perf(kv-router): dense rank slots with atomic coverage bitsets

[PeaBrane]

Status (2026-10-09). John Thomson is taking these over. Merge order: #15609 → #15610 → #15611. The per-PR end-to-end tables are in point 5 of the reviewer block below. Follow-ups: #16000 (dead-leaf memory retention) and #15614 (opt-in race soak, test-only). This layer is end-to-end neutral at the default 4 event threads. Its gains are the overload lookup tail and the fix for main's equal-size-skip overcount.

For reviewers

The stack-wide answers on the allocator, realistic end-to-end load, the page-size-1 limit and glibc results are at the top of #15611 (and #15609). In short:

• No allocator change.
• End to end at page size 16 with about 2,000 workers, the stack is CPU-neutral because transport dominates.
• The case for the stack rests on page size 1 and on indexer memory. The end-to-end sweep had jemalloc in the STACK arm only; the allocator-matched 2x2 at 0.3x and the glibc-only checks at page size 16, 1x and page size 1, 0.1x confirm the page-size-1 and memory results without it.

This PR's own part is the lookup tail: overload lookups no longer queue behind coverage writers. It also removes the equal-size skip that overcounted on main.

End to end, per PR (default allocator; full table in point 5 at the top of #15611): at the default 4 event threads this PR shows no end-to-end gain on its own.

• At page size 1, 0.3x it returns indexer CPU to main's level after perf(kv-router): epoch-reclaimed CRTC child maps and borrowed reads #15609's 3.5% gain (7.55 vs 7.29 cores). Every build below the full stack is saturated there, though, so their CPU is not compared at equal work.
• At page size 16, 1x it is within 1.5% of main.

Its gains appear under overload at high event-thread counts (below). The full stack's end-to-end gains, keeping up at page size 1 and about half the memory, arrive with #15611 on top of it.

Summary

Second of three PRs in the CRTC stack (#15609 → this PR → #15611), based on main, with no allocator change. #15614 is a separate test-only draft with the race soak.

CRTC nodes kept full-edge coverage as an FxHashSet of ranks, every lookup cloned and intersected those sets, and every coverage write took the node's state write lock, so lookups parked behind writers that only added a rank to an edge.

• Rank slots and bitsets: each rank gets a dense slot from the tree's SlotRegistry (coverage.rs); full-edge coverage is one atomic bit per slot (256 inline, then 256-slot chunks installed once). Lookups intersect slot words and map slots back to ranks through one slot table per walk.
• Exact intersection: the equal-size skip is removed. It treated equal-sized sets as equal, which overcounted after head-first evictions and when a concurrent store promoted another rank between hops; a race soak found it overcounting about 0.06% of lookups on main's code. Word intersection costs about the same as the size check.
• Locking change (authorized by the CRTC owner): promoting a slot to full coverage sets its bit under the shared shape gate after validating the shape version, instead of under the state write lock. Removals, splits, extensions, and sweeps keep the exclusive gate and the write lock.
• Slot lifecycle: a removed rank's slot is unmapped, swept out of the tree after pinned threads move on, and freed through the epoch, so a reader never sees it reused. A removal or clear that races another lane's removal of the same rank waits for that release.
• README and AGENTS.md drop the skip exception: lookups may undercount during races but never overcount.

Writer bookkeeping (per-worker lookup maps, grouped removal, lookup validation inside locked operations) is #15611. The commit message has the full design notes.

Reviewer guide: coverage.rs (slots, bitsets, release) and state.rs (cutoffs) are new; then node.rs (shared-gate promote, exact-intersection walk step, split and removal under the write lock), remove.rs (unmap → sweep → release), store.rs, repair.rs, dump.rs, matches.rs.

Validation

• dynamo-kv-router tests pass, including the coverage.rs unit tests, a differential walk against the hash-set walk, random event streams with slot recycling checked against a sequence-hash model, promote/split and drop/read races, sweep address reuse, recycled slots over unlinked subtrees, and removal/clear races on slot release. Clippy and rustfmt are clean.
• Race soak: 0 overcounts, 0 parity mismatches, and 0 strict-mode apply errors across the 4-configuration matrix, including runs that recycle thousands of slots.
• AddressSanitizer: lib tests, the thread-exit test, and a strict soak report no errors.
• Rebased onto main without the allocator change (perf(kv-router): serve the extension's Rust heap from a linked-in jemalloc #15606, closed): the new head is the previous head (e0dd256b8c) minus that PR's files. No file under lib/kv-router changes; cargo check -p dynamo-kv-router passes on it.

Benchmark method as in #15609 (Mooncake replay, fresh process per trial, interleaved arms, ratios of medians with 95% bootstrap CIs).

On the default allocator (glibc). This PR has not been measured alone on glibc. This PR and #15611 together, on top of #15609, both arms on glibc (AMD EPYC 7702P, 10 runs per arm under overload and 6 at keep-up; arms at eeadcb4016 and b3c6c0864d, which carry these PRs' non-test lib/kv-router code):

• Overload lookup p99.9 at 64 event workers (375 ms) falls from 765 µs to 17 µs (0.023×, CI 0.018–0.028, 10/10); main is at 1.09 ms. The per-layer runs on mimalloc (below) put the matching drop in overload lookup p99 in this PR.
• Write capacity rises 1.13× at 64 event workers (375 ms) and 1.19× at 8 (750 ms), 10/10 each.
• At keep-up with 64 event workers, update p99 is 0.74× and lookup p99 is 1.09× (0/6); lookup p99.9 is unchanged (n.s.).

