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Merged
MichaReiser merged 3 commits into from
Aug 9, 2026
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Use mimalloc v3 #27586

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266ca88
Update mimalloc version to 0.1.52
chirizxc Aug 7, 2026
3f9147a
Change mimalloc feature to 'win_direct_tls'
chirizxc Aug 7, 2026
b5594f4
Update Cargo.toml
chirizxc Aug 8, 2026
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2 changes: 1 addition & 1 deletion Cargo.toml
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Expand Up @@ -134,7 +134,7 @@ lsp-server = { version = "0.10.0" }
lsp-types = { package = "gen-lsp-types", version = "0.11.0", features = ["url"] }
matchit = { version = "0.9.0" }
memchr = { version = "2.7.1" }
mimalloc = { version = "0.1.49", features = ["v2"] }
mimalloc = { version = "0.1.52", features = ["win_direct_tls"] }

@chirizxc chirizxc Aug 7, 2026 •
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win_direct_tls enable MI_WIN_DIRECT_TLS

MI_WIN_DIRECT_TLS - Use only direct TLS slots on Windows to avoid extra tests in the malloc fast path (only works if the program uses less than 64 TlsAlloc'd slots in total)

@MichaReiser MichaReiser Aug 8, 2026

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Can you say more about the motivation for enabling this option and why you think it's correct? I'm not familiar with TLS but codex tells me that the number of TLS can be environment specific, meaning, it can be out of Ruff's control.

@chirizxc chirizxc Aug 8, 2026

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mimalloc makes only two calls to TlsAlloc(): _mi_theap_default_slot and _mi_theap_cached_slot, however, what matters here isn't the calls themselves, but which indices have already been occupied by the process prior to mimalloc

In other words, mimalloc calls must fall within the range 0..63.

The first 64 slots are fast because they are physically embedded directly into the TEB (Thread Environment Block) structure at a fixed offset, and accessing them requires only a single read instruction via a segment register (gs/fs), without any additional pointer dereferencing.

Windows guarantees a minimum of 64 TLS indices per process, and modern versions of Windows can have up to 1,088.

@chirizxc chirizxc Aug 8, 2026

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microsoft/mimalloc#1359

Maybe we shouldn't enable this feature, since it's random

@MichaReiser MichaReiser Aug 9, 2026

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I'd prefer not to enable it, unless benchmark differences are significant. If they are, we should then probably dig a little deeper on when it's okay to enable this optimization

natord = { version = "1.0.9" }
notify = { version = "8.0.0" }
ordermap = { version = "1.0.0" }
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