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Subject: Re: [OMPI users] OpenMP + OpenMPI
From: Durga Choudhury (dpchoudh_at_[hidden])
Date: 2007-12-06 09:54:22

Automatically striping large messages across multiple NICs is certainly a
very nice feature; I was not aware that OpenMPI does this transparently. (I
wonder if other MPI implementations do this or not). However, I have the
following concern: Since the communication over an ethernet NIC is most
likely over IP, does it take into account the route cost when striping
messages? For example, host A and B in the MPD ring might be connected via
two NICs, one direct and one via an intermediate router, or one with a large
bandwidth and another with a small bandwidth. Does OpenMPI send a smaller
chunk of data over a route with a higher cost?

Because of this concern, I think the channel bonding approach someone else
suggested is more preferable; all these details will be taken care of at the
hardware level instead of at the IP level.

On Dec 6, 2007 9:42 AM, Jeff Squyres <jsquyres_at_[hidden]> wrote:

> Wow, that's quite a .sig. :-)
> Open MPI will automatically stripe large messages across however many
> NICs you have. So you shouldn't need to use multiple threads.
> The threading support in the OMPI v1.2 series is broken; it's not
> worth using. There's a big warning in configure when you enable it. :-)
> On Dec 5, 2007, at 9:57 PM, Tee Wen Kai wrote:
> > Hi everyone,
> >
> > I have installed openmpi-1.2.3. My system has two ethernet ports.
> > Thus, I am trying to make use of both ports to speed up the
> > communication process by using openmp to split into two threads.
> > However, this implementation always cause error. Then I realized
> > that I need to build openmpi using --enable-mpi-threads and use
> > MPI_Init_thread to initialize. But, the initialization always return
> > MPI_THREAD_SINGLE no matter what value I set. Using ompi_info|grep
> > Thread, it shows that thread support has already been activated.
> > Thus, I seek your help to teach me what other configurations I need
> > to set in order to use multi-threads and what are the parameters to
> > include in mpirun in order to use the two ethernet ports.
> >
> > Thank you very much.
> >
> > Regards,
> > Tee
> >
> >
> >
> > _________________________________________________
> >
> >
> >
> > Many of us spend our time wishing for things we could have if we
> > didn't spend half our time wishing.
> >
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> >
> --
> Jeff Squyres
> Cisco Systems
> _______________________________________________
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