[mpich-discuss] send and recieve not working over ethernet

[D Griffiths]

I am completely new to MPI so I hope I’m not asking anything too silly. I
have two computers, one running Ubuntu and the other running Scientific
Linux. They recognise each other as hosts, I can ssh from each without a
password and have disabled any firewalls.

Although I can run a simple Hello World application, the trouble starts
when I try to use MPI_SEND and MPI_RECV. I am trying a simple program to
send an int variable from one node to the other (see source code below).

If I run the code with two processes on the same computer I get the correct
output:

dan at siriusa:~/mpich2/tutorial2$ mpiexec -n 2 ./result2
Process 0 sending number 4
Process 1 received number 4 from process 0

However, if I try to run the code using both computers, I get:

dan at siriusa:~/mpich2/tutorial2$ mpiexec -n 2 -f hosts ./result2
Process 0 sending number 4
Process 1 received number -1078477356 from process 0

My host file:

dan at siriusa:~/mpich2/tutorial2$ cat hosts
192.168.88.1
192.168.88.2

So there is some sort of problem with using the send/recieve functions over
the network. As I said I am a beginner at this, so any help will be greatly
appreciated. Thanks in advance.

Code:

#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>

 int main(int argc, char** argv) {
    // Initialize the MPI environment
    MPI_Init(NULL, NULL);
    // Find out rank, size
    int world_rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
    int world_size;
    MPI_Comm_size(MPI_COMM_WORLD, &world_size);
    // Get the name of the processor
    char processor_name[MPI_MAX_PROCESSOR_NAME];
    int name_len;
    MPI_Get_processor_name(processor_name, &name_len);

    // We are assuming at least 2 processes for this task
    if (world_size < 2) {
        fprintf(stderr, "World size must be greater than 1 for %s\n",
argv[0]);
        MPI_Abort(MPI_COMM_WORLD, 1);
    }

    int number;
    if (world_rank == 0) {
        // If we are rank 0, set the number to -1 and send it to process 1
        number = 4;
        printf("Process 0 sending number %d\n",number);
        MPI_Send(&number, 1, MPI_INT, 1, 0, MPI_COMM_WORLD);

    } else if (world_rank == 1) {
        MPI_Recv(&number, 1, MPI_INT, 0, 0, MPI_COMM_WORLD,
MPI_STATUS_IGNORE);
        printf("Process 1 received number %d from process 0\n", number);
    }

    MPI_Finalize();
}
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