[Users] hwloc inside virtual machine

Erik Schnetter schnetter at cct.lsu.edu
Sun Sep 29 23:32:25 CDT 2013

On 2013-09-30, at 0:03 , Roland Haas <roland.haas at physics.gatech.edu> wrote:

> Signed PGP part
> Hello all,
> I run a virtual machine (one of the slaves for Jenkins though I think
> it is hardly every used) inside of VirtualBox. I allocated 4 cores
> (and 4GB of RAM) for it. The host is a  Intel(R) Core(TM) i7-2600 CPU
> which supports symmetric multithreading (ht in cpuinfo). hwloc detects
> this also inside the the virtual machine and thus sets the default
> number of threads to 4, however other places in the code disagree and
> I get a warning that more threads than physical SMT units are used:
> INFO (hwloc): Extracting CPU/cache/memory properties:
>   There are 1 PUs per core (aka hardware SMT threads)
>   There are 2 threads per core (aka SMT threads used)
> WARNING: This is larger than the number of hardware SMT threads
> VirtualBox apparently does not fully support this and indeed I end up
> with 8 concurrent threads which make the test unbearably slow (I had
> thought the ML tests were stuck).
> Explicitly setting OMP_NUM_THREADS=2 fixes the problem for me. If
> possible it would be nice though if hwloc detected this situation and
> did not start as many threads (useful eg for new users who use the ET
> virtual machine to try out Cactus).
> I attach the log file output.


Unfortunately, hwloc does not choose the number of MPI processes or OpenMP threads. It seems that you are starting 2 MPI processes with 4 threads each, which are 8 threads altogether, and which over-subscribes the cores you allocated to the VM. hwloc then warns about this, which is correct.

The number of threads is chosen via Simfactory. You probably need to update the machine description's default number of OpenMP threads.

It is a good idea to let hwloc choose the number of OpenMP threads. We should introduce an option for this! Please open a feature request.


Erik Schnetter <schnetter at cct.lsu.edu>

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