Dramatic slowdown for -np 2
siegert at sfu.ca
Tue Feb 12 14:21:23 EST 2002
On Mon, Feb 11, 2002 at 09:58:55PM -0800, J Harrop wrote:
> This may be an MPI problem, but I'm not sure so I'm posting it here and
> We are developing an application on a four node Beowulf while we wait for
> the remaining nodes to arrive. Speed-up has been close to predicted with
> -np 3 and 4 in a master/slave mode. But when I run at -np 2 the speed
> drops to approximately 1/4 of the original serial application. (On 4 nodes
> - that is 1 master and 3 slaves, we get about 2.5 times speed-up relative
> to the original application.) All runs produce the same answer.
> In the MPI code we have basic SEND, RECV, BCAST and REDUCE - nothing
> fancy. Does anyone know if any of these or other MPI functions run into
> problems with one to one master slave ration? Any other enlightenment
> would be welcome.
If you are using mpich: compile using
mpif90 -mpilog ... (or mpicc -mpilog ...)
and then run your program as usual with "mpirun -np 2 ..."
Look at the generated log file with jumpshot (you may have to
convert the log file first: "clog2slog file.clog"). Then do the same with
"mpirun -np 3 ..." This should give you an idea in which part of the
communication pattern your job gets stuck.
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