On Jul 13, 2004, at 1:55 AM, x z wrote:
> Does LAM-MPI optimize a Send whose destination is the local node (and
> the corresponding Recv whose source is the local node)? How?
> That is, if I have:
> if (myID==0)
> for (i=1;i<100;i++)
> MPI_Send(&x, ..., tag, dest, ...)
> else
> for (i=1;i<100;i++)
> MPI_Recv(&y, ..., ANY_TAG,source, ...)
> and in some cases, the send is to the local node, the recv is from
> local node.
> This may happen if the data distribution is irregular and the source
> and destination is not known until run time. If it is local, there is
> no need to copy x to the buffer and then copy the buffer to y. But
> can the LAM-MPI compiler/runtime detect that, and make approopriate
> action, like converting the Send/Recv to simply y=x?
If you call MPI_Send to your rank in MPI_COMM_WORLD and later call
MPI_Recv from your rank in MPI_COMM_WORLD, LAM will deadlock (this
being an erroneous program). However, if you had non-blocking receives
preposted or non-blocking sends or something like that, yes, LAM will
directly copy from the sending buffer to the receiving buffer.
If you are sending between two processes on the same node, LAM will use
shared memory for communication (if support for shared memory is built
into your copy of LAM). In that case, the send buffer is moved into a
temporary shared space and then into the receiving buffer.
Hope this helps,
Brian
--
Brian Barrett
LAM/MPI developer and all around nice guy
Have a LAM/MPI day: http://www.lam-mpi.org/
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