[Numpy-discussion] Windows, blas, atlas and dlls

Dag Sverre Seljebotn d.s.seljebotn@astro.uio...
Mon Feb 18 10:28:36 CST 2013

On 02/18/2013 05:26 PM, rif wrote:
> I have no answer to the question, but I was curious as to why directly
> calling the cblas would be 10x-20x slower in the first place.  That
> seems surprising, although I'm just learning about python numerics.

The statement was that directly (on the Cython level) calling cblas is 
10x-20x slower than going through the (slow) SciPy wrapper routines. 
That makes a lot of sense if the matrices are smalle nough.

Dag Sverre

> On Mon, Feb 18, 2013 at 7:38 AM, Sergio Callegari
> <sergio.callegari@gmail.com <mailto:sergio.callegari@gmail.com>> wrote:
>     Hi,
>     I have a project that includes a cython script which in turn does
>     some direct
>     access to a couple of cblas functions. This is necessary, since some
>     matrix
>     multiplications need to be done inside a tight loop that gets called
>     thousands
>     of times. Speedup wrt calling scipy.linalg.blas.cblas routines is
>     10x to 20x.
>     Now, all this is very nice on linux where the setup script can
>     assure that the
>     cython code gets linked with the atlas dynamic library, which is the
>     same
>     library that numpy and scipy link to on this platform.
>     However, I now have trouble in providing easy ways to use my project in
>     windows. All the free windows distros for scientific python that I have
>     looked at (python(x,y) and winpython) seem to repackage the windows
>     version of
>     numpy/scipy as it is built in the numpy/scipy development sites.
>     These appear
>     to statically link atlas inside some pyd files.  So I get no atlas
>     to link
>     against, and I have to ship an additional pre-built atlas with my
>     project.
>     All this seems somehow inconvenient.
>     In the end, when my code runs, due to static linking I get 3
>     replicas of 2
>     slightly different atlas libs in memory. One coming with
>     _dotblas.pyd in numpy,
>     another one with cblas.pyd or fblas.pyd in scipy. And the last one
>     as the one
>     shipped in my code.
>     Would it be possible to have a win distro of scipy which provides some
>     pre built atlas dlls, and to have numpy and scipy dynamically link
>     to them?
>     This would save memory and also provide a decent blas to link to for
>     things
>     done in cython. But I believe there must be some problem since the
>     scipy site
>     says
>     "IMPORTANT: NumPy and SciPy in Windows can currently only make use
>     of CBLAS and
>     LAPACK as static libraries - DLLs are not supported."
>     Can someone please explain why or link to an explanation?
>     Unfortunately, not having a good, prebuilt and cheap blas
>     implementation in
>     windows is really striking me as a severe limitation, since you
>     loose the
>     ability to prototype in python/scipy and then move to C or Cython
>     the major
>     bottlenecks to achieve speed.
>     Many thanks in advance!
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