[Bugfix][V1] Bound hybrid shared-prefix checkpoints by all full-attention hits

[youweizengumd-cyber]

Purpose

Fix an unnecessary warm-prefill split when a target full-attention group reports a longer reusable prefix than the draft group. The hybrid coordinator currently retains the longest single-group hit as a shared checkpoint, even when another full-attention group cannot reuse that boundary.

Target / draft / Mamba hits	Current shared boundary	Expected shared boundary
96 / 80 / 80	96	0: no missing checkpoint
96 / 80 / 64	96	80: preserve the useful checkpoint

Cap the checkpoint candidate by every full-attention group's discovered reusable hit before truncating the hit lists and overwriting their lengths. The reconciled cache-hit length, EAGLE proof-block drop, and retention policy remain unchanged. Add eight regression cases through KVCacheManager.get_computed_blocks, covering block sizes 16/32, target/draft order, and both checkpoint conditions.

Why the bound is taken here: each full-attention group's discovered hit is still available before final truncation overwrites it. Taking the minimum afterwards would lose a useful checkpoint (80 in the second example). The final reconciled hit is already no larger than every dense hit, so the cap cannot produce a negative gap.

Test Plan

The focused regression uses cache-manager fixtures and synthetic token IDs; it does not download model weights:

.venv/bin/python -m pytest tests/v1/core/test_prefix_caching.py \
  -k hybrid_mamba_shared_prefix_is_bounded_by_draft_hit -q

On base c69d5d72a6018db6ff55efc66097d377c25828cc with just the regression added: 4 failed / 4 passed. With this fix: 8 passed. The failures are the target-first boundary cases; reversed order is included to guard existing behavior.

Broader validation:

.venv/bin/python -m pytest \
  tests/v1/core/test_prefix_caching.py \
  tests/v1/core/test_mamba_align_chunk_split.py \
  tests/v1/core/prefix_cache/test_mamba_eagle_resume_checkpoint.py -q

.venv/bin/pre-commit run --files \
  tests/v1/core/test_prefix_caching.py \
  vllm/v1/core/kv_cache_coordinator.py \
  vllm/v1/core/kv_cache_manager.py

Test Result

• 165 passed in the three cache suites (41.24 s; 14 dependency deprecation warnings); applicable pre-commit hooks passed.
• Supplemental real-manager matrix: 64/64 passed, versus 10 failures with the original method. It covers coarse/fine hash sizes, EAGLE on/off, spec grouping, lookup order and checkpoint position.
• Public target/draft model validation on two H100s: raw main and raw main plus this fix each pass four generation smoke requests. The conditional experiment below removes the extra warm chunk in 160/160 measurements.
• Public LongBench evaluation is included below. Cold outputs are unchanged across the patch; observed cold/warm equality rises to 32/32. Warm subset scores decrease to the cold baseline; this is not an accuracy or general bitwise-determinism claim.

Dependency and evidence scope: this code change and its unit regression stand alone. The GPU performance comparison uses the separate draft-ownership fix #56026 on both sides; that PR is still open and is not included here. Raw main's ownership fallback masks the GPU symptom in the selected model configuration. The 39–37% timing reductions below are conditional, instrumented, sequential-run results, not a stock-main or production-throughput claim. The clean CUDA bootstrap recipe remains unvalidated.

Public scripts, pinned model/input manifests and results · Pre-outreach audit

Public GPU reproduction, model evaluation, measurements and limitations

Working and GPU benchmark base: upstream main c69d5d72a6018db6ff55efc66097d377c25828cc, fetched on 2026-09-09 (commit time 18:10:26 UTC).

Public GPU reproduction: 2 × H100 80 GB, TP=2, BF16, Python 3.12.11, Torch 2.13.0+cu129, Transformers 5.16.1, FlashInfer 0.6.18, Triton 3.7.1. FA3, DFlash 15 draft tokens, prefix caching, chunked prefill, async scheduling, -O3, max model length 65536 and max batched tokens 32768.

Component	Public repository	Pinned revision
Target	Qwen/Qwen3.5-35B-A3B	59d61f3ce65a6d9863b86d2e96597125219dc754
Draft	z-lab/Qwen3.5-35B-A3B-DFlash	a6ab3a277f856d91c43f28711611e7929073d56d
Dataset	zai-org/LongBench	5e628be450b7e67fb7ae6e201bd6d8f7056f7672
Prompts/scorers	THUDM/LongBench	2e00731f8d0bff23dc4325161044d0ed8af94c1e

The draft is the public 8-layer full-attention version explicitly paired with this target in its model card. Both models load and generate successfully, with nonzero speculative acceptance. Select 16 Qasper and 16 GovReport examples using seed 20260909; disable thinking, temperature 0, output limits 512/2048. The public reproduction files include download/file manifests, input preparation, pinned launch and measurement scripts, test logs and aggregate results. Running the scripts regenerates request-level records and raw GPU traces.

Native extensions were built from 474839f8469ed4d47bffa18c6a063547a148bf2c and reused after verifying no differences in native sources, build configuration, requirements or custom-op bindings at the new base. The bundle records this check and binary SHA256 values. All Python source uses the new base and the explicitly listed variant patches. The provided fresh-environment bootstrap recipe has not itself been validated end to end.

