perf(kv-router): epoch-reclaimed CRTC child maps and borrowed reads

[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 approved and CI is green.

For reviewers

This block answers the review questions for the whole stack (#15609 → #15610 → #15611), in order. Per-PR detail follows under Summary and Validation. Every number names its allocator, host CPU and sample size.

Short version. The stack changes no allocator. At page size 16 with about 2,000 workers it does not reduce indexer CPU end to end, because transport dominates there. Three separate ingest PRs (#15875, #15876, #15877) go after that cost instead. The case for the stack is page size 1, SGLang's default. At 0.3x the reviewer-scale token rate (about 22M block writes/s into one indexer), main's indexer falls behind on events, and jemalloc alone does not fix that: main with jemalloc still fell behind in 2 of 3 trials. The stack, with no allocator change, kept up in 3 of 3 trials on the default glibc and in 3 of 3 with jemalloc, and at that load it cut peak indexer memory about in half under both allocators (2-socket AMD EPYC 7742, n = 3 per arm; point 3). With both arms on glibc, the stack left indexer CPU unchanged at page size 16, 1x (1.00×; n = 3 rules out an increase above about 4%), and at page size 1, 0.1x it used 0.91× the indexer CPU and 0.56× the memory with equal lookup latency. In these runs the stack in the indexer added no measurable cost to the frontend or the rest of the system (2-socket AMD EPYC 7742, n = 3 per arm; point 4).

1. Allocator: no change

• The default stays glibc malloc. The linked-in jemalloc PR (perf(kv-router): serve the extension's Rust heap from a linked-in jemalloc #15606) is closed, and no mimalloc integration is proposed.
• jemalloc stays opt-in:
  • DYN_FRONTEND_JEMALLOC=1 covers the frontend today.
  • feat(components): add a DYN_JEMALLOC preload opt-in for the router and indexer #15880 adds a generic DYN_JEMALLOC=1 for python -m dynamo.router (including --serve-indexer) and python -m dynamo.indexer, and keeps DYN_FRONTEND_JEMALLOC as a frontend alias.
  • A preloaded jemalloc also serves the extension's Rust allocations, so no linked allocator is needed.
  • docs(frontend): document DYN_FRONTEND_JEMALLOC and MALLOC_CONF guidance #15884 documents the opt-in and changes the recommended MALLOC_CONF to background_thread:true.
• Against jemalloc, mimalloc was within 2% on write capacity in every cell that was not harness-limited. jemalloc used 2–6% more indexer CPU; at light load, most of the difference was its background threads. This is the indexer microbench with the full stack in both arms (2-socket AMD EPYC 7742, 5–10 runs per arm). That gap does not justify a second allocator integration.
• The stack's own effect on the frontend on glibc, the default: 0.97× throughput at concurrency 128 (paired 0.92–1.02, not significant) and 3% more frontend CPU per request. That was an earlier head in the dynamo-frontend-benchmark mocker topology (AMD EPYC 9654P, 4–5 runs per arm).

2. Realistic end to end: at page size 16, CPU-neutral, as the review expected

The setup (gist): one python -m dynamo.router --serve-indexer (4 event threads, pinned to 32 cores of a 2-socket AMD EPYC 7742) is loaded the way about 10 frontends and 2,000 workers would load it.

• About 1/10 of the load is real: a KV-routed frontend serves an AIPerf AgentX replay with 200 live mockers.
• About 9/10 is synthetic: 1,800 publishers replay captured AgentX worker timelines over the production direct-ZMQ event wire, and a driver sends the production kv_indexer_query RPC.
• A trial counts only if the indexer admitted exactly what every publisher sent. n = 3 per arm, interleaved, with a fresh indexer per trial.

Results:

• Page size 16 at 1x (3.49M write blocks/s, 1,944 driver lookups/s): indexer CPU 0.99× (9.69 vs 9.78 cores; the ranges overlap, so the difference is not significant). Lookup RTT is equal. STACK had jemalloc and MAIN ran on glibc; the run with both on glibc is under point 4.

• Why there is no gain. At this point CRTC lookups take about 0.7% of indexer CPU (uncounted perf profiles) and CRTC apply about 17% (the event-worker threads in the counted trials). The rest is ZMQ receive and signaling, TCP ACKs, tokio scheduling and MessagePack decode. So the network path dominates, as the review suggested. At 3x load from the same 2,000 sources, four ZMQ I/O threads each ran at 0.995 of a core, in both arms.

• The ingest PRs. They target those costs, and each was measured with this harness on main and its default allocator (page size 16, 1x, 2-socket AMD EPYC 7742, n = 3 per build). They are independent of this stack:

  • perf(zmq): use the eventfd signaler in the vendored libzmq on Linux #15875, eventfd signaler in the vendored libzmq: −5.1% indexer CPU;
  • perf(runtime): drain event-plane SUB sockets without per-message ZMQ_EVENTS #15876, drain event-plane SUB sockets without per-message ZMQ_EVENTS: −3.8%;
  • perf(kv-router): decode KV event batches with a typed MessagePack walker #15877, typed MessagePack decode of KV event batches: about −1.5%, which is within run-to-run noise on its own;
  • combined build: −9.4% (9.29 → 8.41 cores). It also carried a fourth change that had no measurable effect on its own and is not proposed.

  This A/B ran on a newer main with 300 s windows, so compare its ratios, not its absolute cores, with the 1x result above.

3. The limit the stack removes: page size 1, and memory

Both measurements here use the harness above (2-socket AMD EPYC 7742, n = 3 per arm, interleaved). The 2x2 at 0.3x is allocator-matched. The earlier sweep from 0.05x to 0.3x was measured with jemalloc linked into the STACK arm only (jemalloc 5.3.1 from the closed #15606, against MAIN on glibc), so its deltas include an allocator effect.

Allocator-matched 2x2 at 0.3x. Page size 1 with SGLang's event cadence, at 0.3x the reviewer-scale token rate: about 22M block writes/s into one indexer (stores plus removals; 16.7M from the synthetic publishers and 5–6.5M from the live mockers), 11.9k KV events/s and 780 lookups/s, with the default 4 event threads. The arms were {main e61319d830, the stack's head f13016868a with no allocator change} × {glibc, jemalloc}. jemalloc is Ubuntu's libjemalloc2 5.3.0, preloaded into the indexer only, with MALLOC_CONF=background_thread:true. Delivery was exact in all 12 trials. An arm keeps up if its event backlog at the end of the window drains within 1 s.

