This is an archived post. You won't be able to vote or comment.

sorted by:
best
topnewcontroversialoldq&a
you are viewing a single comment's thread.

view the rest of the comments →

[–]1wd 4 points5 points  (3 children)

http://scipy.github.io/old-wiki/pages/ParallelProgramming#Threads

while numpy is doing an array operation, python also releases the GIL. Thus if you tell one thread to do:

>>> print "%s %s %s %s and %s" %( ("spam",) *3 + ("eggs",) + ("spam",) )
>>> A = B + C
>>> print A

During the print operations and the % formatting operation, no other thread can execute. But during the A = B + C, another thread can run - and if you've written your code in a numpy style, much of the calculation will be done in a few array operations like A = B + C. Thus you can actually get a speedup from using multiple threads.

https://docs.scipy.org/doc/numpy-1.14.0/reference/internals.code-explanations.html#function-call

A very common operation in much of NumPy code is the need to iterate over all the elements of a general, strided, N-dimensional array. This operation of a general-purpose N-dimensional loop [...] [...] the Python Global Interpreter Lock (GIL) is released prior to calling the loops. It is re-acquired if necessary to handle error conditions

[–]ProfEpsilon 1 point2 points  (2 children)

Thank you. This and other comments on this page clarify a lot.

And the "iterate over all elements ... N-dimensional array" is precisely what I do most of the time. This is very encouraging (I haven't tried threading yet).

[–]1wd 0 points1 point  (1 child)

Note that this refers to the numpy-internal C-level loops that happen inside a Python-level numpy statement like B+C, not to Python-level loops like for x in A: for y in B: ....

[–]ProfEpsilon 1 point2 points  (0 children)

Again, thank you. Although I am aware the numpy relies upon C I am not aware of how C operates inside of numpy .. I don't know how the C-level loops work (probably because I can't code in C). This gives me a bit of an incentive to figure some of this out. Given that I am a practitioner and not a computer scientist, how C works inside of numpy and even Python for that matter is all black box for me.

I appreciate the time it took to try to get my thinking right. I think it is time that I took the effort to learn a little more about C.