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Subject: Re: [OMPI users] Running Open MPI on Ethernet
From: Ralph Castain (rhc_at_[hidden])
Date: 2008-08-08 08:37:12


Best guess, without seeing the mpirun cmd line, is that all the procs
are executing on one computer. In that scenario, we default to using
shared memory, so there would be no traffic going across the network.

On Aug 8, 2008, at 6:11 AM, Rayne wrote:

> Hi all,
> I'm running openmpi-1.2.6, and my computer is connected to a
> Ethernet network. I have no experience in setting up a network that
> supports parallel computing using MPI before, nor do I know much
> about networking. So please excuse me if my questions seem too
> simple or silly.
>
> What I simply did was install the Open MPI package on my computer,
> and executed the following simple code to test if I can successfully
> run MPI programs:
>
> #include <stdio.h>
> #include <mpi.h>
>
> int main(int argc, char** argv)
> {
> int rank, size, i = 1, j, total = 1, total2;
> MPI_Status status;
> MPI_Init(&argc, &argv);
> MPI_Comm_rank(MPI_COMM_WORLD, &rank);
> MPI_Comm_size(MPI_COMM_WORLD, &size);
> printf("Hello... %d of %d processors\n", rank, size);
> if (rank != 0)
> MPI_Send(&i, 1, MPI_INT, 0, 1, MPI_COMM_WORLD);
> else
> {
> for (j = 1 ; j < size ; j++)
> {
> MPI_Recv(&i, 1, MPI_INT, j, 1, MPI_COMM_WORLD, &status);
> total += i;
> }
> printf("%d Calculated total: %d\n", rank, total);
> }
> MPI_Reduce(&i, &total2, 1, MPI_INT, MPI_SUM, 0, MPI_COMM_WORLD);
> if (!rank)
> printf("%d Reduced total: %d\n", rank, total2);
> MPI_Finalize();
> return 0;
> }
>
> I got the correct output, i.e.
>
> Hello... 0 of 3 processors
> Hello... 1 of 3 processors
> Hello... 2 of 3 processors
> 0 Calculated total: 3
> 0 Reduced total: 3
>
> together with the warning message about OpenIB and UDAPL, which
> after some Googling, I was able to resolve using "-mca btl
> ^openib,udapl".
>
> I was trying to see if my MPI program is really running on the
> Ethernet network, so I deactivated the network, but the program
> still works. Then I unplugged the network cable and still the
> program works. Then I ran Wireshark, which is a kind of packet
> sniffer program, and ran the MPI program again to see if the Send/
> Receive functions work right, as I thought if the program is sending/
> receiving the variable i, something should show up in Wireshark.
> However, no packets are captured by Wireshark.
>
> Can someone help me understand what is going on here, as it seems
> like the program is working when it shouldn't?
>
> Thank you.
>
> Regards,
> Rayne
>
>
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