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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