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Subject: Re: [hwloc-users] hwloc-ps output - how to verify process binding on the core level?
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2011-02-14 08:26:51

On Feb 13, 2011, at 4:07 AM, Brice Goglin wrote:

>> $ mpirun -np 4 hwloc-bind socket:0.core:0-3 ./test
>> 1. Does hwloc-bind map the processes *sequentially* on *successive* cores of the socket?
> No. Each hwloc-bind command in the mpirun above doesn't know that there are other hwloc-bind instances on the same machine. All of them bind their process to all cores in the first socket.

To further underscore this point, mpirun launched 4 copies of:

    hwloc-bind socket:0.core:0-3 ./test

Which means that all 4 processes bound to exactly the same thing.

If you want each process to bind to a *different* set of PU's, then you have two choices:

1. See Open MPI 1.5.1's mpirun(1) man page. There's new affinity options in the OMPI 1.5 series, such as --bind-to-core and --bind-to-socket. We wrote them up in the FAQ, too.

2. Write a wrapper script that looks at the Open MPI environment variables OMPI_COMM_WORLD_RANK, or OMPI_COMM_WORLD_LOCAL_RANK, or OMPI_COMM_WORLD_NODE_RANK and decides how to invoke hwloc-bind. For example, something like this:

    mpirun -np 4 ./test

where is:


if test "$OMPI_COMM_WORLD_RANK" = "0"; then
hwloc-bind $bind_string $*

Something like that.

Jeff Squyres
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