I'm new to this list and wonder if anyone can help. I'm trying to measure communication time between parallel processes using openmpi. As an example I might be running on 4 dual core processors (8 processes in total). I was hoping that communication using shared memory (comms between dual cores on the same chip) would be faster than that over the network. To measure communication time I'm sending a block of data to each process (from each process) using a blocking send, and am timing how long it takes. I repeat this 50 times (for example) and take the average time. The code is something like:

for(int i=0;i<numProcs;i++)
    for(int j=0;j<numProcs;j++)
      if(i!=j)
         {
           // // // i is the sending proc to j, others wait
             double time = 0.0;
             for(int kk=0; kk<50; kk++)
             {
                  if(i==my_rank)
                  {
                      double start = MPI::Wtime();
                      MPI::COMM_WORLD.Send(&sendData[0],dataSize,MPI::DOUBLE,j,i);
                      double end = MPI::Wtime();
                      time+=(end-start);
                  }
                  if(j==my_rank)
                  {
                      MPI::COMM_WORLD.Recv(&recvData[0],dataSize,MPI::DOUBLE,i,i);
                  }
             }
             if(i==my_rank)
                  out << i << " " << j << " " << time/50.0 << std::endl;
             MPI::COMM_WORLD.Barrier();
         }

The problem I am having is that I'm not noticing any appreciable difference in communication times between shared memory and network protocols. I expected shared memory to be faster(!?!).