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Subject: [OMPI users] MPI jobs ending up in one node
From: Peter Teoh (htmldeveloper_at_[hidden])
Date: 2009-03-13 06:00:13

I saw the following problem posed somewhere - can anyone shed some
light? Thanks.

I have a cluster of 8-sock quad core systems running Redhat 5.2. It
seems that whenever I try to run multiple MPI jobs to a single node
all the jobs end up running on the same processors. For example, if I
were to submit 4 8-way jobs to a single box they all end up in CPUs 0
to 7, leaving 8 to 31 idle.

I then tried all sorts of I_MPI_PIN_PROCESSOR_LIST combinations but
short of explicitly listing out the processors at each run, they all
end up still hanging on to CPUs 0-7. Browsing through the mpiexec
script, I realise that it is doing a taskset on each run.
As my jobs are all submitted through a scheduler (PBS in this case) I
cannot possibly know at job submission time which CPUs are not used.
So is there a simple way to tell mpiexec to set the taskset affinity
correctly at each run so that it will choose only the idle processors?

Peter Teoh