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.
>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.
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.
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.
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.
