Lucas,
the number of MPI tasks started by mpirun is either
- explicitly passed via the command line (e.g. mpirun -np 2306 ...)
- equals to the number of available slots, and this value is either
a) retrieved from the resource manager (such as a SLURM allocation)
b) explicitly set in a machine file (e.g. mpirun -machinefile
<machinefile> ...) or the command line
(e.g. mpirun --hosts host0:96,host1:96 ...)
c) if none of the above is set, the number of detected cores on the
system
Cheers,
Gilles
On Tue, Sep 13, 2022 at 9:23 PM Lucas Chaloyard via users <
users@lists.open-mpi.org> wrote:
> Hello,
>
> I'm working as a research intern in a lab where we're studying
> virtualization.
> And I've been working with several benchmarks using OpenMPI 4.1.0 (ASKAP,
> GPAW and Incompact3d from Phrononix Test suite).
>
> To briefly explain my experiments, I'm running those benchmarks on several
> virtual machines using different topologies.
> During one experiment I've been comparing those two topologies :
> - Topology1 : 96 vCPUS divided in 96 sockets containing 1 threads
> - Topology2 : 96 vCPUS divided in 48 sockets containing 2 threads (usage
> of hyperthreading)
>
> For the ASKAP Benchmark :
> - While using Topology2, 2306 processes will be created by the application
> to do its work.
> - While using Topology1, 4612 processes will be created by the application
> to do its work.
> This is also happening when running GPAW and Incompact3d benchmarks.
>
> What I've been wondering (and looking for) is, does OpenMPI take into
> account the topology, and reduce the number of processes create to execute
> its work in order to avoid the usage of hyperthreading ?
> Or is it something done by the application itself ?
>
> I was looking at the source code, and I've been trying to find how and
> when are filled the information about the MPI_COMM_WORLD communicator, to
> see if the 'num_procs' field depends on the topology, but I didn't have any
> chance for now.
>
> Respectfully, Chaloyard Lucas.
>
