[Users] hwloc inside virtual machine

[Roland Haas]

Hello Erik, all,

> 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.
True.

> The number of threads is chosen via Simfactory. You probably need to
> update the machine description's default number of OpenMP threads.
Well actually, I was not using simfactory to run the test, only to
compile the executable. Thinking about this now, I think I was simply
not looking at this correctly. Since OMP_NUM_THREADS was not set I got 4
threads (as many as there are cores which is the OpenMP default) per
process. However since I was running 2 MPI processes I was obviously
oversubscribing. This had nothing to do with SMT or virtual machines.
The error was sitting right in front of the keyboard.

> 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.
Oh, I had no intention claiming that hwloc is doing anything wrong. A
feature request has been created.

Yours,
Roland

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