[Numpy-discussion] numarray and SMP

Christopher T King squirrel at WPI.EDU
Thu Jul 1 13:37:13 CDT 2004

(I originally posted this in comp.lang.python and was redirected here)

In a quest to speed up numarray computations, I tried writing a 'threaded 
array' class for use on SMP systems that would distribute its workload 
across the processors. I hit a snag when I found out that since the Python 
interpreter is not reentrant, this effectively disables parallel 
processing in Python. I've come up with two solutions to this problem, 
both involving numarray's C functions that perform the actual vector 

1) Surround the C vector operations with Py_BEGIN_ALLOW_THREADS and 
   Py_END_ALLOW_THREADS, thus allowing the vector operations (which don't 
   access Python structures) to run in parallel with the interpreter.
   Python glue code would take care of threading and locking.

2) Move the parallelization into the C vector functions themselves. This 
   would likely get poorer performance (a chain of vector operations
   couldn't be combined into one threaded operation).

I'd much rather do #1, but will playing around with the interpreter state 
like that cause any problems?

Update from original posting:

I've partially implemented method #1 for Float64s. Running on four 2.4GHz
Xeons (possibly two with hyperthreading?), I get about a 30% speedup while
dividing 10 million Float64s, but a small (<10%) slowdown doing addition
or multiplication. The operation was repeated 100 times, with the threads
created outside of the loop (i.e. the threads weren't recreated for each
iteration). Is there really that much overhead in Python? I can post the
code I'm using and the numarray patch if it's requested.

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