Dramatic slowdown for -np 2

[J Harrop]

This may be an MPI problem, but I'm not sure so I'm posting it here and 
comp.parallel.mpi

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.

Cheers,

John Harrop