CODEWITHSUNDEEP
mpi
AHM200
AHM200

Reputation: 145

MPI:point-to-point blocking (MPI_Send & MPI_Recv)

I'm a little bit confused about MPI point-to-point blocking modes.
now i have the following code.

y = id;
j = id - 1;
if(j<0) j = p -1;

k = id + 1;
if(k>p-1) k = 0;

MPI_Recv(&x, 1, MPI_INT, k, 0, MPI_COMM_WORLD, &status);
MPI_Send(&y, 1, MPI_INT, j, 0, MPI_COMM_WORLD);

z = x + y;

That will be blocked as MPI_Recv is a blocking function, but when i change the order to call MPI_Send first everything goes ok and i do not know the reason.
Thanks in advance and sorry for my English.

Upvotes: 0

Views: 320

Answers (1)

Gilles Gouaillardet
Gilles Gouaillardet

Reputation: 8395

MPI_Send() blocks until the send buffer can be reused.

from a pragmatic point of view, MPI_Send() returns immediatly if the message is short enough, and otherwise, blocks until a matching MPI_Recv() is posted. this is a general behavior, but you should not rely on that.

in this case, your best option is to use a single MPI_Sendrecv() call instead of two blocking send/recv calls

an other option is to MPI_Irecv(); MPI_Send(); MPI_Wait();

Upvotes: 2

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