fix(kv-router): check a member fingerprint in CRTC's equal-size skip

[jthomson04]

Summary

CRTC's lookup walk skipped intersecting its active workers with a child's full-edge coverage whenever the two sets had the same size. That shortcut can overcount. Removal doesn't cascade to children (see the TODO(CORRECTNESS) in remove.rs), so when a worker evicts a head or middle block on a node that another worker keeps alive, the child nodes still list the evicting worker. An equal-sized coverage set can then name different workers, and the walk credits a worker past its cached prefix.

The minimal case:

1. Worker 0 stores [1, 2] and worker 1 stores [1, 3]. The tree is [1]{0,1}, with children [2]{0} and [3]{1}.
2. Worker 0 evicts block 1. The tree is now [1]{1}, but [2] still holds {0}.
3. Query [1, 2]: after the first node the active set is {1}, and the child [2] holds {0}. Both have one member, so the walk skipped the check and scored worker 1 at 2 when it holds 1.

This was the only documented exception to "lookups never overcount" in the CRTC AGENTS.md. #15610 removed it by moving coverage to atomic bitsets, which made always intersecting cheap. That needed #15609's epoch layer, and the stack was closed after #16134 carried #15611's win to main. This is the standalone fix promised in #15610's closing note.

Fix: keep the O(1) skip, but also require a matching member fingerprint. The check is probabilistic, not exact.

• full_edge_workers and the walk's active set become a WorkerSet (worker_set.rs). It is an FxHashSet plus the XOR of a 64-bit splitmix64 mix of each member.
• The fingerprint changes only through insert, remove and retain, and only when membership actually changes. The fields are private, so it can't fall out of step with the members. Reads go through Deref, so existing call sites are unchanged.
• The walk now skips the intersection only when both the size and the fingerprint match.
  • Equal sets always match.
  • Unequal sets collide with probability about 2⁻⁶⁴ per comparison. A collision overcounts exactly as the size-only check did, so it remains the one overcount path.
  • Debug builds assert the sets are equal whenever the skip fires, so every test run checks the bookkeeping.
• Cost:
  • one splitmix64 per coverage change;
  • one extra u64 compare per hop when the sizes match;
  • one splitmix64 per worker dropped, only on the intersection path that already does a hash probe per worker;
  • 8 bytes per node.
• No locking, versioning or removal semantics change.

Why not always intersect, or port the bitsets?

• Always intersecting: the README recorded +34% lookup p50 at 128 workers, and the cost grows with workers × hops.
• Bitsets: they need safe slot reuse, which perf(kv-router): dense rank slots with atomic coverage bitsets #15610 got from the epoch layer. Our 2,048-worker e2e runs put all of CRTC at about 1.7% of frontend CPU, so a cheaper intersection can't buy much end to end.

Docs: the CRTC README's "Equal-size skip" section becomes "Equal-set skip" and states the collision case. The AGENTS.md exception to "never overcount" is narrowed from any equal-size skip to a fingerprint collision in it.

Out of scope:

• Invalidating descendants on a mid-edge removal (the remove.rs TODO). It would fix the root cause and the TODO's other symptoms, but it costs a subtree walk on the writer.
• The positional indexer's similar size check (positional.rs).

Validation

Tests

• equal_sized_coverage_of_another_worker_does_not_extend_match: the 3-event case above. On main it fails, scoring 2 against an expected 1.
• random_hole_streams_never_overcount, ported from perf(kv-router): cut CRTC writer per-block work #15611: every rank stores, evicts runs in any order (leaving holes), and clears, over two lanes. On main it fails with 85,149 overcounting lookups in all 40 of 40 seeds.
• worker_set unit tests: the fingerprint follows the members through 10K random inserts, removes and retains; equal-sized sets with different members differ.
• With the fix:
  • cargo test -p dynamo-kv-router --lib: 1,073 passed, with the skip's debug assertion active.
  • CRTC_REMOVE_FUZZ_SEEDS=200 random-hole: 0 overcounts.
  • Clippy -D warnings (default, standalone-indexer, bench,metrics) and rustfmt: clean. Also re-run on aarch64 (tyche).

E2e on tyche

Same harness as #16134: 2,048 mocker workers across 12 processes, two NUMA-pinned frontends, an AIPerf agentic replay at fixed concurrency 6,144, and a 90 s measurement window. One allocation; order: main rehearsal (discarded), main timing, fix timing, fix profile, main profile.

The fix doesn't change lookup results on this workload. A bench-only build of this branch (never merged) counted the walk's hops in a separate one-run allocation:

Measurement window, both frontends
Lookups	485,462
Skip-eligible hops	5,924,519
…with equal sizes	5,145,982 (86.9%)
…with equal sizes but different members (where main's skip misfires)	0

So the skip matters here (87% of hops), but on main it never misfires on this workload. Main and the fix return the same scores, which agrees with the README's 0-in-4.3M Mooncake measurement. The overcount needs evictions of a block that another worker still holds, from the head or mid-chain, as in the random-hole test.

