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From: Allan, Mark \(UK Filton\) (Mark.Allan2_at_[hidden])
Date: 2007-06-04 10:00:47


Hi,
 
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(!?!).
 
Does anyone have a better way of measuring communication times?
 
Thanks,
 
Mark.

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