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by Brian Schrader

Python 3.5 and multitasking

Posted on Fri, 18 Sep 2015 at 08:19 AM

Python has a long and complicated history with multitasking. Strictly speaking the GIL prevents a lot of classical attempts at multithreading for arguably simpler application implementation. Therefore true multitasking in Python is available with multiprocessing, but the community tends to avoid it like the plague (for not undeserved reasons). With the release of Python 3.5, the community has very firmly cemented it's stance that multitasking should be done cooperatively using coroutines and generators, but as of yet the actual process of writing multitasking code is still very complicated and I don't really understand why.

Back when I was doing iOS development, I got quite familiar with Apple's Grand Central Dispatch API and I guess it spoiled me. The syntax for kicking off calculations to separate threads, or returning to the main thread for come quick UI updates is so simple. By comparison, Python's approach seems overly complex. Granted, the two languages/environments are used for different things.

Regardless, the newest version of Python (v3.5) adds support for a new asynchronous function syntax. In short, this means that the old Python generator-based coroutines have been codified into a full on language feature. If you're like me, then you probably aren't to familiar with generator-based coroutines, so this new syntax brings with it a host of new concepts to grok.

    async def some_function():
        pass

With the new release, I thought I'd do some testing to see, not only how the new syntax works, but how well these new coroutines perform various tasks. At work I need to manipulate fairly large files (~2-5GB of text) and I thought that I might be able to get some performance improvements by switching to using coroutines in Python 3.5. Currently, most of my code is plain-old sequential, but I do have some auxiliary files that I generate during my analysis, and I'd like to be able to write out without stopping the main set of calculations to wait for a slow-ass disk. At my first glance, coroutines seemed perfect.

Time to test the new thing

Right off the bat, the new syntax threw me into a torrent of confusion. I won't get into my issues here, but suffice to say that having a coroutine await some other coroutine which awaits some other coroutine is likely a way to get yourself confused fairly quickly.

One thing I wanted was to be able to use the new async/await syntax without using asyncio. In my mind I shouldn't have to use a library to make the basic syntax work, but that doesn't seem to be the case, and in the end I ended up having to use the event loop functions to get my code working. Once I discovered the secret of coroutines hidden in the documentation (and hidden quite well I'd say), I had some code that could finally be run asynchronously.

My goal was to have a few long-running tasks that would normally block, and kick them off into a coroutine that would spin away while I did other things. This is not how Python's coroutines work, and that becomes clear right away.

In short: Everything is asynchronous, or nothing is.

This mantra makes it really difficult to make Python code simple and serial and still kick side work off to the coroutines. I understand that Python can only do one thing at once, but the design of the async/await system seems really punishing for little gain.

Results

Once I finally got the whole thing working, I devised some performance tests. My 3 sets of test covered CPU bound, I/O bound, and a mix of CPU and I/O bound computations, and each test would attempt all 5 of the usual Python multitasking techniques:

  • Serial execution (as a control)
  • Classical multithreading (which Python's GIL basically kills)
  • Classical multiprocessing
  • Python 3.5 Coroutines (with a threading pool, the default)
  • Python 3.5 Coroutines (with a process pool)

I've published the test suite I used over on GitHub if you want to check it out for yourself. Overall, the results were really surprising.

CPU bound I/O bound Both CPU/IO bound
Test Time Time Time
Serial 7.804 1.924s 4.789s
Coro Multi 4.739s 0.951s 13.574s
Coro Thread 8.221s 1.97s 7.724s
Multiprocess 4.488s 1.320s 4.513s
Threading 8.251s 1.807s 4.976s

While for I/O bound computations the coroutines improved performance quite nicely with a multiprocessing pool behind it (even over conventional multiprocessing), the moment you introduce CPU bound operations into the mix everything goes to hell. In the case of using threading (which because of the GIL was doomed before it began) it made the test ~9% slower. While the pure CPU bound case did actually get somewhat faster (~40%) this is an unrealistic benchmark since eventually you'd need to write something to disk. Once CPU and I/O operations get mixed together, for some reason, the execution time went up ~300%. This is certainly not the kind of behavior that I was expecting and it made me rethink my tests. On a second look everything seemed to be in order, but if you see something glaring out, please let me know.

One of the flaws that I see with my tests is that they don't use asyncio's native IO lib to write to the files, instead they wrap the synchronous open and write calls from the standard library. I did this for two reasons, the first being that I cannot, for the life of me, figure out how to use asyncio to do local file IO and not a network request, but maybe I'm just an idiot. The second reason was because that's not how I'd imagine that I'd have to write my code if I wanted to use this new functionality. I wrote this code how I would expect that I would have to in Python. To me, this is an important reason why I use Python. It's simple and expressive. I'd argue that, while asyncio is amazingly powerful, simple and expressive it isn't.

asyncio and the new syntax in Python 3.5 have all of the pieces needed for very simple multitasking, but they both stop short of making them easy to use for newcomers and typical users. Actually, thinking about it, they both feel a lot like urllib: extremely powerful, but fairly complex for most use cases. Maybe, with Python 3.5, it'll finally be time for Async for humans.

Checkout my tests for yourself →

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