Free threaded - #23437
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June 23, 2026 09:04
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In its current iteration this change increases runtime by about 4x but increases core-engagement by about +5 on my 8 core machine. I'm investigating why. |
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Closed in favor of #23477 |
tobni
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Aug 9, 2026
Switches Pants' own runtime to free-threaded CPython and makes the engine and Pants' Python code thread-safe without the GIL. This has been validated as running without error in my companies CI, and nets great speedups. Most important callouts: - Keep a persistent per-thread PyThreadState in the executor. Without this, every `Python::attach` cycle tears down the thread state, and on 3.14t that abandons the thread's mimalloc heaps. Created per tokio thread, torn down in `on_thread_stop` (blocking threads are recycled, so the state must not leak per thread creation). - Makes run-scoped internals thread-safe: rule_visitor, exception_sink, logging, build_root, run_tracker. I used #23437 as a launching point. I have on speculation set this as shipping in 2.34.x, as I believe 2.33 is too far along the dev-cycles. But I also do not see any reason to delay further. Blocks pantsbuild/scie-pants#541 Edit: This branch now serves as proof of green CI of the accumulation of changes, and will be rebased as its parts are accepted incrementally: - #23487 - #23488 - #23489 - #23490 - #23491 - #23492 - #23497 - #23514 Edit2: Now that all pre-requisites are in, this PR contains CI/CD changes and the new PANTS_FREE_THREADED option. Current proposal for consumer UX is | Consumer | `pants.toml#[GLOBAL].pants_free_threaded` | Resolves to | Bootstrap warning | |---|---|---|---| | **scie-pants - not updated** | absent / `false` | `cp314` (GIL 3.14) | - | | **scie-pants - not updated** | `true` | `cp314` (GIL 3.14) | **"update scie-pants"**. Old launcher can't honor the field | | **scie-pants - updated** | absent (default) | `cp314t` (free-threaded) | - | | **scie-pants - updated** | `false` | `cp314` (GIL 3.14) | - (escape hatch) | | **scie-pants - updated** | `true` | `cp314t` (free-threaded) | - | | **Fat scie - `…-cp314`** | `true` | GIL 3.14 | **"no effect on a fat scie; download the binary you want"** | | **Fat scie - `…-cp314t`** | `false` | free-threaded | **"no effect on a fat scie; download the binary you want"** | This PR is coupled to pantsbuild/scie-pants#541.
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yeet