CRTC CPU (cycles:u profile runs, both frontends combined; 2σ is sampling error only):

Group	main ms/req	fix ms/req	Δ	2σ
CRTC_all	0.3134	0.2942	−6.1%	±4.2%
CRTC_apply_event	0.2474	0.2243	−9.3%	±4.8%
CRTC_match	0.0344	0.0406	+18.1%	±12.0%
KV_indexer	0.8062	0.7950	−1.4%	±2.6%
KV_all_routing	1.5287	1.5142	−1.0%	±1.9%

CRTC_match grows by 0.006 ms per request (the extra intersections, fingerprint compares and drop mixes). The other moves are run-to-run variation: the change adds work to the apply path and doesn't remove any.

Timing:

Run	Req/s	FE cores	ITL p99 (ms)
main rehearsal	5,222	127.3	3.49
main timing	5,135	127.5	3.51
fix timing	5,031	126.8	4.76
fix profile	5,025	128.7	8.46
main profile	5,177	127.8	4.43
fix + counters (separate allocation)	4,929	125.1	3.71

• Frontend CPU is pinned at about 127 cores in every run, so CPU per request just tracks requests/s.
• The fix runs in the first allocation came in about 3% lower on requests/s, and had higher ITL tails. With identical lookups and no frontend CPU group consistently higher, nothing in this change explains it. The instrumented fix build's ITL p99 (3.71 ms) falls inside main's range.
• Requests: 663K–672K per run, 0 errors in all six.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Match results no longer extend beyond a valid reachable prefix when different workers have equally sized coverage.
  • Added randomized checks for matching behavior after partial stores, removals, and clears.

[jthomson04]

/ok to test 874d5e6

[coderabbitai]

Walkthrough

Compressed radix tree matching now uses a worker set with a 64-bit XOR fingerprint to compare membership. Match traversal filters workers when sets differ. New tests and documentation describe the comparison and match-result constraints.

Changes

Compressed Radix Worker Matching

Layer / File(s)	Summary
Fingerprint worker sets lib/kv-router/src/indexer/concurrent_radix_tree_compressed/worker_set.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/mod.rs	WorkerSet maintains an XOR fingerprint through member changes and compares sets by size and fingerprint. Tests check fingerprint updates and selected comparisons.
Use WorkerSet in matching lib/kv-router/src/indexer/concurrent_radix_tree_compressed/types.rs, 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/matches.rs	Match state and traversal now use WorkerSet. find_match_step filters active workers when sets differ and returns the active count used by traversal.
Validate and document matching lib/kv-router/src/indexer/concurrent_radix_tree_compressed/tests.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/state.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/README.md, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/AGENTS.md	Tests cover equal-sized sets with different members and randomized store, removal, and clear operations. Documentation updates the result constraint and equal-set skip condition.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: 🔵 Low · up to 874d5

The fix correctly stops treating equal-sized worker sets as identical. However, the new membership check relies on a hash fingerprint. In rare constructed cases, two different sets can share a fingerprint, so a cached-prefix match could be overcounted. Adding an exact set comparison when fingerprints match would close this gap. Otherwise the change is safe to merge.

Pre-merge checks · 3 · 2

Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 54.55% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 22 functions across 7 files. (2 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.
Description check		The description gives detailed context, implementation details, validation results, performance data, and scope limits. However, it does not include the required Related Issues section with a closing …	Add a Related Issues section with the applicable issue reference, such as Closes #1234 or Relates to #1234. Also add a Where should the reviewer start? section that names the primary files or code paths for review.

Passed checks (3 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 clearly identifies the CRTC equal-size skip change and the added member fingerprint check.

Full details: Docstring Coverage

Explanation

Docstring coverage is 54.55% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 22 functions across 7 files. (2 skipped: 2 unsupported.)

Full details: Description check

Explanation

The description gives detailed context, implementation details, validation results, performance data, and scope limits. However, it does not include the required Related Issues section with a closing or relationship reference, and it does not identify where the reviewer should start.

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Inline comments:
Review comments at
@lib/kv-router/src/indexer/concurrent_radix_tree_compressed/worker_set.rs:
- Line 53: Update WorkerSet::same_members to compare the worker collections
after the length and fingerprint checks, so the shortcut only succeeds for exact
membership equality. Add a regression test confirming the identified colliding
singleton sets are not considered the same.

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📒 Files selected for processing (9)

• 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/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/state.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/indexer/concurrent_radix_tree_compressed/worker_set.rs

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[jthomson04]

/ok to test 5380af3

[devin-ai-integration]

✅ Dynamo PR CI passed — run 38337507269 (attempt 1) on 5380af3b93

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