[Numpy-discussion] Using multiprocessing (shared memory) with numpy array multiplication

[Christopher Barker]

Sturla Molden wrote:
> IMHO, trying to beat Intel or AMD performance library developers with 
> Python, NumPy and multiprocessing is just silly. Nothing we do with 
> array operator * and np.sum is ever going to compare with BLAS functions 
> from these libraries.

I think the issue got confused -- the OP was not looking to speed up a 
matrix multiply, but rather to speed up a whole bunch of independent 
matrix multiplies.

> Sometimes we need a little bit more course-grained parallelism. Then 
> it's time to think about Python threads and releasing the GIL or use 
> OpenMP with C or Fortran.
> 
> multiprocessing is the last tool to think about. It is mostly 
> approproate for 'embarassingly parallel' paradigms, and certainly not 
> the tool for parallel matrix multiplication.

I think this was, in fact, an embarrassingly parallel example. But when 
the OP put it on two processors, it was slower than one -- hence his 
question.

I got the same result on my machine as well.

I'm not sure he tried python threads, that may be worth a shot.

It would also would be great if someone that actually understands this 
stuff could look at his code and explain why the slowdown occurs (hint, 
hint!)

-CHB





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