Arm	Backlog at window end, s, median [range]	Kept up	Event-worker busy, per thread	Indexer CPU, cores	Peak RSS, GiB
main, glibc	12.9 [8.1–18.2]	0/3	79%	7.19	196
main, jemalloc	2.2 [0.36–3.5]	1/3	76%	7.27	190
stack, glibc	0.30 [0.09–0.69]	3/3	63%	6.63	104
stack, jemalloc	<0.001	3/3	59%	6.65	97

• jemalloc alone is not enough. It cut main's backlog about 6x, but main still fell behind in 2 of 3 trials. The stack kept up on either allocator, and even on glibc it ended every rotation round with less backlog than main with jemalloc.
• Under either allocator, the stack used 20–23% less event-worker CPU, 8–9% less indexer CPU and about half the peak memory. jemalloc alone cut event-worker CPU 4–7%, did not reduce total CPU (+0.2% to +1.1%), trimmed peak memory 3–7% and raised lookup RTT (p50 +7–10%, p99 +23–31%). The stack is RTT-neutral.
• main on glibc kept up at 0.2x in the sweep below, so its limit on this host class lies between 0.2x and 0.3x.
• Limits: n = 3 per arm, at one load point, on one host class. The margin on glibc is thin (the stack still queued 0.09–0.69 s). The live mockers were in a KV-thrash regime and supplied about 25% of block writes, so read the limit as a block-write rate, not a request rate. Only the default 4 event threads were tested. The stack's own limit was not measured: with jemalloc its event-worker load projects about 0.4x, and the 25 GbE NIC nears line rate around 0.45–0.5x. Details are in perf(kv-router): cut CRTC writer per-block work #15611 and the gist.

Sweep from 0.05x to 0.3x (jemalloc linked in the STACK arm only):

• Backlog. At 0.3x MAIN ended every window 8.5–14.8 s behind on events (3 of 3 trials), so it was routing on stale state. STACK kept up in 3 of 3 (end-of-window drain under 0.4 ms). At 0.05x, 0.1x and 0.2x both arms kept up. 0.3x is not a clean CPU comparison: MAIN fails the drain gate there, and the live mockers thrash their KV caches, so live traffic is 25–26% of writes instead of the designed 7.6%.

• Memory. STACK's indexer RSS started at 0.56–0.59× MAIN's at every point. MAIN's RSS then grew linearly through the window (about 15, 41, 72 and 88 MiB/s at 0.05x, 0.1x, 0.2x and 0.3x, against 3–8 MiB/s for STACK), so the end-of-window ratio (0.56× at 0.05x down to 0.43× at 0.3x, 85 vs 197 GiB) depends on load and window length. The 2x2 at 0.3x shows that the saving is the stack's: jemalloc alone trims peak RSS 3–7%. At page size 16, 1x, with both arms on glibc, the end-mark ratio is 0.68× (point 4).

• CPU. STACK used about 5–7% less serving-indexer CPU than MAIN at every point. The ranges separate at every point, only barely at 0.05x:

  • 0.05x: 0.945× (1.19 vs 1.26 cores);
  • 0.1x: 0.938× (2.24 vs 2.39);
  • 0.2x: 0.931× (4.61 vs 4.96; 0.926× before a rerun of one MAIN trial);
  • 0.3x: 0.944× (6.79 vs 7.20; 0.936× if MAIN is credited with its unapplied backlog).

  These points establish no load trend: the differences between them are within noise, and the live workload changes from point to point. At page size 16 the ratio was 0.99× at 1x, 0.98× at 3x (15.68 vs 16.02 cores) and 0.95× at 3x spread over 5,400 publishers (16.76 vs 17.57).

• Where the CPU goes. STACK cut event-worker (CRTC apply) CPU by 26–29% at every point (0.71–0.74×). It added tokio + ZMQ CPU, where lookups run inline: +4% at 0.05x, +5% at 0.1x, +6% at 0.2x and +10% at 0.3x (+0.03 to +0.39 cores). Nearly all of it is on the tokio workers (+5% to +12%); ZMQ I/O CPU moved by 1.3% or less at every point. Lookup RTT was also higher on STACK:

  • p50: +0% to +6.5% from 0.05x to 0.3x;
  • p99: +1% to +4% at 0.05x–0.2x (11.4 vs 11.2 ms at 0.1x; the ranges overlap at 0.05x), and 0.83 vs 0.64 s at 0.3x (the ranges overlap; both arms are past the latency knee there).

  The points differ in mix as well as scale, so these penalties are not established as load trends either. Allocator-matched at 0.3x, the stack alone added about 2% tokio + ZMQ CPU and no lookup latency, while jemalloc alone added about 4% tokio + ZMQ CPU and 7–10% to RTT p50. The sweep's RTT penalty therefore fits the allocator rather than CRTC; that reading spans two runs and is not a decomposition.

Allocator-matched microbench (jemalloc 5.3.1 in both arms; 2-socket AMD EPYC 7742; 10 runs per arm under overload, 6 at keep-up and 5 at a fixed load):

• write capacity ≥2.04× at 64 event workers and 1.66× at 8;
• indexer CPU per 1k block ops: 79.2 → 63.2 µs (−20%);
• keep-up update p99: 0.44–0.57×;
• overload lookup p99: 636 → 16 µs.

4. On the default allocator (glibc)

• Microbench, stack vs main, both on glibc (AMD EPYC 7702P; 10 runs per arm under overload, 6 at keep-up and 5 at a fixed load):

  • write capacity 1.25× / 1.30× at 64 / 8 event workers;
  • indexer CPU per 1k block ops at a fixed 26.7M block ops/s: 87.9 → 70.1 µs (−20%);
  • keep-up update p99: 0.46×;
  • overload lookup p99.9: 1.09 ms → 17 µs.

  The cost: keep-up lookup p99 and p99.9 are 5% and 8% higher, and the stack was worse in all 6 rounds. malloc's arena locks still serialize the event workers, which limits the gain on glibc.

• End to end, both arms on glibc:

  • page size 1 at 0.3x: the 2x2 in point 3. The stack kept up in 3 of 3 trials and main in 0 of 3, with 0.92× the indexer CPU, 0.80× the event-worker CPU, 0.53× the peak memory and 0.98× the lookup RTT p50 (2-socket AMD EPYC 7742, n = 3 per arm);
  • page size 1 at 0.1x, SGLang cadence (5.56M synthetic plus 0.45–0.47M live write blocks/s): indexer CPU 0.91× (2.20 vs 2.41 cores; paired 0.910–0.913), event-worker CPU 0.73×, RSS at the end of the window 0.56× (90.6 vs 161.5 GiB), and equal lookup RTT (p50 3.52 vs 3.50 ms). Tokio and ZMQ I/O CPU moved by less than 1%. The sweep's 0.1x point, which had jemalloc in the stack arm only, showed +5% tokio + ZMQ CPU and +1.3% RTT p50; on glibc both are gone, which fits the 2x2's reading that they came with the allocator, though that comparison spans two runs;
  • page size 16 at 1x (2.67M synthetic plus 0.55M live write blocks/s, 300 s windows as in the ingest A/B): no measurable change in indexer CPU or lookup latency. Indexer CPU 1.00× (9.33 vs 9.31 cores; paired 0.99–1.02), equal lookup RTT, RSS 0.68× (13.4 vs 19.6 GiB), and event-worker CPU 1.03× (+0.04 core) with total CPU unchanged.

