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Subject: Re: [OMPI users] Progress of the asynchronous messages
From: Eugene Loh (Eugene.Loh_at_[hidden])
Date: 2008-11-06 12:52:28


vladimir marjanovic wrote:
In order to overlap communication and computation I don't want to use MPI_Wait.
Right.  One thing to keep in mind is that there are two ways of overlapping communication and computation.  One is you start a send (MPI_Isend), you do a bunch of computation while the message is being sent, and then after the message has been sent you call MPI_Wait just to clean up.  This assumes that the MPI implementation can send a message while control of the program has been returned to you.  The experts can give you the fine print, but my simple assertion is, "This doesn't usually happen."

Rather, the MPI implementation typically will send data only when your code is in some MPI call.  That's why you have to call MPI_Test periodically... or some other MPI function.
For sure the message is being decomposed into chucks and the size of chuck is probably defined by environment variable.
Maybe do you know how can I control size of chuck?
I don't.  Try running "ompi_info -a" and looking through the parameters.  For the shared-memory BTL, it's mca_btl_sm_max_frag_size.  I also see something like coll_sm_fragment_size.  Maybe look at the parameters that have "btl_openib_max" in their names.