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Subject: [OMPI users] mpi_paffinity_alone and Nehalem SMT
From: Noam Bernstein (noam.bernstein_at_[hidden])
Date: 2009-10-23 11:30:34

Hi all - we have a new Nehalem cluster (dual quad core), and SMT is
enabled in the BIOS (for now). I do want to do benchmarking on our
applications, obviously, but I was also wondering what happens if I just
set the number of slots to 8 in SGE, and just let things run. It
how will things be laid out if I do "mpirun --mca mpi_paffinity_alone

1. Will it be clever enough to schedule each process on its own core,
    and only resort to the second SMT virtual core if I go over 8
    processes per node (dual quad core)?
2. If it's not that clever, can I pass a rank file?
3. If I do have to do that, what is the mapping between core numbers
     and processor/core/SMT virtual cores?