  Both points compare main e61319d830 with the stack's head f13016868a, and no trial's indexer mapped jemalloc or mimalloc (2-socket AMD EPYC 7742 indexer, n = 3 per arm, interleaved; all 12 trials valid). Only the serving indexer differed, and frontend CPU (1.01×), AIPerf throughput (within 0.4%) and AIPerf errors (0 in every window) were unchanged, so the stack in the indexer adds no measurable cost elsewhere; these runs do not test it inside a frontend's own indexer (point 1). At page size 16, n = 3 rules out an indexer CPU increase above about 4% (Welch 95% interval 0.97–1.04, with a conservative 2 degrees of freedom), not smaller ones. At page size 1, the stack's AIPerf TTFT p99 and request-latency p99 medians were 7% and 5% higher, with overlapping ranges and equal lookup RTT at the indexer, which n = 3 cannot resolve. Details are in perf(kv-router): cut CRTC writer per-block work #15611 and the gist, point 4.

5. Per PR, end to end (default allocator)

The same end-to-end harness, run with four indexer builds: main, #15609, #15609 + #15610, and the full stack (#15611).

• All builds on glibc, with one serving indexer (default 4 event threads) on a 2-socket AMD EPYC 7742 host.
• n = 3 per build, Williams rotation; 24 of 24 trials valid.
• Only the indexer differs between builds.

Page size 1, SGLang cadence, 0.3x (the limit point):

	main	#15609	+ #15610	+ #15611 (full stack)
Kept up with events	0/3	0/3	0/3	3/3
Backlog at end of window, s	15.5	11.7	13.6	0.46
Indexer cores	7.56	7.29	7.55	6.71
Event-worker cores	3.18	3.19	3.19	2.54
Peak RSS, GiB	198	198	198	105

Page size 16, 1x:

	main	#15609	+ #15610	+ #15611 (full stack)
Indexer cores	9.47	9.56	9.61	9.56
RSS at end of window, GiB	19.3	19.0	19.3	13.2

What each PR does end to end:

• perf(kv-router): cut CRTC writer per-block work #15611 carries the end-to-end gain.
  • At page size 1 it is the only build that keeps up. Against the build below it, it cuts indexer CPU by 11%, event-worker CPU by 20% and memory by 47%.
  • At page size 16 it cuts memory by 32% at unchanged CPU.
  • These match the earlier full-stack runs.
• perf(kv-router): epoch-reclaimed CRTC child maps and borrowed reads #15609 alone:
  • At page size 1: 3.5% less indexer CPU, but it still falls behind and memory is unchanged.
  • At page size 16: CPU within about 1% of main.
• perf(kv-router): dense rank slots with atomic coverage bitsets #15610 on top of perf(kv-router): epoch-reclaimed CRTC child maps and borrowed reads #15609: no end-to-end gain at the default 4 event threads.
  • At page size 1 it gives back perf(kv-router): epoch-reclaimed CRTC child maps and borrowed reads #15609's CPU gain (7.55 vs 7.29 cores). Every build below the full stack is saturated at that point, though, so their CPU is not compared at equal work.
  • Its own gains are under heavy overload at high event-thread counts in the microbench: lookup p99 535 → 17.8 µs and 1.13× write capacity at 64 event threads, mimalloc in both arms.
  • It also removes the equal-size skip that makes main overcount.

Caveats:

• n = 3 per build.
• The full stack's CPU saving at page size 1 has ranged from 6% to 11% across runs: 0.944× in the sweep, 0.922× in the 2x2, and 0.887× here.
• Lookup latency is not compared here. On this run's hosts main's lookups were unusually slow, and its query driver fell behind schedule, so the per-build RTT differences do not carry over between runs.
• Each PR was measured only on top of the PRs below it.

Summary

First of three PRs in a stack that raises the CRTC KV indexer's write capacity: this PR → #15610 → #15611, based on main, with no allocator change. #15614 is a separate test-only draft with the race soak used to validate the stack.

CRTC child maps published compact snapshots through ArcSwap; every store walked each thread's debt list (arc_swap::Debt::pay_all, 18–22% of event-worker time at 64 event workers, profiled on mimalloc), lookups cloned an Arc per hop, and a lookup that dropped the last reference to an unlinked subtree freed the whole subtree inline.

• Child maps publish snapshots through crossbeam-epoch: writers swap in a successor and retire the predecessor once pinned readers move on, so a store no longer scans other threads.
• Reads pin once per walk (LookupPin) and borrow each child under the guard (get_ref/child_ref, find_match_step taking &'g self) instead of cloning its Arc.
• Stale-leaf cleanup stays exact: retired snapshots' child references are counted in retired_snapshot_refs and subtracted from a bracketed strong-count read, and a budgeted drain releases each count only as it drops that child's Arc.
• Reclamation placement: expired garbage goes to the collecting thread's graveyard and is freed in small budgets, counting graves opened plus node references dropped. A lookup frees at most 8 units and hands the rest to a shared queue that the lanes drain; a lane frees 256 per task, and all of it when idle or on Flush. Threads that pin the global collector without running CRTC work (rayon, moka) also hand garbage to the shared queue.
• Thread exit: NodeChildren::drop frees inline instead of pinning, which recursed at thread exit; tests/crtc_thread_exit.rs covers it.
• Teardown: dropping the tree frees every child map inline.

Coverage and writer lookups are unchanged here: lookups still use per-node worker sets with the equal-size skip (#15610 replaces both), and writer bookkeeping is #15611. The commit message has the full design notes.

Reviewer guide: children.rs (epoch maps, graveyards, LookupPin, take_exclusive) is the core; then node.rs (retired_snapshot_refs, stale-leaf check, borrowed find_match_step), matches.rs, remove.rs (sweep), mod.rs (Drop), sync_impl.rs (lane drains). compressed_radix.rs has a small retain-based demotion that RadixTree shares.

