Unable to use all cores with mpirun

Question

I\'m testing a simple MPI program on my desktop (Ubuntu LTS 16.04/ Intel® Core™ i3-6100U CPU @ 2.30GHz × 4/ gcc 4.8.5 /OpenMPI 3.0.0) and mpirun won\'t let me use all of the cor

Answer 1

Your processor has 4 hyperthreads but only 2 cores (see the specs here).

By default, Open MPI does not run more than one MPI task per core. You can have Open MPI run up to one MPI task per hyperthread with the following option

mpirun --use-hwthread-cpus ...

FWIW

The command you mentioned reports the number of hyperthreads.

A better way to figure out the topology of a machine is via the lstopo command from the hwloc package.

MPI tasks are not bound on cores nor threads on OS X, so if you are running on a Mac, the --oversubscribe -np 4 would lead to the same result.