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• Issue: Mimalloc differences from upstream #113141

[DinoV]

https://github.com/facebookexperimental/free-threading-benchmarking/actions/runs/39860993693

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Based on an analysis of python -c "import re", with help from a script
written with Claude and run under gdb.

The effect of reducing the page size differs by object size.

Scope	AS-IS pages	AS-IS resident	AS-IS live	AS-IS util	TO-BE pages	TO-BE resident	TO-BE live	TO-BE util
All heaps	149	5480 KiB	4106 KiB	75 %	309	4964 KiB	4102 KiB	83 %
MEM	46	1668 KiB	1146 KiB	69 %	77	1420 KiB	1142 KiB	80 %
OBJECT	45	1604 KiB	1083 KiB	68 %	88	1344 KiB	1083 KiB	81 %
GC	47	2108 KiB	1841 KiB	87 %	133	2088 KiB	1841 KiB	88 %
GC_PRE	11	100 KiB	35 KiB	35 %	11	112 KiB	35 KiB	32 %
<=256 B	68	2540 KiB	2228 KiB	88 %	174	2556 KiB	2228 KiB	87 %
256 B-2 KiB	47	1636 KiB	1346 KiB	82 %	109	1608 KiB	1346 KiB	84 %
2-64 KiB	30	1088 KiB	315 KiB	29 %	22	584 KiB	311 KiB	53 %
>64 KiB	4	216 KiB	216 KiB	100 %	4	216 KiB	216 KiB	100 %

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cc @nascheme

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@DinoV @nascheme @mpage
According to the benchmark results, this change reduces memory usage by 3% compared to the baseline, with no performance regression. Although there is no comparison with the default build, the results do confirm a reduction in memory usage.

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cc @daanx

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We may need to tune a 32-bit system, but I prefer to pass at this time.

[corona10]

@nascheme @DinoV
The results from Tools/gcbench from Thomas show that reducing the page size from 64 KiB to 16 KiB increases GC time by 6–8%, since each collection visits four times as many pages. GC-bound pathological cases are 5–8% slower, while steady-state workloads and pyperformance show no change in total runtime.

Peak RSS is generally unchanged on these large-heap tests, although bigcollect/container shows an 11% increase. That case would be worth a separate follow-up if we decide to merge this PR.

This PR aims to reduce the memory overhead relative to the default build from the current 18% to 15%, as measured by pyperformance. I think we should also compare against the default build using the GC benchmarks to make sure both the performance and memory overheads remain within the guardrails outlined in PEP 779. If both criteria are met, I would like to propose merging this PR.

I’ll share the results once I’ve run the GC benchmarks against the default build.