HI, I'm running openmpi on Rackspace cloud over Internet using MPI_Spawn. IT means, I run the parent on my PC and the children on Rackspace cloud machines. Rackspace provides direct IP addresses of the machines (no NAT), that is why it is possible.
Now, there is a communicator involving only the children and some communications involve only communication between children (on Rackspace cloud, in this scenario). When we conducted experiments, we experienced more than expected delays in this operation - communication between children alone. My assumption is that openMPI is looking at the direct IP addresses at the hostfile and try to communicate between Rackspace children over Internet. What I would want/expect is the Rackspace children communicate between themselves internally, using the internal Rackspace hostnames. Rackspace provide internal IP addresses. But if I use that in the hostfile at my home PC, the parent wont be able to access the children (there is a communicator involving parent and children). Can I anyway tell openMPI to look into the internal IP addresses of Rackspace machines (another hostfile, may be) for the sub-group (communicator) involving Rackspace children? In that case we will get performance improvement, I guess. Thanks in advance for your valuable suggestions. Jaison Australian National University.