Validation

• dynamo-kv-router tests pass, including new children.rs tests (concurrent promotion races, deferred Arc drops, budgeted drains that keep undropped children counted) and the thread-exit and synchronous-drop regression tests. Clippy (default and bench,metrics) and rustfmt are clean.
• Race soak (16 writer and 6 reader threads, adversarial per-rank event streams, exact parity checks against a sequence-hash model at quiescence): 0 parity mismatches and 0 apply errors in strict mode. Overcounts appear at the same rate as on main (the equal-size skip is still present here; with the skip disabled they are 0).
• AddressSanitizer: the crate's lib tests, the thread-exit test, and strict soaks 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 (b3fabcabb5) minus that PR's files. No file under lib/kv-router changes, so every result below applies to this head; cargo check -p dynamo-kv-router passes on it.

Method for the tables below: Mooncake trace replay from lib/bench/kv_router/INDEXER_BENCH.md, fresh process per trial, arms interleaved, ratios of medians with 95% bootstrap CIs, k/n rounds in which this PR did better. "Overload" compresses the trace until the indexer falls behind, so the achieved rate measures capacity; "keep-up" is a window both arms sustain. The benches link no allocator on main, so they run on glibc unless jemalloc is preloaded.

On the default allocator (glibc): this PR vs main. AMD EPYC 7702P, 10 runs per arm under overload and 6 at keep-up, at eeadcb4016 (an earlier head with this PR's lib/kv-router tree and crossbeam-epoch 0.9.21):

Cell	Achieved block ops/s	Lookup p99	Lookup p99.9	Update p99
Overload, 64 event workers (375 ms)	49.3M → 54.3M, 1.10× (1.10–1.11), 10/10	0.93 (n.s.)	0.70	0.90
Overload, 8 event workers (750 ms)	45.1M → 49.3M, 1.09× (1.08–1.10), 10/10	0.97	1.06	0.90
Keep-up, 64 event workers	equal	0.96	1.08 (1.01–1.22)¹	0.63

On glibc the event workers still wait on malloc's arena locks, which bounds what CRTC changes can show. The full stack on glibc gives 1.25× / 1.30× (#15611).

With a faster allocator. This PR alone has not been measured with jemalloc. The per-layer measurements used mimalloc linked into the bench in both arms, as a measurement configuration only; nothing in this stack ships mimalloc. On the full stack, jemalloc and mimalloc were within about 2% on write capacity (#15606 has that comparison).

This PR vs main, both with mimalloc, at an earlier head of this PR. AMD EPYC 7702P (64 cores), 20 runs per arm, trials that overlapped another job on the host excluded:

Cell	Achieved block ops/s	Lookup p50	Lookup p99	Lookup p99.9	Update p99
Overload, 64 event workers (375 ms)	400M → 563M, 1.41× (CI 1.36–1.45, 18/18)	1.08	0.88	0.75	0.53
Overload, 8 event workers (750 ms)	139M → 153M, 1.10× (CI 1.09–1.12, 20/20)	0.96	0.99 (n.s.)	1.03	0.86
Keep-up, 64 event workers	equal	0.96	0.97 (n.s.)	1.03 (n.s.)	0.82
Keep-up, 8 event workers	equal	0.96	0.94	0.98	0.95

The overload lookup tail is still dominated by lookups queued behind coverage writers (p99 613 → 539 µs at 64 event workers); #15610 removes that queueing.

The 750 ms gain does not depend on the allocator (1.10× with mimalloc, 1.09× on glibc); most of the 375 ms gain does (1.41× against 1.10×). With jemalloc in both arms, the three PRs together add at least 2.04× at 375 ms and 1.66× at 750 ms, measured on a different host (2-socket AMD EPYC 7742; #15611).

Review follow-up (lookups hand leftover garbage to the lanes; the drain budget counts every dropped reference) vs this PR's first head, mimalloc in both arms, 10 runs per arm: throughput is unchanged in every overload cell on EPYC 7702P and 9654P (0.97–0.99, n.s.). Lookup p99.9 at 750 ms with 8 event workers is 1.07× on 7702P and 1.06× on 9654P; the other lookup and update percentiles are unchanged or lower.

¹ Marginal at n=6. Every other arm in this run shows the same 1.07–1.09× against main in this cell.

[PeaBrane]

@coderabbitai review

[coderabbitai]

Walkthrough

The pull request migrates compressed radix-tree child storage from ArcSwap to crossbeam-epoch, adds deferred reclamation and guarded traversal, updates cleanup and worker draining, expands reclamation tests and documentation, and simplifies worker coverage updates.

Changes

Epoch reclamation and guarded traversal

Layer / File(s)	Summary
Epoch-protected child storage lib/kv-router/Cargo.toml, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/children.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/node.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/types.rs	Child snapshots use epoch atomics. Retired snapshots and unlinked children use local or shared graveyards. Child access can return guard-scoped borrows. Insertion and removal return updated result types.
Guarded matching and event access 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/mod.rs, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/sync_impl.rs	Matching pins once per lookup and passes borrowed nodes through traversal. Event handling and anchored matching use epoch guards.
Cleanup, draining, and reclamation validation lib/kv-router/src/indexer/concurrent_radix_tree_compressed/{mod.rs,node.rs,remove.rs,sync_impl.rs,README.md}, lib/kv-router/src/indexer/concurrent_radix_tree_compressed/tests.rs, lib/kv-router/tests/crtc_thread_exit.rs	Tree destruction, stale-leaf cleanup, sweeps, worker loops, and flushes drain deferred references. Tests cover deferred drops, drain budgets, shared-queue collection, tree destruction, and thread exit.
Worker coverage update lib/kv-router/src/indexer/compressed_radix.rs	append_blocks_to_leaf uses retain to preserve the appending worker’s full-edge coverage and update other workers to the old edge cutoff.

Priority: ➖ Normal

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

Merge Risk: 🔵 Low · up to 92c2d

Lookups now pin once per walk and borrow child nodes under that pin instead of cloning a reference at each step. Removed child maps are freed later, after readers move on. One bounded memory-retention gap remains: a lookup thread can keep detached tree memory after traffic stops, until that thread runs another lookup or exits. The change is mergeable with this follow-up.

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 52.44% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 82 functions across 10 files. (2 skipped:…	Write docstrings for the functions missing them to satisfy the coverage threshold.
Description check		The description provides detailed overview, implementation details, reviewer guidance, validation results, and performance data. However, it does not include the required Related Issues section with a…	Add a Related Issues section containing an explicit reference such as "Closes #15609". Keep the existing technical details and validation information unchanged unless needed for clarity.

✅ 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 main change: epoch-reclaimed CRTC child maps and borrowed reads. It is concise and specific.

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Full details: Docstring Coverage

Explanation

Docstring coverage is 52.44% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 82 functions across 10 files. (2 skipped: 2 unsupported.)

