Objects: amortize uncontended named-property synchronization

[chrisbbreuer]

Summary

Split from #531 after first-class Shape attribution proved that partitioned transition publication has zero per-Shape writer yields. The end-to-end shared-realm row remains limited by repeated ordinary-property synchronization around fresh, lane-local receivers.

Related: #478, #480, #471, #493. Parent benchmark/Shape work: #531.

Measured diagnosis

For 1,000 unique add/read/delete-to-base operations per lane, the 8-lane invocation records 8,016 Shape transition requests, 8,008 misses, and 0 transition writer yields. After private-activation Environment read elision and combined named-descriptor probes, the same 8,000 scored edges still record approximately:

• 72,531 Environment binding-lock acquisitions;
• 96,403 Object property-lock acquisitions;
• 33,048 Object backing-lock acquisitions;
• 24,073 Object element-lock acquisitions.

The combined descriptor snapshot lowers property-lock acquisitions from about 22.1 to 12.1 per edge, but quick wall samples remain noisy and aggregate 1/2/4/8 throughput is not monotonic. Lower counters alone are not acceptance evidence.

Requirements

• Preserve exact OrdinaryGet/OrdinarySet/Delete semantics for accessors, attributes, prototype setters, Proxies, module namespaces, integer-indexed exotics, lazy function prototypes, host hooks, and JIT invalidation ordering.
• Preserve engine-state race freedom under the no-GIL default and concurrent marking; ordinary JavaScript program races remain permitted as documented.
• Do not use predictable hashing, disable locks globally, add benchmark-only fast paths, weaken validation, or hide work outside the claimed boundary.
• Do not modify the user-owned stacks/queues/jobs work.

Acceptance

• Committed base/variant/contended benchmark rows retain frozen checksums and equal timed work.
• Partitioned total throughput is non-decreasing across 1/2/4/8 lanes on stable repeated samples; same-parent contention remains bounded and visible.
• Exact attribution explains remaining Environment/property/backing/element acquisitions, contentions, worker CPU, allocation, GC, and RSS.
• Shared/captured/root Environments and genuinely shared Objects keep synchronization; any private fast path has a written publication/revocation proof.
• OOM rollback, bounded heap, moving/concurrent GC, same-name convergence, no-GIL TSan, threadfuzz profiles, and full units pass.
• Object + Array-prototype + Proxy test262 show no regressions against the exact parent.
• Exact-parent raw evidence and report are checked in before performance claims are published.

[chrisbbreuer]

The separately reviewable Environment work discovered during #532 is complete in #533 / 6a6b508a. #532 remains open and scoped to ordinary named-property synchronization, JIT-safe mutation/publication ordering, and its own acceptance gates; no object fast path is being claimed from the Environment result.

[chrisbbreuer]

Landed perf(object): snapshot named descriptors once (#532) on main. Ordinary named accessor/data/attribute observations now use one property-lock snapshot across [[Get]], prototype [[Set]], and receiver validation; indexed/lazy/Proxy/module/TypedArray and virtual Array/String descriptors retain explicit semantic paths. Exact 1,000-edge attribution preserved checksums while property-lock invocation deltas fell 104,376 -> 64,230 at 4 lanes and 208,688 -> 128,422 at 8 lanes (about 38.5%); Environment/backing counts were unchanged. Validation: full units 1,589/1 skipped/0 failed/0 leaked; two focused shared-property/JIT race TSan tests clean; threadfuzz 400/0; exact Object/Reflect/Proxy/Array/TypedArray/String test262 9,625 -> 9,625, 0 failures/flips. #532 remains open: monotonic end-to-end scaling and the remaining mutation/publication design are not yet proven.

[chrisbbreuer]

Landed perf(object): fuse ordinary named mutations (#532) on main.

The same-receiver ordinary data-write path now revalidates accessor/data attributes and extensibility, then publishes the write under one property_lock when no native artifact is published. Ordinary named deletion similarly removes accessor/data representations in one transaction. Published-JIT mutation keeps the pre-mutation invalidation path; Proxy/module/host, canonical-index, TypedArray/String, lazy-function-prototype, and real-Array length behavior stays on the dedicated paths.

Exact-parent (2d895edb) 8-lane attribution for 8,000 scored representative_shape_transition_fanout edges, frozen checksum 18,026,000:

• Object property-lock acquisitions: 128,422 -> 80,384 (-37.4%)
• Environment locks: 16,340 -> 16,340
• Object backing locks: 33,048 -> 33,048
• Object element locks: 24,073 -> 24,073
• Shape requests/hits/misses/writer yields: 8,016 / 8 / 8,008 / 0
• Final variant allocation: 41,503 backing allocations, 41,402 GC cells; retained RSS 45,088,768 bytes

Final-code seven-sample medians and aggregate throughput:

lanes	median ns	edges/s	checksum
1	3,584,292	278,995.1	501,500
2	6,225,166	321,276.6	1,503,500
4	14,681,083	272,459.5	5,009,000
8	30,657,959	260,943.7	18,026,000

Compared with the exact-parent medians recorded in #531, elapsed time changes by +1.35% / -2.74% / -19.75% / -23.88% at 1/2/4/8 lanes. This is a material high-lane improvement, but total throughput still declines after two lanes. The monotonic-scaling acceptance item therefore remains unchecked and #532 stays open.

