[Cython] [Re] OpenMP thread private variable not recognized (bug report + discussion)

[Leon Bottou]

On Tue, 12 Aug 2014 14:26:31, Sturla Molden wrote:
> Cython does not do an error here:[...
> - i is recognized as private
> - r is recognized as reduction
> - w is (correctly) recognized as shared

Not according to the documentation.
http://docs.cython.org/src/userguide/parallelism.html documentation for
cython.parallel.parallel says "A contained prange will be a worksharing loop
that is not parallel, so any variable assigned to in the parallel section is
also private to the prange. Variables that are private in the parallel block
are unavailable after the parallel block.".  Variable w is such a variable.

Furthermore, if cython is correct, why does GCC report an error on the
cython generated C code?  

My point here is that there is a bug because (a) cython does not behave as
documented, and (b) it generates invalid C code despite not reporting an
error.

> Personally I prefer to avoid OpenMP and just use Python threads and an
> internal function (closure) or an internal class. If you start to use
OpenMP,
> Apple's libdispatch ("GCD"), Intel TBB, or Intel clikplus, you will soon
discover
> that they are all variations over the same theme: a thread pool and a
closure.

I am making heavy uses of OpenBlas which also uses OpenMP.
Using the same queue manager prevents lots of CPU provisioning problem.
Using multiple queue managers in the same code does not work as well because
they are not aware of what the other one is doing.


- L.