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Comment by ollien | Hacker News Reader

How do single-threaded programs benefit from a lack of GIL?

They don't benefit much from a lack of GIL, perhaps a small reduction in overhead. This feature is a first step towards being able to disable the GIL completely. It is intended to be implemented in a very conservative manner, bit by bit and so for this first step it should work for thread free code.

Disabling the GIL can unlock true multi-core parallelism for multi-threaded programs, but this requires code to be restructured for safe concurrency, which isn't that difficult it seems:

> When we found out about the “nogil” fork of Python it took a single person less than half a working day to adjust the codebase to use this fork and the results were astonishing. Now we can focus on data acquisition system development rather than fine-tuning data exchange algorithms.

https://peps.python.org/pep-0703/

>We frequently battle issues with the Python GIL at DeepMind. In many of our applications, we would like to run on the order of 50-100 threads per process. However, we often see that even with fewer than 10 threads the GIL becomes the bottleneck. To work around this problem, we sometimes use subprocesses, but in many cases the inter-process communication becomes too big of an overhead. To deal with the GIL, we usually end up translating large parts of our Python codebase into C++. This is undesirable because it makes the code less accessible to researchers.

Maybe they should look in to translating parts of their code base to Shedskin Python. It compiles (a subset of) Python to C++.

How's it different from Cython, which compiles a subset of Python to C or C++?

Shedskin has stricter typing, and about 10-100 times performance vs Cython.

Speed. Admittedly not quite as much so the way this patch is implemented, since it just short circuits the extra function calls, doesn’t omit them entirely.

Removing the GIL results in slower execution. Without the guarantees of single thread action, the interpreter needs to utilize more locks under the hood.

Not in single threaded code.

Umm, yes it does? For the longest time, Guido’s defense for the GIL was that all previous efforts resulted in an unacceptable hit to single threaded performance.

Read PEP-703 (https://peps.python.org/pep-0703/#performance) where the performance hit is currently 5-8%

That's to make it thread safe without the GIL.

If you only care about single thread there's all kinds of stuff you can do.

How to ensure there are no other threads, confidently enough that one can turn thread safety off?

From when I was reading the proposal, the idea is that until a C extension is loaded, you can assume that there are no other threads. Then when a module is loaded, by default you assume that it uses threads but modules that are thread free can indicate that using a flag, so if a module indicates it's thread free then you continue running without the thread safety features.

It could remove the locking/unlocking operations.

Doesn't removing the GIL imply adding back new, more granular locks?

Sort of, but the biased reference counting scheme they’re using avoids a lot of locks for the common case.

Removing the GIL requires more locking/unlocking operations. For single-threaded program, it's a performance penalty on average: https://peps.python.org/pep-0703/#performance

They don't.

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