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Full details: Description check

Explanation

The description provides detailed overview, implementation details, reviewer guidance, validation results, and performance data. However, it does not include the required Related Issues section with an explicit issue-closing reference.

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• Fix all pre-merge checks with AI

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Commit to this branch
• Create a new PR

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[coderabbitai]

Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
Review comments at
@lib/kv-router/src/indexer/concurrent_radix_tree_compressed/children.rs:
- Around line 184-189: Update LookupPin::drop to hand off remaining local
graveyard entries to SHARED_GRAVEYARD when the scope was not already draining,
after the bounded drain. Keep the handoff best-effort with the existing TLS
borrow safeguards, and avoid pushing leftovers from scopes that were already
draining.

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between ee55ac5 and 92c2db5.

⛔ Files ignored due to path filters (3)

• Cargo.lock is excluded by !**/*.lock
• lib/bindings/kvbm/Cargo.lock is excluded by !**/*.lock
• lib/bindings/python/Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (12)

• lib/kv-router/Cargo.toml
• lib/kv-router/src/indexer/compressed_radix.rs
• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/README.md
• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/children.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/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/tests/crtc_thread_exit.rs

Included review availability: This review used your included allowance. Your plan provides up to 12 included reviews per hour; 10 remain after this review.

[dynamo-review-agent]

Questions for the author:

• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/matches.rs: The supported inference lookup paths now pin crossbeam-epoch once per radix walk and run the lookup graveyard drain on every LookupPin drop. Please provide the PR's pending A/B throughput and tail-latency results, including shallow/short-query lookups and a run with reclaimed CRTC garbage present, so the runtime impact of the new per-request pin/drain work can be assessed.

[coderabbitai]

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[dynamo-review-agent]

Previously reported defects still present:

• Original discussion: The previously reported redundant doc comment above NodeChildren::insert is still present at lib/kv-router/src/indexer/concurrent_radix_tree_compressed/children.rs:545; it still just restates that the helper inserts or replaces the keyed child.
• Original discussion: The previously reported redundant doc comment above NodeChildren::remove is still present at lib/kv-router/src/indexer/concurrent_radix_tree_compressed/children.rs:619; the signature and body already show that it unlinks the keyed child and returns whether it was present.
• Original discussion: The previously reported redundant doc comment above run_lane is still present at lib/kv-router/tests/crtc_thread_exit.rs:54; the helper body already shows that it runs the tasks on a fresh worker lane and waits for termination.
• Original discussion: Verified still present: LookupPin::drop drops the epoch guard and drains only LOOKUP_GRAVEYARD_BUDGET local graveyard units with include_shared=false, but it does not hand off any remaining LOCAL_GRAVEYARD entries before the lookup DrainScope exits. A long-lived lookup thread that goes idle can therefore retain up to LOCAL_GRAVEYARD_CAP graves holding detached subtrees, and event-lane idle/flush drains cannot reach that TLS. The narrow correction is to move remaining local graves from non-lane lookup scopes to SHARED_GRAVEYARD after the bounded drain.
• Original discussion: Verified still present: a lookup thread enters a temporary DrainScope, and LookupPin::drop only drains up to LOOKUP_GRAVEYARD_BUDGET local graves with include_shared=false before restoring draining=false. Any remaining LOCAL_GRAVEYARD entries on a non-lane thread are not reachable by event-lane idle drains or Flush and can retain detached subtree memory until that thread performs another lookup or exits.
• Original discussion: LookupPin::drop still only drains 8 local graves and leaves any remaining lookup-thread local graveyard entries in TLS when the thread goes quiet, so detached subtrees can remain retained until another lookup on the same thread or thread exit.

Questions for the author:

• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/matches.rs: The supported inference lookup paths now pin crossbeam-epoch once per radix walk and run the bounded lookup graveyard drain on every LookupPin drop. The PR includes aggregate Mooncake A/B results, but please provide the shallow/short-query lookup results and a run with expired CRTC garbage present so the overhead of the new per-request pin/drain work can be assessed for those supported paths.

[dynamo-review-agent]

Previously reported defects still present:

• Original discussion: The doc comment above NodeChildren::compare_and_swap remains at children.rs:409 and still only restates the compare-and-swap behavior evident from the helper and compare_exchange call.
• Original discussion: The doc comment above NodeChildren::replace remains at children.rs:286 and still only restates the helper's swap-and-retire implementation.
• Original discussion: Verified still present with an additional teardown case: ConcurrentRadixTreeCompressed::drop flushes and drains only garbage that is already eligible. Crossbeam 0.9.18 does not guarantee deferred callbacks run during flush while another participant remains pinned. A callback that expires after all event lanes and the tree have been dropped sends its grave to SHARED_GRAVEYARD, leaving no guaranteed consumer and retaining the detached subtree until another CRTC lane drains it or the process exits. Teardown needs a reclamation owner that remains available for callbacks completing after tree drop.
• Original discussion: Verified still present: LookupPin::drop drains only eight local graves with include_shared=false. If that non-lane lookup thread then becomes idle, remaining LOCAL_GRAVEYARD entries can retain detached subtrees until another lookup runs on the same thread or the thread exits; event-lane idle and Flush drains cannot reach them.
• Original discussion: LookupPin::drop still drains only eight local graves and leaves any remainder in thread-local storage when a non-lane lookup thread becomes idle. Event-lane flushes cannot reach that storage, so detached subtrees can remain retained until the same thread performs another lookup or exits; hand remaining graves from a temporary lookup scope to SHARED_GRAVEYARD after the bounded drain.
• Original discussion: LookupPin::drop still drains only eight local graveyard units and leaves any remainder in lookup-thread TLS; if that long-lived thread becomes idle, detached subtrees remain unreachable by event-lane idle and flush drains until another lookup or thread exit.

Questions for the author:

• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/matches.rs: The supported inference lookup paths now pin the global epoch collector and run a bounded graveyard drain for every radix walk. Please provide shallow/short-query throughput and tail-latency results, including a run with expired CRTC garbage present; the aggregate Mooncake results do not isolate this per-request overhead.

