Do you have different IB subnet IDs?  That would be the only way for Open MPI 
to tell the two IB subnets apart.



> On Jun 16, 2015, at 1:25 PM, Tim Miller <btamil...@gmail.com> wrote:
> 
> Hi All,
> 
> We have a set of nodes which are all connected via InfiniBand, but all are 
> mutually connected. For example, nodes 1-32 are connected to IB switch A and 
> 33-64 are connected to switch B, but there is no IB connection between 
> switches A and B. However, all nodes are mutually routable over TCP.
> 
> What we'd like to do is tell OpenMPI to use IB when communicating amongst 
> nodes 1-32 or 33-64, but to use TCP whenever a node in the set 1-32 needs to 
> talk to another node in the set 33-64 or vice-versa. We've written an 
> application in such a way that we can confine most of the bandwidth and 
> latency sensitive operations to within groups of 32 nodes, but members of the 
> two groups do have to communicate infrequently via MPI.
> 
> Is there any way to tell OpenMPI to use IB within an IB-connected group and 
> TCP for inter-group communications? Obvoiously, we would need to tell OpenMPI 
> the membership of the two groups. If there's no such functionality, would it 
> be a difficult thing to hack in (I'd be glad to give it a try myself, but I'm 
> not that familiar with the codebase, so a couple of pointers would be 
> helpful, or a note saying I'm crazy for trying).
> 
> Thanks,
> Tim
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Jeff Squyres
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