Wednesday, May 11, 2022

The 2022 Python Language Summit: Python without the GIL

If you peruse the archives of language-summit blogs, you’ll find that one theme comes up again and again: the dream of Python without the GIL. Continuing this venerable tradition, Sam Gross kicked off the 2022 Language Summit by giving the attendees an update on nogil, a project that took the Python community by storm when it was first announced in October 2021.

The GIL, or “Global Interpreter Lock”, is the key feature of Python that prevents true concurrency between threads. This is another way of saying that it makes it difficult to do multiple tasks simultaneously while only running a single Python process. Previously the main cheerleader for removing the GIL was Larry Hastings, with his famous “Gilectomy” project. The Gilectomy project was ultimately abandoned due to the fact that it made single-threaded Python code significantly slower. But after seeing Gross’s proof-of-concept fork in October, Hastings wrote in an email to the python-dev mailing list:

Sam contacted me privately some time ago to pick my brain a little. But honestly, Sam didn’t need any helphe’d already taken the project further than I’d ever taken the Gilectomy.

The current status of nogil

Since releasing his proof-of-concept fork in October – based on an alpha version of Python 3.9 – Gross stated that he’d been working to rebase the nogil changes onto 3.9.10.

3.9 had been chosen as a target for now, as reaching a level of early adoption was important in order to judge whether the project as a whole would be viable. Early adopters would not be able to use the project effectively if third-party packages didn’t work when using nogil. There is still much broader support for Python 3.9 among third-party packages than for Python 3.10, and so Python 3.9 still made more sense as a base branch for now rather than 3.10 or main.

Gross’s other update was that he had made a change in his approach with regard to thread safety. In order to make Python work effectively without the GIL, a lot of code needs to have new locks added to it in order to ensure that it is still thread-safe. Adding new locks to existing code, however, can be very difficult, as there is potential for large slowdowns in some areas. Gross’s solution had been to invent a new kind of lock, one that is “more Gilly”.

The proposal

Gross came to the Summit with a proposal: to introduce a new compiler flag in Python 3.12 that would disable the GIL.

This is a slight change to Gross’s initial proposal from October, where he brought up the idea of a runtime flag. A compiler flag, however, reduces the risk inherent in the proposal: “You have more of a way to back out.” Additionally, using a compiler flag avoids thorny issues concerning preservation of C ABI stability. “You can’t do it with a runtime flag,” Gross explained, “But there’s precedent for changing the ABI behind a compiler flag”.


Gross’s proposal was greeted with a mix of excitement and robust questioning from the assembled core developers.

Carol Willing queried whether it might make more sense for nogil to carry on as a separate fork of CPython, rather than for Gross to aim to merge his work into the main branch of CPython itself. Gross, however, responded that this “was not a path to success”.

"A lot of the value of Python is the ecosystem, not just the language… CPython really leads the way in terms of the community moving as a block.

"Removing the GIL is a really transformative step. Most Python programs just don’t use threads at the moment if they want to run on multiple cores. If nogil is to be a success, the community as a whole has to buy into it."

– Sam Gross

Samuel Colvin, maintainer of the pydantic library, expressed disappointment that the new proposal was for a compiler flag, rather than a runtime flag. “I can’t help thinking that the level of adoption would be massively higher” if it was possible to change the setting from within Python, Colvin commented.

There was some degree of disagreement as to what the path forward from here should be. Gross appeared to be seeking a high-level decision about whether nogil was a viable way forward. The core developers in attendance, however, were reluctant to give an answer without knowing the low-level costs. “We need to lay out a plan of how to proceed,” remarked Pablo Galindo Salgado. “Just creating a PR with 20,000 lines of code changed is infeasible.”

Barry Warsaw and Itamar Ostricher both asked Gross about the impact nogil could have on third-party libraries if they wanted to support the new mode. Gross responded that the impact on many libraries would be minimal – no impact at all to a library like scikit-learn, and perhaps only 15 lines of code for numpy. Gross had received considerable interest from scientific libraries, he said, so was confident that the pressure to build separate C extensions to support nogil mode would not be unduly burdensome. Carol Willing encouraged Gross to attend scientific-computing conferences, to gather more feedback from that community.

There was also a large amount of concern from the attendees about the impact the introduction of nogil could have on CPython development. Some worried that introducing nogil mode could mean that the number of tests run in CI would have to double. Others worried that the maintenance burden would significantly increase if two separate versions of CPython were supported simultaneously: one with the GIL, and one without.

Overall, there was still a large amount of excitement and curiosity about nogil mode from the attendees. However, significant questions remain unresolved regarding the next steps for the project.