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Subject: [OMPI users] Oversubscribing a subset of a machine's cores
From: Torje Henriksen (torjeh_at_[hidden])
Date: 2008-02-07 06:07:07


Hi,

I have a slightly odd problem, that you might not think is important
at all. Anyways, here it goes:

I'm using a single eight-core machine. I want to oversubscribe four of
the cores and leave the other four idle. My approach is to make a
hostfile:

localhost slot=4 # shouldn't this limit the core count to 4?

and run the command:

$mpirun -np 8 --hostfile my_hostfile ./my_mpiprog

or the command:

$mpirun -np 8 --host localhost,localhost,localhost,localhost ./
my_mpiprog

Still, all eight cores are being used. I can see why you would want to
use all cores, and I can see that oversubscribing a sub-set of the
cores might seem silly. My question is, is it possible to do what I
want to do without hacking the open mpi code?

Guess I just wanted to know is there is a solution I overlooked before
I start hacking like a madman :)

Thanks

Torje Henriksen