[pull] master from ruby:master#1044
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Adds MMTK_HEAP_MODE=cpu, a dynamic heap-sizing policy that grows or shrinks the heap after each GC cycle to keep measured GC CPU overhead near a configurable target. The control law follows Tavakolisomeh et al., 'Heap Size Adjustment with CPU Control', MPLR '23: a sigmoid of the (averaged) GC CPU overhead error in (-inf, +inf) maps to a heap-size adjustment factor in (0.5, 1.5). Implementation lives alongside the existing 'ruby' delegated trigger in gc/mmtk/src/heap/. T_GC is wall-clock GC duration; T_APP is process CPU time delta read via clock_gettime(CLOCK_PROCESS_CPUTIME_ID), which correctly credits multi-threaded mutator parallelism. Nursery-only generational GCs are skipped so the trigger only re-sizes at full collections. Configuration: MMTK_GC_CPU_TARGET target GC CPU overhead, percent. Default 5. MMTK_GC_CPU_WINDOW number of recent cycles averaged. Default 3. The default differs from the paper's recommended 15. The paper targets ZGC, a concurrent generational collector; MMTk-Ruby currently ships stop-the-world Immix, where every percent of GC CPU also blocks the mutator. An empirical sweep of MMTK_GC_CPU_TARGET across ruby-bench (railsbench, lobsters, psych-load, liquid-render, lee) found 5-6 to be Pareto-optimal vs the existing 'ruby' heap mode: about 6 percent geomean throughput improvement at essentially equal peak RSS. Targets >=10 trade large amounts of throughput for modest RSS savings on this collector. bin/smoke-test, bin/ruby-mmtk-mode, bin/compare-heap-modes, and doc/testing-cpu-heap-mode.md are included so reviewers and future contributors can reproduce the sweep against ruby/ruby-bench. ruby/mmtk@1f223f5ad5 Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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