[dynamo-review-agent]

Previously reported defects still present:

• Original discussion: The lookup drain budget is still bypassed when Arc::into_inner fails: popped nodes do not increment work, so an 8-unit drain can drop references for an entire high-fanout grave on the inference path. Count every popped node toward the budget and rebury the unprocessed remainder.
• Original discussion: The previously reported doc comment above NodeChildren::compare_and_swap remains at children.rs:409 and still restates the function name and compare_exchange call.
• Original discussion: The previously reported doc comment above NodeChildren::replace remains at children.rs:286 and still only narrates the helper's swap-and-retire implementation.
• Original discussion: Verified still present: LookupPin::drop drains only eight local graveyard units and leaves any remainder in thread-local storage. If that non-lane lookup thread becomes idle, detached subtrees remain retained until another lookup runs there or the thread exits. Hand remaining local graves to SHARED_GRAVEYARD after the bounded drain when the scope was not already draining.
• Original discussion: Verified still present: drain_graveyard_with increments its budget only when Arc::into_inner succeeds. A lookup draining a large Grave::Nodes whose children have other references can therefore drop the entire vector despite the eight-unit budget, adding O(child fanout) reference-count operations to one inference request. Count every popped node toward the budget and rebury the remainder.
• Original discussion: Verified still present: a non-lane LookupPin drains only eight local graveyard units, then restores draining to false while leaving the remainder in thread-local storage. If that long-lived thread becomes idle, detached subtrees remain unreachable by event-lane idle and flush drains until another lookup runs there or the thread exits. Hand remaining local graves to SHARED_GRAVEYARD when a temporary lookup scope ends.
• Original discussion: Verified still present: the drain budget advances only when Arc::into_inner succeeds. A Grave::Nodes containing many children with other strong references can therefore drop every Arc on a lookup thread despite the nominal eight-unit budget. Count each popped node as work and rebury the unprocessed remainder.
• Original discussion: LookupPin::drop still leaves graves beyond its eight-unit budget in lookup-thread TLS. If a non-lane thread then becomes idle, event lanes and Flush cannot reclaim those detached subtrees; hand the remaining local graves to SHARED_GRAVEYARD when the temporary lookup drain scope ends.

Questions for the author:

• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/matches.rs: The supported inference lookup paths now pin the global epoch collector and perform graveyard draining for every radix walk. Please provide shallow or short-query throughput and tail-latency results, including a run with expired CRTC garbage present, to isolate this new per-request overhead from the aggregate Mooncake results.
• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/matches.rs: The supported inference lookup paths now pin the global epoch collector and run a bounded graveyard drain for every radix walk. The aggregate Mooncake results do not isolate shallow or short-query overhead or the cost when expired CRTC garbage is present; please provide throughput and tail-latency measurements for those cases.

[dynamo-review-agent]

Previously reported defects still present:

• Original discussion: The teardown retention defect remains. ConcurrentRadixTreeCompressed::drop flushes the global epoch collector once and drains only callbacks that have already run. If an unrelated crossbeam participant remains pinned, this tree's retired snapshots can become reclaimable only after its lanes and tree have been destroyed; their callbacks then enqueue detached subtrees in SHARED_GRAVEYARD, with no remaining lane guaranteed to drain them. That memory can remain for the process lifetime unless another CRTC worker later runs. Teardown needs ownership-aware pending-retirement tracking or another drainer that survives until this tree's deferred retirements complete.

[dynamo-review-agent]

Previously reported defects still present:

• Original discussion: The doc comment above LocalGraveyard::draining is still present and only restates that the flag tracks whether the thread frees its graveyard, while the nearby DrainScope documentation contains the actual rationale.
• Original discussion: The doc comment above Node::into_child_arcs is still present and repeats what the forwarding helper name, return type, and delegated children.into_child_arcs() call already show; the caller already makes the iterative graveyard use explicit.
• Original discussion: The teardown retention concern remains present: ConcurrentRadixTreeCompressed::drop still flushes the global epoch collector once and drains only graves whose deferred callbacks have already run. If another epoch participant is pinned during drop, this tree's retired snapshots can become reclaimable only after the event lanes and tree are gone; their callbacks then enqueue detached subtrees in the process-wide graveyard with no remaining lane guaranteed to drain them, so that memory can remain retained until a later CRTC worker runs or the process exits.
• Original discussion: Verified still present: ConcurrentRadixTreeCompressed::drop still only flushes the global epoch participant once and drains callbacks that have already run. If another crossbeam participant remains pinned while this tree drops, this tree's retired snapshots can become eligible only after its workers and tree are gone; their deferred callbacks then enqueue detached subtrees in SHARED_GRAVEYARD with no remaining lane guaranteed to drain them, retaining that memory until another CRTC lane runs or the process exits.

Questions for the author:

• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/matches.rs: The supported lookup paths now pin the global epoch collector and run LookupPin cleanup once per radix walk. The PR describes aggregate Mooncake results and a follow-up overload comparison, but I could not find isolated shallow/short-query throughput and tail-latency numbers, including both negligible-garbage and expired-garbage cases; please provide those measurements so the per-request pin/drain overhead can be assessed for these supported inference paths.

[devin-ai-integration]

✅ Dynamo PR CI passed — run 37732346463 (attempt 1) on c1b7110c75

Gate checks: ✅ backend-status-check · ✅ deploy-status-check · ✅ dynamo-status-check

_{Posted automatically by Devin for run 37732346463. Updated on every full-CI run of this PR.}

[dynamo-review-agent]

Previously reported defects still present:

• Original discussion: The previously raised redundant doc comment above Node::into_child_arcs is still present; the helper name, return type, and delegated children.into_child_arcs() call already show that it consumes the node and returns child Arcs, and the caller documents the iterative graveyard use.
• Original discussion: The previously raised redundant field comment above LocalGraveyard::draining is still present; it only says the flag tracks whether this thread frees its graveyard, while the nearby DrainScope documentation explains the actual rationale.
• Original discussion: The reviewed head still only calls NodeChildren::flush_retired() once and then drains currently available graves during ConcurrentRadixTreeCompressed::drop. Retired snapshot callbacks delayed by another pinned participant in the global epoch can run after this drop path and enqueue detached subtrees into SHARED_GRAVEYARD when no CRTC lane remains to drain them, so the previously reported teardown retention issue remains present.
• Original discussion: The teardown path still only flushes/drains once in ConcurrentRadixTreeCompressed::drop; if a retired child snapshot is still blocked by an unrelated pinned epoch participant, its deferred callback can run after the tree and worker lanes have exited and enqueue the detached subtree into SHARED_GRAVEYARD with no remaining drainer, retaining that memory until another tree/lane appears or process exit.
• Original discussion: Verified still present: ConcurrentRadixTreeCompressed::drop still flushes the global epoch participant once and drains only callbacks that have already run. If another epoch participant remains pinned while this tree drops, this tree's retired snapshots can become eligible only after its workers and tree are gone; their deferred callbacks then enqueue detached subtrees in SHARED_GRAVEYARD with no remaining lane guaranteed to drain them, retaining that memory until another CRTC lane runs or the process exits.

Questions for the author:

• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/matches.rs: The supported lookup paths now pin the global epoch collector and run LookupPin cleanup once per radix walk. The PR describes aggregate Mooncake results and a follow-up overload comparison, but I could not find isolated shallow/short-query throughput and tail-latency numbers, including both negligible-garbage and expired-garbage cases; please provide those measurements so the per-request pin/drain overhead can be assessed for these supported inference paths.

