| [eigen] tests taking forever when run with -j8 and OpenMP is enabled |
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- To: eigen <eigen@xxxxxxxxxxxxxxxxxxx>
- Subject: [eigen] tests taking forever when run with -j8 and OpenMP is enabled
- From: Benoit Jacob <jacob.benoit.1@xxxxxxxxx>
- Date: Sat, 3 Jul 2010 11:44:05 -0400
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No, I'm not reporting about a bug here!
But I just stumbled across something interesting. With OpenMP enabled,
when I run
ctest -j8
that is, 8 tests run simultaneously, the tests using large matrix
product take above of 2 minutes each to complete.
When I run only 1 test at a time (which is the default for ctest),
they complete very quickly.
Of course the reason is that running 8 tests at a time on my 4-core
Core i7 is a recipe for making them fight for the CPU's cache memory.
This is an illustration that setting cache sizes is ultimately
something that only the user can do; the defaults we're using can only
rely on rather unsafe assumptions about competing processes.
Benoit