| [eigen] Re: tests taking forever when run with -j8 and OpenMP is enabled |
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- To: eigen <eigen@xxxxxxxxxxxxxxxxxxx>
- Subject: [eigen] Re: tests taking forever when run with -j8 and OpenMP is enabled
- From: Benoit Jacob <jacob.benoit.1@xxxxxxxxx>
- Date: Sat, 3 Jul 2010 11:47:25 -0400
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Ah, just one thing. It remains that I only get this phenomenon when
using OpenMP. Is it really the case that with OpenMP enabled we rely
on the availability of more cache memory? That would be a bug.
Benoit
2010/7/3 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
> 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
>