[dynamo-review-agent]

Previously reported defects still present:

• Original discussion: The comment above LocalGraveyard::draining remains and merely restates the field; the nearby DrainScope documentation already preserves the rationale.
• Original discussion: The comment above LocalGraveyard::drop remains and narrates the immediately following hand_off call; nearby graveyard documentation already records the lasting lifecycle rationale.
• Original discussion: The comment above Node::into_child_arcs remains and repeats the forwarding helper's name, return type, and delegated call; its caller already documents the iterative-reclamation reason.
• Original discussion: The teardown path still flushes the epoch collector once and drains only callbacks that have already run. A snapshot blocked by another pinned participant can become reclaimable after the tree and its lanes are gone, enqueueing detached nodes in SHARED_GRAVEYARD with no guaranteed drainer until another CRTC lane runs or the process exits.
• Original discussion: The teardown retention defect remains: ConcurrentRadixTreeCompressed::drop flushes the epoch collector once and drains only callbacks that have already run. If another participant remains pinned, delayed callbacks can later enqueue detached subtrees into SHARED_GRAVEYARD after all CRTC lanes have exited, retaining them until another lane runs or the process exits.
• Original discussion: Verified still present: tree teardown flushes once and drains only graves whose deferred callbacks have already run. A callback blocked by another pinned epoch participant can run after the tree and all event lanes are gone, enqueue detached nodes into SHARED_GRAVEYARD, and retain them until another CRTC lane runs or the process exits.

Questions for the author:

• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/matches.rs: The supported lookup paths now pin the global epoch collector and perform bounded cleanup on every radix walk. Please provide shallow/short-query throughput and tail-latency measurements with both negligible and expired CRTC garbage; the aggregate results and reported 6–7% p99.9 increase in one overload cell do not isolate this per-request cost.

[dynamo-review-agent]

Previously reported defects still present:

• Original discussion: The previously reported comment above LocalGraveyard::draining is still present and still just points to nearby DrainScope documentation for the same draining rationale.
• Original discussion: The previously reported doc comment above LocalGraveyard::drop is still present and still narrates the handoff performed by the destructor/helper rather than preserving additional lasting constraints.
• Original discussion: The previously reported doc comment above Node::into_child_arcs is still present and still describes the forwarding helper's direct delegation to the child-map helper.
• Original discussion: The teardown retention issue remains present: ConcurrentRadixTreeCompressed::drop still calls NodeChildren::flush_retired() once and drains only graves whose deferred callbacks have already run. If another crossbeam epoch participant is pinned during drop, this tree's retired snapshots can become reclaimable only after the tree and its worker lanes are gone; their callbacks then enqueue detached subtrees in SHARED_GRAVEYARD with no remaining CRTC lane guaranteed to drain them, retaining that memory until another CRTC lane runs or process exit.
• Original discussion: Verified still present: ConcurrentRadixTreeCompressed::drop still only calls NodeChildren::flush_retired() once and drains graves whose deferred callbacks have already run. If another epoch participant is pinned during teardown, this tree's retired snapshots can become reclaimable only after the tree and worker lanes are gone; their callbacks then enqueue detached subtrees into SHARED_GRAVEYARD with no remaining lane guaranteed to drain them, retaining that memory until another CRTC lane runs or process exit.

Questions for the author:

• lib/kv-router/src/indexer/concurrent_radix_tree_compressed/matches.rs: The supported inference lookup paths now create a LookupPin for every radix walk and run bounded graveyard cleanup when it drops. Please provide isolated query_service throughput and tail-latency measurements for shallow/short-query keep-up replay with negligible garbage, plus a run with expired CRTC garbage present, so the per-request pin/drain overhead can be assessed apart from the aggregate overload results.

[dmitry-tokarev-nv]

I approve 87b40b2d5f24231ff65e3c0db14c532a9d140f63. The head moved to this SHA during my review. It merges base commit 4c3e7e280f into eeadcb4016 with no edits, and the PR diff did not change. I ran the checks below on eeadcb4016, and I ran again the ones that the merge can change. The crossbeam-epoch bump and the three lock files are clean. If I revert the fix that a new regression test guards, that test fails. A soak, which reruns the race tests many times, found no failure on an x86_64 host. One P3 and one open item follow.

[P3] The thread-exit bullet in the description ends with "tests/crtc_thread_exit.rs covers it". If "it" means the inline free in NodeChildren::drop (children.rs:250), the test does not cover it. If that drop calls epoch::pin() or defers its map through the epoch, the test still passes. Please reword the bullet so that it names what the test guards.

Mutants of the head, each a copy with one change, show what the test guards.

The bullet in the description:

Thread exit: NodeChildren::drop frees inline instead of pinning, which recursed at thread exit; tests/crtc_thread_exit.rs covers it.

Change to the head	crtc_thread_exit	42 CRTC lib tests
None	pass, 50 of 50	pass
epoch::pin() as the first line of the retire closure	SIGSEGV, 3 of 3	not run
epoch::pin() as the first line of NodeChildren::drop	pass, 3 of 3	pass
NodeChildren::drop calls guard.defer_destroy(state) instead of the inline free	pass, 3 of 3	pass

The two deferred destructors, at children.rs:327 and children.rs:344, only call bury. So this test never runs NodeChildren::drop inside a deferred destructor. Of these mutants, the test fails only for the one that calls epoch::pin() inside a deferred destructor.

Open item: this PR stacks on #15606 at 4c3e7e280f.

What I read, what I ran, and what I did not run.

