I apologise if this is a naive question, but I'm new to this world of
beowulfs.
I'm using C++/mpi, to get a feel for communication costs I ran tests using
mpptest and my own programs.
For 2 processor blocking calls, mpptest indicates a latency of about 30
microseconds.
However when I measure communication times in my own program using a loop as
follows....
MPI_Barrier(MPI_COMM_WORLD);
start = MPI_Wtime();
for (unsigned t=1; t<=5000; t++)
{
if (my_rank==0)
{
MPI_Send(data, size, MPI_INT, 1, tag, MPI_COMM_WORLD);
}
else
{
MPI_Recv(data, size, MPI_INT, 0, tag, MPI_COMM_WORLD, &status);
}
}
end = MPI_Wtime();
for size>=4, I get a latency of about 30 microseconds as expected, however for
size<4, communication costs increase massively, and latency now appears to be
1ms!
Firstly, I assume this isn't normal?
Secondly, can anyone suggest what's going on, or where I can go for more
information.
Many thanks.
We're using mpich.
Processors are Intel(R) Xeon(TM) CPU 3.60GHz.
Interconnects are Dell PowerConnect 5324 24-port gigabit switches.
_______________________________________________
Beowulf mailing list, [email protected]
To change your subscription (digest mode or unsubscribe) visit
http://www.beowulf.org/mailman/listinfo/beowulf
