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并行计算的例子程序 环境需要lam/Linux 可从lam-mpi.org找到

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This file is part of the LAM/MPI software package.  For license
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LAM/MPI source distribution.

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The alltoall program is a canonical example of MPI usage.  It sends a
message from each MPI rank to each other MPI rank.

Note that we carefully distinguish between the order of sending and
receiving.  It is *not* sufficient to have a single loop where each
rank does a simple "MPI_Send(...); MPI_Recv(...);" -- doing so can
lead to deadlock.  This is a subtlety in the MPI standard --
MPI_Send() *may* or *may not* block.  In LAM, MPI_Send() will block if
a message is too long; it will wait until the message has been started
to be received on the target rank.  Under that size, MPI_Send() will
[most likely] return immediately regardless of what the target rank is
doing.

Note that this is not a LAM-specific issue; every MPI implementation
is coded in this way -- that MPI_Send tries to complete immediately,
but above a certain threshold (usually related a combination of
message size and the cumulative size of unreceived messages to that
target), MPI_Send may block until the target rank starts receiving. */

Finally, notice that this program is actually a poor example of the
all-to-all communication pattern -- there is some degree of
serialization in the sending of messages.  For example, rank N has to
wait for rank 0 to send to it before it can continue.  That is, rank 0
causes a daisy-chain of reactions that allow the rest of the ranks to
continue -- each rank will not "start" until rank 0 contacts it.

Use "make" to compile this example.  Make will use mpicc to compile
the program:

        mpicc -o alltoall alltoall.c 

This program must be run on 2 or more MPI processes.  For example, if
your LAM universe has more than two CPUs listed, you can simply use:

        mpirun C alltoall