• I read every changed line of the 15 files, all of children.rs at the head, and each lock-file change line by line. I sampled the gate locks around child-map writes in node.rs, the lane loop in sync_impl.rs, and ThreadPoolIndexer::drop. I did not read the unchanged parts of the other CRTC files.
• crossbeam-epoch 0.9.21 is on crates.io and is not yanked. taiki-e published it on 2026-09-05. Its .crate sha256 equals the checksum in all three locks. Its source, build.rs, and manifest equal tag crossbeam-epoch-0.9.21 (2920c984) on the v0.8 branch of crossbeam-rs/crossbeam.
• From 0.9.18 to 0.9.21, the MSRV (1.61), the features, and the dependencies did not change. The behavior changes are: Atomic::fetch_update returns the previous pointer, fmt::Pointer no longer dereferences, defer_destroy accepts unsized types, and Shared::null is const. If the build is a TSan build, the new build.rs turns on the ThreadSanitizer orderings. Other builds keep the 0.9.18 orderings. No crate in the three graphs calls fetch_update.
• The commit message names ThreadSanitizer and the fmt::Pointer fix as the reasons for 0.9.21. The head kv-router and its tests also type-check against 0.9.18, so no API in this PR needs 0.9.21.
• Each lock changes only the version, the checksum, and the new crossbeam-epoch entry of dynamo-kv-router. cargo tree --locked passes in the workspace and both bindings at ee55ac529e, at eeadcb4016, and at a trial merge with main at def3b79b15.
• Before this PR, 31 workspace crates used 0.9.18 through crossbeam-deque, moka, or crossbeam. So did 7 crates in the KVBM binding and 13 crates in the Python binding. They all get 0.9.21 now. The Pre Merge rust-tests jobs for the workspace and both bindings passed on eeadcb4016.
• cargo test -p dynamo-kv-router --locked passes at the old base ee55ac529e (1042 lib tests) and at eeadcb4016 (1049 lib tests and crtc_thread_exit). With --all-features, the counts are 1295 and 1302. The only new test names are the 7 new tests and crtc_thread_exit.
• Reverting the fix in 60f5944548, 22a362b364, or eeadcb4016 fails budgeted_drain_counts_references_it_cannot_free, lookup_hands_graveyard_leftovers_to_the_lanes, or idle_drain_yields_between_chunks, in that order. If live_strong_count keeps the retired references, 2 cleanup tests fail.
• The soak ran the 42 CRTC tests 4100 times, with 32 test threads and also as 8 parallel processes. It also ran crtc_thread_exit 50 times and the full lib suite 11 times. No run failed or hung. One run took 1.4 s, and in 1500 timed reruns no test took more than 0.3 s.
• cargo check --locked --all-targets passes for the workspace and both bindings. A full cargo build --workspace --all-targets did not fit the disk limit of the host. The Pre Merge rust-tests (.) job ran cargo test --locked --all-targets on eeadcb4016 merged with bf6597d901, which is the old base merged into main. It passed. At the time of this review, CI on 87b40b2d5f was still running.
• On 87b40b2d5f, the three locks and lib/kv-router/Cargo.toml equal those of eeadcb4016. There, cargo tree --locked passes in all three workspaces, and cargo test -p dynamo-kv-router --locked passes (1049 lib tests and crtc_thread_exit). cargo check --locked --all-targets passes for the Python binding, which the merge changed.
• I did not repeat the race soak, AddressSanitizer, and ThreadSanitizer runs from the description, or its benchmarks.

[dmitry-tokarev-nv]

I approve b3fabcabb514a2cda3a80cbd4d8d3c3eb3168430. My approval moves from 87b40b2d5f to this head. The three new merges of layer 1 hold no hand edits, and the diff of this PR did not change.

[P3] Still open from my review of 87b40b2d5f: the thread-exit bullet in the description ends with "tests/crtc_thread_exit.rs covers it". The test does not cover the inline free in NodeChildren::drop (children.rs:250). If that drop pins or defers its map, the test still passes at this head. Please reword the bullet so that it names what the test guards.

Mutants of this head, each a copy with one change, show what the test guards.

Change to this head	crtc_thread_exit	42 CRTC lib tests
None	pass	pass
epoch::pin() as the first line of the retire closure (children.rs:327)	SIGSEGV, 3 of 3	not run
epoch::pin() as the first line of NodeChildren::drop	pass, 3 of 3	pass
NodeChildren::drop calls guard.defer_destroy(state) instead of the inline free	pass, 3 of 3	pass

With jemalloc as the global allocator, the mutant in the retire closure also fails with SIGSEGV in 3 of 3 runs. The bullet and the test are the same as at 87b40b2d5f.

Open item: this PR stacks on #15606 at ca51e42427.

What I measured at this head, and what I did not run.

• I merged again the parents of 1011fa8933, 3cbb3f4ebb and b3fabcabb5 with git merge-tree. Each result has the same tree as its commit.
• I applied the diff that I approved (4c3e7e280f to 87b40b2d5f) to the new base ca51e42427. The result is the head tree. All 15 files have the same changed lines as before.
• The two lock files have new blobs only because layer 1 changed their base side. Each lock still changes only crossbeam-epoch 0.9.18 to 0.9.21 and the new crossbeam-epoch entry of dynamo-kv-router.
• cargo metadata --locked and cargo tree --locked pass in the workspace, lib/bindings/python and lib/bindings/kvbm. They pass at the base, at this head, and at a trial merge with main at f787ba7568.
• In all three, the head tree differs from the base tree only in crossbeam-epoch and the new edge. The workspace and Python graphs resolve crossbeam-epoch 0.9.21 together with tikv-jemallocator 0.7.0 and tikv-jemalloc-sys 0.7.1.
• As a control, I pinned crossbeam-epoch = "=0.9.18" in lib/kv-router/Cargo.toml. Then both --locked commands fail.
• The CRTC runs inside _core, and layer 1 makes jemalloc the Rust global allocator of _core. --router-event-threads defaults to 4. Any value above 1 makes dynamo-llm build ThreadPoolIndexer<ConcurrentRadixTreeCompressed> (lib/llm/src/kv_router/indexer/mod.rs:152 and :205).
• In a copy of this head, I made tikv-jemallocator 0.7.0 the global allocator of the kv-router lib tests and of crtc_thread_exit. It has the features that the extension uses on Linux with glibc.
• With jemalloc, all 1049 lib tests and crtc_thread_exit pass. Both test binaries contain _rjem_malloc, and jemalloc prints its statistics with background_thread:true. The test binaries of the unchanged head contain no _rjem_malloc.
• On an x86_64 Linux host, cargo test -p dynamo-kv-router --locked passes at this head. That is 1049 lib tests, crtc_thread_exit and plugin_api_compat.
• The kv-router source at this head is the same as at eeadcb4016, where my earlier review ran its soak. It is also the same at the old and the new base. So the earlier base runs and soak apply to this code.
• Pre Merge passed on a5f6e76c0f, which merges this head into the base tip merged with main at 548a010548. Its rust-tests and rust-clippy jobs for the workspace and both bindings passed.
• I did not repeat the soak, because it needs parallel runs. I did not repeat the sanitizer runs or the benchmarks of the description.

[jthomson04]

Closing the stack (#15609 → #15610 → #15611). #15611's writer per-block work carried the e2e win; it was ported standalone onto main as #16134 and has merged.

Per-layer attribution for this PR: about 3.5% CPU at page size 1, neutral at page size 16. Frontend RSS at matched stored events (page size 16, 30M events) was about 7–10% higher than base. #16134 doesn't need the epoch-reclaimed child maps, so this foundation isn't worth carrying on its own. The branch is kept for reference.