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Tracing and visualizing the Python GIL with perf and VizTracer

264 points · submitted Jan 2021 by u/maartenbreddels · Tutorial · archived

Tracing and visualizing the Python GIL with perf and VizTracer

9 comments · 97% upvoted · source · other discussions

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flying-sheep 6 points · Jan 2021

Nice! A convenient alternative (but maybe not as powerful?):

I recently discovered py-spy for profiling python.

py-spy record --format=speedscope -o profile.speedscope.json -- python my-script.py

And then visualize it via speedscope.app

maartenbreddels [OP] 3 points · Jan 2021

I think profiling and tracing (what I do here) are complementary. I use py-spy a lot, but it only gives you a 'statistical' view of where time is spent or how much the GIL is held. To gain a deep understanding of the flow of your program and the GIL in a multithreaded environment I needed a tool as I presented, and unfortunately had to make it myself :)

flying-sheep 3 points · Jan 2021

Thank you! I know the feeling: “This should exist, why doesn’t it yet? Sigh.” *rolls up sleeves*

chirisalex 2 points · Jan 2021

Helpful article

[deleted] 2 points · Jan 2021

Good stuff, thanks

apockill 2 points · Jan 2021

This looks awesome! How does it handle cythonized libraries and/or C bindings such as tensorflow?

maartenbreddels [OP] 1 points · Jan 2021

Thanks! You will see the call to the C library (this is what CPython and VizTracer do), and you should see if the GIL is being released, like with NumPy, one of the examples in the article.

apockill 1 points · Jan 2021

That's fantastic. Actually, my application for this is to find naughty C bindings that don't release the GIL. I have a big application that's heavily multithreaded with many C bindings (think gstreamer + tensorflow + OpenVINO), and something is causing a lockup somewhere. It's quite frustrating!

I'll give this tutorial go.

maartenbreddels [OP] 1 points · Jan 2021

Indeed, that's one of the things I'd like to automate (and I'm very close now), feel free to contact me, we could maybe collaborate on this https://github.com/maartenbreddels/per4m