Validation on the final code:

• Full units: 1,593 passed, 1 skipped, 0 failed, 0 leaked.
• Focused no-GIL TSan: fused descriptors + transparent Proxy/Array boundary; shared named deletion/compaction; optimized reads racing delete/redefine/prototype replacement — clean.
• threadfuzz: default 400/0 and verify 300/0.
• Exact-parent Object + Reflect + Proxy + Array.prototype + TypedArray + String test262: 9,354 -> 9,354, 0 failures/flips.
• OOM deletion rollback, descriptor/creation-order conversion, shared deep-shape convergence, targeted JIT retirement, and moving-GC named sidecars pass.

The exact-parent comparison caught and prevented an intermediate regression where transparent Proxy forwarding exposed real Array length to the ordinary delete path; the landed code has a dedicated regression and retains the Array exotic boundary.

[chrisbbreuer]

The amplified 8-lane representative_shape_transition_fanout profile separated another shared bottleneck from named-property synchronization: repeated serviceDebugStatement source-location cache misses contend on Context.debug_registry_lock, even with no debugger hook installed. I split that independent, bounded cache/registry problem into #541. The existing attribution rows still show fixed named-property-lock acquisitions per edge, so #532 remains open and scoped to object synchronization; no stacks, queues, jobs, Promise, or Worker lifecycle code is part of this child.

[chrisbbreuer]

#541 is complete in 59b34db6. On the 8-lane base invocation, debugger-registry acquisitions fell from 440,593 to 814 while the #532 counts remained exactly 80,384 named-property, 33,048 backing, 24,073 element, and 16,340 environment lock acquisitions. The checksum and 8,016 shape requests were unchanged.

This removes the dominant uncounted shared lock from the profile and sharpens #532: named-property acquisition is still fixed per published edge, but no longer hidden behind hundreds of thousands of debugger-registry entries. I’m continuing here without touching the separately owned stacks, queues, jobs, Promise, or Worker lifecycle areas.

[chrisbbreuer]

Instrumentation landed on main in 8df448eb (bench(object): classify property lock acquisitions (#532)). Every opt-in property-lock acquisition is now exactly one of named descriptor snapshot, ordinary receiver set, ordinary named delete, or residual “other”; the attribution validator rejects rows whose categories do not sum to the aggregate.

ReleaseFast 1,000-job invocation counts (identical for base, variant, and contended controls):

lanes	total	named snapshots	receiver sets	named deletes	other
1	10,076	6,033	1,005	1,000	2,038
2	20,120	12,055	2,007	2,000	4,058
4	40,208	24,099	4,011	4,000	8,098
8	80,384	48,187	8,019	8,000	16,178

The distribution is deterministic and scales with equal work, while contention counts remain scheduling-dependent. Named snapshots are 59.9% of acquisitions; residual metadata locks are another 20.1%. The next change will take only atomic-null fast paths for absent accessor/attribute sidecars, retaining locks for every non-null mutable map. This is an engine-state-safe optimization, not a global lock disable or benchmark recognition.

[chrisbbreuer]

Landed on main in 65df8bc8 (perf(object): elide absent sidecar locks (#532)).

The ordinary descriptor helpers now consume only an atomically published null sidecar without locking: null attributes mean the complete default descriptor, and a null accessor map means no accessor exists. Every non-null map lookup remains serialized against grow/mutation. A dedicated four-reader TSan witness covers concurrent first publication and permits only the pre-publication null/default state or the fully published state.

Exact invocation attribution:

lanes	total before	total after	other before	other after
1	10,076	9,074	2,038	1,036
2	20,120	18,116	4,058	2,054
4	40,208	36,200	8,098	4,090
8	80,384	72,368	16,178	8,162

This removes exactly one lock per scored edge (−9.97% at eight lanes). Named snapshot, receiver-set, and named-delete counts and all frozen checksums are unchanged across base, variant, and contended controls.

Seven order-balanced exact-parent pairs per lane (diagnostic, not publication evidence):

lanes	parent median	candidate median	ratio	instruction ratio	peak-RSS ratio
1	3.396 ms	3.413 ms	1.005×	1.0001×	0.9984×
2	4.635 ms	4.656 ms	1.005×	1.0079×	0.9979×
4	7.123 ms	7.240 ms	1.017×	1.0007×	0.9979×
8	35.505 ms	29.192 ms	0.822×	0.9780×	0.9996×

Low-lane changes are within noise; the eight-lane wall/cycle improvement is also noisy. Final candidate throughput is approximately 293k / 430k / 552k / 274k edges/s at 1/2/4/8 lanes, so the required monotonic scaling is still not achieved and #532 remains open.

Validation:

• full units: 1,603 passed, 1 skipped, 0 failed, 0 leaked
• null fast-path/populated fallback unit and concurrent first-publication TSan witness
• exact-parent test262: Object 3,411 → 3,411; Array 3,081 → 3,081; Proxy 311 → 311; zero failures/flips
• exact-parent benchmark identities, checksums, dependency revisions, and nominal thermal states matched

No stacks, queues, jobs, Promise, microtask, or Worker-lifecycle code changed.