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On Linux the runner pins a benchmark from outside with taskset, but on Windows the benchmark pinned itself in harness-common.rb. That loaded Fiddle and kept kernel32 open in the process being measured, only covered harnesses that load harness-common.rb, and left harness-spinel unpinned even though pin? selects Spinel. A child process inherits the affinity mask of its parent when it is created, so the runner now narrows its own mask around Process.spawn and restores it right after. The child starts on the CPU it would be given with taskset. The priority class is not inherited, so the runner sets HIGH_PRIORITY_CLASS and the power throttling opt-out on the child by its PID while Ruby boots, before the harness takes any timings. If pinning fails, the benchmark runs unpinned with a warning as before. RUBY_BENCH_PIN_CPU is gone, and the harness no longer has Windows-specific code. The load_fiddle fix from ruby#549 is kept.
k0kubun
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October 13, 2026 23:04
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Follow-up to #549. On Linux, the runner pins a benchmark from outside with `taskset`, but on Windows the benchmark pinned itself in `harness-common.rb`. That loaded Fiddle and kept kernel32 open in the process being measured. It also only covered harnesses that load `harness-common.rb`, so `harness-spinel` was left unpinned even though `pin?` selects Spinel.
A child process inherits its parent's affinity mask when it is created. So the runner now narrows its own mask around `Process.spawn` and restores it right after, and the child starts on the CPU `taskset` would give it. The priority class is not inherited, so the runner sets `HIGH_PRIORITY_CLASS` and the power-throttling opt-out on the child by PID while Ruby boots, before the harness takes any timings. If pinning fails, the benchmark runs unpinned with a warning, as before.
`RUBY_BENCH_PIN_CPU` is gone, and the harness no longer has Windows-specific code. The `load_fiddle` fix from #549 is kept.