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GeezerVison is showing github.com — gh-158660: Avoid locking for new lists with preallocated space by ZeroIntensity · Pull Request #160156 · python/cpython

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@ZeroIntensity

@ZeroIntensity ZeroIntensity commented Oct 22, 2026 •
edited by bedevere-app Bot
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The idea here is that when a list is created with PyList_New(n) where n is greater than zero, the list cannot be shared across threads, because it's unusable as an object (due to the NULL pointers that it would try to access upon use), and therefore we can skip locking.

This was generated by AI without my review; I'm putting this up as a proof of concept that we can use for benchmarking and such.

  • Issue: PyList_SetItem is ~2.7x slower per item on the free-threaded build #158660

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@ZeroIntensity
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@ZeroIntensity ZeroIntensity added sprint topic-free-threading labels Oct 22, 2026
@github-project-automation github-project-automation Bot added this to Sprint Oct 22, 2026
@github-project-automation github-project-automation Bot moved this to Todo in Sprint Oct 22, 2026
@bedevere-app bedevere-app Bot mentioned this pull request Oct 22, 2026
PyList_SetItem is ~2.7x slower per item on the free-threaded build #158660
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@ZeroIntensity ZeroIntensity changed the title gh-158660: Avoid locking for newly created lists gh-158660: Avoid locking for new lists with preallocated space Oct 22, 2026
@ZeroIntensity

ZeroIntensity commented Oct 22, 2026

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Benchmarks, using Nathan's fill_list script from the issue:

  • This PR, free-threaded: 3.4 ns/item
  • Main, free-threaded: 4.6 ns/item
  • This PR, GIL-icious: 2.5 ns/item
  • Main, GIL-icious: 2.5 ns/item

So, with this fix, PyList_SetItem is now about 35% slower than the default build, in contrast to about 84% slower without it. I'll look into ways to optimize this further tomorrow.
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@ZeroIntensity
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