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Subject: [OMPI users] Naming MPI_Spawn children
From: Jaison Paul Mulerikkal (jmulerik_at_[hidden])
Date: 2012-06-18 00:01:51


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

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.

Australian National University.