With jemalloc enabled, measured on the full stack against main with jemalloc (2-socket AMD EPYC 7742, 10 runs per arm): at 64 event workers (375 ms), overload lookup p99 falls from 636 µs to 16 µs (#15611).

Per layer, with mimalloc in both arms (a measurement configuration only, see #15609). This PR vs #15609, measured with an earlier head of each (trials that overlapped another job on the host excluded), 10 runs per arm, both arms built with the same toolchain:

Host	Cell	Achieved block ops/s	Lookup p50	Lookup p99	Lookup p99.9	Update p99
EPYC 7702P	Overload, 64 event workers (375 ms)	566M → 639M, 1.13× (CI 1.08–1.25, 10/10)	0.99 (n.s.)	0.033 (535 → 17.8 µs)	0.86	0.70
EPYC 7702P	Overload, 8 event workers (750 ms)	154M → 160M, 1.04× (CI 1.02–1.08, 10/10)	0.99 (n.s.)	0.87	0.90	0.94
EPYC 7702P	Keep-up, 64 event workers	equal	0.96	1.03 (n.s.)	1.01 (n.s.)	0.95
EPYC 7702P	Keep-up, 8 event workers	equal	0.94	1.01 (n.s.)	0.99 (n.s.)	0.93
EPYC 9654P	Overload, 64 event workers (187 ms)	1.37G → 1.55G, 1.14× (CI 1.05–1.19, 10/10)	0.86	0.45 (21.4 → 9.6 µs)	0.14	0.39
EPYC 9654P	Overload, 8 event workers (750 ms)	225M → 233M, 1.04× (CI 1.03–1.06, 10/10)	0.94	0.88	0.91	0.91

Overload lookups no longer queue behind coverage writers: at 64 event workers, lookup p99 falls from 535 µs to 17.8 µs on the 7702P and from 21.4 µs to 9.6 µs on the 9654P.

[PeaBrane]

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Walkthrough

The compressed radix tree now tracks coverage with reusable rank slots, full-edge bitsets, and per-slot cutoffs. Store, removal, lookup repair, matching, and dump paths use these structures. The change also adds slot lifecycle handling and tests for coverage transitions and concurrent cleanup.

Changes

Slot-based compressed radix tree

Layer / File(s)	Summary
Slot coverage and registry lib/kv-router/src/indexer/concurrent_radix_tree_compressed/coverage.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/mod.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/types.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/README.md	Adds atomic full-coverage bitsets, slot sets, worker-to-slot mappings, and epoch-deferred slot reuse. The tree initializes a SlotRegistry.
Node coverage and edge changes lib/kv-router/src/indexer/concurrent_radix_tree_compressed/state.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/node.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/types.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/README.md, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/tests.rs	Node state stores full coverage and slot cutoffs. Store, split, extension, promotion, and removal paths update those structures. Tests cover coverage transitions and promotion behavior.
Event handling and lookup redirects lib/kv-router/src/indexer/concurrent_radix_tree_compressed/store.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/sync_impl.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/mod.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/repair.rs, lib/kv-router/src/lookup_update.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/types.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/README.md, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/tests.rs	Store and anchor events acquire slots for tree updates. Split and repair paths redirect lookup entries only when they still point to the node being replaced.
Matching and filtered tree dumps lib/kv-router/src/indexer/concurrent_radix_tree_compressed/matches.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/node.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/dump.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/types.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/AGENTS.md, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/README.md, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/tests.rs	Matching intersects active slots with full coverage at each node and scores cutoff slots at their covered prefix. Dumps filter ranks by full coverage of the parent edge.
Worker cleanup and slot release lib/kv-router/src/indexer/concurrent_radix_tree_compressed/remove.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/sync_impl.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/tests.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/README.md	Clear and worker-removal paths erase lookup entries and coordinate slot clearing, tree sweeping, and release. Tests cover slot reuse and concurrent cleanup.

Priority: ➖ Normal

Estimated code review effort: 5 (Critical) | ~120 minutes

Severity of issue fixed: Low

Merge Risk: 🔵 Low · up to 67ded

The indexer's coverage tracking and lookup intersection were reworked for performance, and the reported correctness soak tests are clean. The repository's required before/after performance numbers for these hot paths are still missing. Publish them before merging to confirm there is no regression.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 51.42% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 212 functions across 13 files. (2 skipped…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Title check		The title is concise, specific, and accurately identifies the main change: dense rank slots with atomic coverage bitsets.
Description check		The description provides detailed scope, reviewer guidance, validation results, benchmarks, and related issue references. It does not use the template headings, and the required Related Issues section…

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Full details: Docstring Coverage

Explanation

Docstring coverage is 51.42% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 212 functions across 13 files. (2 skipped: 2 unsupported.)

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📒 Files selected for processing (15)

• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/AGENTS.md
• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/README.md
• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/coverage.rs
• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/dump.rs
• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/matches.rs
• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/mod.rs
• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/node.rs
• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/remove.rs
• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/repair.rs
• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/state.rs
• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/store.rs
• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/sync_impl.rs
• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/tests.rs
• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/types.rs
• lib/kv-router/src/lookup_update.rs

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[jthomson04]

Closing along with the rest of the #15609 stack. The writer-side win from #15611 already landed on main via #16134, without this layer.

Per-layer attribution showed no e2e gain from the dense rank slots and atomic coverage bitsets at 4 event threads; they only helped the overloaded 64-thread case. The indexer memory saving is about 2% (~43 MiB of ~1.8 GiB live heap).

The one piece worth keeping is the fix for the equal-size-skip overcount, which main still has (L3's random_hole fails on main because of it). That will be split out as its own small fix PR. The branch is kept for reference.