Reproduction scope: raw main's default ownership fallback marks all groups as draft in this model configuration, masking the GPU symptom. The conditional GPU comparison applies #56026 (506a4bdcead0520301f67e455391b8f71b4eb7c4) on both sides. The unit regression reproduces on raw main with explicit draft ownership. A separate raw-main versus raw-main-plus-this-patch control checks the standalone change.

Test Result

• 165 passed in 41.24 s in the pre-outreach rerun on the pinned base with only this patch, 14 dependency deprecation warnings; pre-commit passed again. The original recorded run took 37.40 s.
• A supplemental 64-case real-manager matrix also passes on the pinned base (10 cases fail with the original method), covering coarse/fine hash sizes, EAGLE on/off, equal/distinct full-attention specs, lookup order, and both checkpoint conditions. This matrix is in the reproduction gist; the PR keeps the eight focused regression cases.
• On unmodified new main, the eight added regression cases yield 4 failed / 4 passed, failing on the expected target-first boundary cases.
• Both conditional variants complete 96 full-generation requests and 320 single-token requests. All 160 formal warm prefill measurements change from two chunks to one. Cold requests retain two chunks.
• A sibling-prefix control restores a hit from 0 to 3504 tokens; repeating it hits 25696 tokens, and changing the cache salt hits 0.

Conditional warm engine prefill with audit wrappers enabled, one output token, five repetitions per input:

Task	Before ms	After ms	Reduction	Paired-input bootstrap 95% CI
LongBench Qasper (16)	165.128	99.891	39.51%	33.31–44.29%
LongBench GovReport (16)	162.978	102.596	37.05%	30.45–42.37%

Warm TTFT changes from 173.117 to 107.219 ms (Qasper) and 173.701 to 112.369 ms (GovReport). Cold single-token prefill increases by 1.01% and 1.47%; sequential variant runs leave temporal drift unresolved. Bootstrap intervals resample prompts, keeping all five repetitions together.

Full-output cold/warm equality improves from 12/32 to 32/32; warm/repeat remains 32/32. Cold outputs are identical across the patch. Paired first-token equality improves from 155/160 to 160/160; maximum cold/warm top-20 logprob delta decreases from 1.007612 to 0, and minimum overlap increases from 18 to 20. No formal generation reaches its output cap.

Official LongBench subset score	Cold, unchanged across patch	Before warm	After warm
Qasper QA F1	37.8218	39.9790	37.8218
GovReport ROUGE-L	32.4515	32.5920	32.4515

Scores return to the cold baseline and decrease relative to the previous warm path. This is an unnecessary-split fix with improved observed cold/warm agreement, not an accuracy-improvement claim. Changed chunking can change floating-point execution; output differences alone do not establish a cache-corruption bug, and these results do not guarantee bitwise invariance across other inputs or batches. Changed output lengths confound full-generation decode/E2E comparisons, so the performance claim uses one-token measurements.

Separate representative TP0 warm traces show fused MoE calls 160 → 80 for both tasks. Some asynchronous GPU activities remain unattributed to steps in these traces; they are retained, not silently excluded from inventories. Profiling is separate from latency measurements, and TP-rank times are not added as wall time.

Raw main and raw main plus only this patch each pass a four-request public generation smoke test and retain one warm chunk. These controls do not constitute a full stock-main performance comparison. All reported 39–37% improvements are conditional on correct draft ownership.

Limits: concurrency one, sequential variant runs, 16 examples per task, and incomplete trace step attribution. Timing includes synchronous JSON audit writes at cache lookup and prefill scheduling. Both variants use the same wrappers, but removing a split also removes an audit write. A wrapper-free, interleaved A/B run is still needed to isolate production latency without instrumentation or temporal drift. Confidence intervals describe input variation, not temporal machine drift. This does not establish production throughput or broad model-quality equivalence.

Related work

#56026 fixes draft ownership annotation; #53479 changes Mamba state materialization/back-off; #52527 adds shared-prefix loss metrics; #52371 adds other Mamba/EAGLE boundary tests; #54163 changes DFlash block-drop policy. #55403 changes sparse block eviction priority; #55868 targets GDN batch-invariant execution. These changes do not contain this full-attention hit cap. This PR does not include #56026.

AI assistance

OpenAI Codex assisted with code inspection, implementation, tests, and experiment analysis. The numerical logs above were produced by the agent. The submitting human separately confirmed reviewing every changed line, running and passing the relevant cache tests, and agreeing to the DCO. The commit includes AI attribution and the human submitter's Signed-off-by trailer.

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[youweizengumd-cyber]

Hi @s3woz, I’ve opened this PR as a follow-up fix to the shared-prefix checkpoint detection introduced in your Marconi-style admission work (#37898).

With separate target and draft full-attention groups, the longest single-group hit can overestimate the useful checkpoint boundary. For example, target/draft/Mamba hits of 96/80/80 currently trigger a checkpoint at 96 even though no Mamba checkpoint is missing within the jointly reusable prefix. For 96/80/64, the useful checkpoint is 80.

This patch caps the candidate by all full-attention groups’ discovered hits before truncation, without changing the reconciled cache hit or EAGLE block-drop policy. It includes focused regression tests.

Given your work on this mechanism, would you have time to review the boundary logic? If someone else would be better placed to review it, I’d appreciate a pointer. Thanks!