Avoid using multiple processes for downloads #375
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@bemoody please could you fix the conflict? |
The standard multiprocessing module is used to distribute a task to multiple processes, which is useful when doing heavy computation due to the limitations of CPython; however, making this work is dependent on the ability to fork processes or else to kludgily emulate forking on systems that don't support it. In particular, it tends to cause problems on Windows unless you are very scrupulous about how you write your program. Therefore, as a rule, the multiprocessing module shouldn't be used by general-purpose libraries, and should only be invoked by application programmers themselves (who are in a position to guarantee that imports have no side effects, the main script uses 'if __name__ == "__main__"', etc.) However, downloading a file isn't a CPU-bound task, it's an I/O-bound task, and therefore for this purpose, parallel threads should work as well or even better than parallel processes. The multiprocessing.dummy module provides the same API as the multiprocessing module, but uses threads instead of processes, so it should be safe to use in a general-purpose library.
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@lbugnon does this solve the windows downloading issue? |
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multiprocessingis messy; it's a pretty nifty tool but requires you to write your entire application with multiprocessing in mind.For simply downloading files in parallel, there's little reason to use processes rather than threads.
This should, though I haven't tested it, solve the issues mentioned in pull #330 and probably also issue #306. I can't guarantee that this will solve every problem, but it should be strictly better than what we have now.