[Users] hwloc inside virtual machine

[Roland Haas]

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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.

Yours,
Roland

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