Multiprocessing performance question

[Israel Brewster]

I have the following code running in python 3.7:

def create_box(x_y):
    return geometry.box(x_y[0] - 1, x_y[1],  x_y[0], x_y[1] - 1)

x_range = range(1, 1001)
y_range = range(1, 801)
x_y_range = list(itertools.product(x_range, y_range))

grid = list(map(create_box, x_y_range))

Which creates and populates an 800x1000 “grid” (represented as a flat list at this point) of “boxes”, where a box is a shapely.geometry.box(). This takes about 10 seconds to run.

Looking at this, I am thinking it would lend itself well to parallelization. Since the box at each “coordinate" is independent of all others, it seems I should be able to simply split the list up into chunks and process each chunk in parallel on a separate core. To that end, I created a multiprocessing pool:

pool = multiprocessing.Pool()

And then called pool.map() rather than just “map”. Somewhat to my surprise, the execution time was virtually identical. Given the simplicity of my code, and the presumable ease with which it should be able to be parallelized, what could explain why the performance did not improve at all when moving from the single-process map() to the multiprocess map()?

I am aware that in python3, the map function doesn’t actually produce a result until needed, but that’s why I wrapped everything in calls to list(), at least for testing.

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Israel Brewster
Software Engineer
Alaska Volcano Observatory 
Geophysical Institute - UAF 
2156 Koyukuk Drive 
Fairbanks AK 99775-7320
Work: 907-474-5172
cell:  907-328-9145