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Subject: Re: [OMPI users] [torqueusers] which eth interface does mpi use by default when torque supplies it with a hostfile?
From: George Wm Turner (turnerg_at_[hidden])
Date: 2010-05-28 17:45:10

Open MPI is very aggressive about looking for and using any tcp
communications device it can find. In your case it will use both the
10.0.. network and the 192.168.. network at the same time. Open MPI
does not pay attention to the hosts names for the communications
channel. You want to do something like:

mpirun --mca btl_tcp_if_include lo,eth1

Please see the Open MPI FAQ:

Tuning the run-time characteristics of MPI TCP communications

7. How do I tell Open MPI which TCP networks to use?

george wm turner
high performance systems
812 855 5156

On May 28, 2010, at 5:29 PM, Rahul Nabar wrote:

> Each of our servers has twin eth cards: 1GigE and 10GigE. How does
> openmpi decide which card to use while sending messages on? One of the
> cards is on a 10.0. IP address subnet whereas the other cards are on a
> 192.168 adress subnet. Can I select one or the other by specifying the
> --host option with the correct IP addresses?
> How does it select the default though? Frequently I call mpirun from
> within a PBS wrapper and then there is no explicit --host directive.
> (I think PBS somehow communicates to mpirun what the assigned hostfile
> is) In such a case though, which interface will mpirun use?
> --
> Rahul
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