Isend(). To my understanding this should always be quicker than MPI_Send() and MPI_Ssend(), yet in my program both MPI_Send() and MPI_Ssend() reproducably perform quicker than SSend(). Is there something

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