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… Windows A section map (CreateFileMapping + MapViewOfFile) costs a kernel section object plus soft page faults on first touch. For the small files require feeds the parser this is slower than a single ReadFile into a heap buffer. Read files up to 16 MB directly; keep the map for larger ones and as the fallback on read failure. PM_WIN32_MMAP=1 restores the old behavior for A/B testing. Measured (warm, best-of-7, this binary A/B via env var): require rspec 117 -> 109 ms median; require active_support/all ~570 -> 559 ms median. Byte-identical correctness suite output on both paths, DATA/__END__ intact.
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A section map (
CreateFileMapping+MapViewOfFile) costs a kernel section object plus soft page faults on first touch. For the small filesrequirefeeds the parser, that is slower than a singleReadFileinto a heap buffer.Read files up to 16 MB directly and keep the mapping for larger ones and as the fallback on read failure.
PM_WIN32_MMAP=1restores the old behavior for A/B testing.Measured on this VM (warm, best-of-7, same binary, A/B via the env var):
require "rspec"117 → 109 ms median;require "active_support/all"~570 → 559 ms median. Correctness suite output byte-identical on both paths,DATA/__END__intact.Windows-only. Note this touches
prism/, which is vendored from ruby/prism, so it would need to go there rather than here.