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fix(heap): stranded large spans after adoption; perf: segment scrub, large cache (2.2.4) - #47
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…large cache Fixes (docs/plans/huge-free-retention.md, a consumer report): - adopt_segment retired ONE dead large span per adopted segment and left the rest carved forever (a large span is never queued, so no collect reaches it). A model loaded on a thread that then exited, and dropped on another, kept ~0.6 of its size resident whatever purge_delay said. Now every dead span is retired; the walk resumes at the merged span. Adoption also no longer re-counts and releases the empty segment that retire_span had just parked. - huge_free purges a dead huge block (> 32 MiB) before its chunks rejoin the arena when purging is on. It stayed resident: the consumer's unexplained ~150 MB was Whisper small's 152 MiB embedding. Performance (callgrind; memset counted as real instructions): - scrub_recycled: a recycled chunk zeroes the owner table and the slots read before a carve, not the whole 45 KB header; slots are zeroed at their first carve. Huge segments skip 511 strided slot writes and wait_no_remote_in_flight scans slot 1 only. 64 MiB malloc+free 8,959 -> 824 Ir; thread life -12.4 %; Rust thread-pool workload -15.0 %. - Per-heap large-span cache (<= 4 MiB, purging off, lone frees only, so a teardown run never leaves a span to be refilled out of order): 2 MiB malloc/free 293 -> 138 Ir. - Multi-chunk arena claim: one bitmap load per word, a one-word run claimed with one masked fetch_or. find_arena replaces three copies of the scan. Every opscan op unchanged or better; perl/sqlite/Endless Sky allocator Ir flat. Refuted and recorded in docs/opps.md: segment_map RMW batching (+15), masked chunk release (+9), flush-on-miss in large_alloc (+3 on every generic op). Validated: workspace --all-features, linux-gates (42 suites), embedded matrix 15/15, miri gate, semgrep, mimalloc-bench sweep 19/19, churn, real-world programs byte-identical across ra/mi/sys, alloc-eval checks on 8 game traces, downstream corpus (spacedb x3, rusty_alloc_default, rusty_zstd). Unsafe census 973 -> 977 (UNSAFE.md); wasm baseline 20321 -> 20873 B gz (deliberate). corpus/linux-gates.sh no longer hard-codes a /mnt/c path. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
The heartbeat in Heap::malloc_generic_body called options::deferred_free while malloc_generic's `&mut self` was live and protected. A hook that allocates (the openheimer test's) or frees re-entered the same heap through a raw pointer under that borrow: undefined behaviour under Stacked Borrows. Miri has reported it on every main run since 2.2.3, which kept the CI badge red. The heartbeat now only marks the hook DUE (Heap::deferred_due, set when options::deferred_registered() peeks a hook). Every allocation entry in alloc.rs fires a due hook BEFORE forming &mut Heap (fire_deferred_if_due); malloc_slow keeps both arms in tail position, jumping to the cold malloc_slow_deferred when due. Upstream fires the hook at the start of the generic path; here it runs at the start of the next entry, within the hook's "called occasionally, with a heartbeat" contract. Cost, measured: big 91 -> 94, mixed 92.04 -> 94.26 Ir/op; perl/sqlite allocator Ir +0.16 % / +0.22 %. The first shape (a call inside malloc_slow) gave it a frame and cost big +9; refuted, recorded. Miri: tests/openheimer.rs now passes, 37/37 (it aborted at the first UB before, so the suites after it had not run under Miri in CI since 2.2.3). Unsafe census 977 -> 981 (UNSAFE.md). Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
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Intended release: 2.2.4.
Fixes (consumer report,
docs/plans/huge-free-retention.md)adopt_segmentretired one dead large span per adopted segment and left the rest carved for the life of the process. A model loaded on a thread that exited and dropped on another kept ~0.6 of its size resident, whateverpurge_delaysaid (probe: 1.62 -> 1.01 models). Adoption also stopped re-counting and releasing the empty segmentretire_spanhad just parked.Performance (callgrind, memset counted as real instructions; vs 2.2.3)
Every opscan op unchanged or better (big 92 -> 91, mixed 92.70 -> 92.04). perl/sqlite allocator Ir flat; Endless Sky start-up output identical, allocator Ir flat. wasm +361 B gz (baseline moved deliberately).
Validated locally: workspace
--all-features177/177,corpus/linux-gates.sh42 suites, embedded matrix 15/15, miri gate, semgrep, mimalloc-bench sweep 19/19, churn 5/5, real-world programs byte-identical across ra/mi/sys, alloc-eval checks on 8 game traces, downstream corpus (spacedb x3, rusty_alloc_default, rusty_zstd; rusty_maplibre is the known 2.0.0 break).Refutations recorded in
docs/opps.md. Unsafe census 973 -> 977, each site inUNSAFE.md.🤖 Generated with Claude Code