fix(prefix-cache): allow hash-aligned DCP hybrid hits

[myshytf]

Bug Description

Hybrid Mamba/attention models blanket-disable fine-grained prefix hits when
dcp_world_size > 1. This coarsens local APC hits to the DCP-expanded
full-attention block even when the recurrent manager already materializes a
complete state at every hash boundary.

For the reproduced geometry:

local/cache block       1,536 tokens
DCP world size              8
attention block        12,288 tokens
producer prompt        44,449 tokens
consumer prompt        44,609 tokens
pre-fix local hit      36,864 tokens
safe recurrent boundary 43,008 tokens

Root Cause

HybridKVCacheCoordinator enabled partial hash hits only when
dcp_world_size == 1. That guard treated two different cases as equivalent:

1. a DCP hash boundary where recurrent state would itself be partial; and
2. a DCP hash boundary that is already a complete aligned Mamba state, while
   only the DCP-sharded attention block is partial.

The second case does not need a new recurrent-state representation. It can use
the existing full Mamba state plus the existing full-attention partial-block
copy-on-write path.

Fix

• Detect aligned Mamba managers participating in the hybrid coordinator.
• Require at least one cache group with a block larger than the hash unit.
• Preserve existing DCP1 behavior.
• Under DCP, enable fine-grained hits only when every aligned recurrent
  manager block exactly equals the hash block size.
• Keep the coarse scheduler-block fallback when recurrent state would be
  partial.

This does not change tensor layouts, kernels, DCP sharding, cache keys, or
sampling behavior.

How to Verify

1. Run the exact DCP8 regression:

   pytest -q \
     tests/v1/core/prefix_cache/test_partial_prefix_cache_hits.py::test_dcp8_hybrid_reuses_prefix_at_recurrent_block_boundary

   Before the fix it fails with 36864 != 43008; after the fix it passes.

2. Run the preserved-negative regression:

   pytest -q \
     tests/v1/core/prefix_cache/test_partial_prefix_cache_hits.py::test_dcp_hybrid_keeps_coarse_hits_when_recurrent_state_is_partial

   It verifies that a DCP recurrent block larger than the hash unit remains on
   the coarse boundary.

Test Plan

• Regression observed RED before implementation (36864 != 43008)
• Partial-prefix suite: 24 passed
• Prefix-caching + Mamba split suites: 113 passed
• Single-type manager + primitives + KV geometry suites: 116 passed
• Related scheduler/connector selectors: 7 passed
• ruff check and ruff format --check
• Independent pre-commit review: approved, no blocking issue
• Runtime DCP8 exact-payload token-source and deterministic local equivalence
• Compatible-restart external equivalence

Runtime Qualification

Immutable image:

sha256:c8d565820ef3736a866ee9d9f323414ca429061ec85ed528969b44ed28fc648b

Exact captured 44,449 -> 44,609 token pair, greedy sampling, seed 0:

Arm	Local hit	External hit	Computed	TTFT	Semantic SHA-256
cold control	0	0	44,609	49.099 s	57b2c985277ccd4463b67fd51d24ec7d0c75753d543fc566de6f7629511eb47c
same-process local	43,008	0	1,601	2.053 s	57b2c985277ccd4463b67fd51d24ec7d0c75753d543fc566de6f7629511eb47c
compatible-restart external	0	36,864	7,745	11.690 s	57b2c985277ccd4463b67fd51d24ec7d0c75753d543fc566de6f7629511eb47c

The local arm was byte/semantic equivalent and improved TTFT by 23.91x.
The restarted external arm also remained byte/semantic equivalent and proves
that this change is compatible with LMCache's unchanged 12,288-token object
geometry. The local APC improvement is intentionally finer-grained than the
external L2 hit.

The immutable restart used the same OCI digest, logged preserve-compatible
with zero entries removed, retained 13,248 L2 files / 272,512,843,776 bytes,
and completed with restart count 0, no fatal log matches, and no inference-time
Triton JIT notices.

Risk Assessment

Low to medium. The behavior change is restricted to hybrid aligned-Mamba
coordinators. DCP remains disabled whenever a candidate hash boundary lacks a
complete recurrent state. The negative regression locks that fallback in.

LMCache's external 12,288-token object geometry is unchanged; this PR improves
same-process local APC reuse only.

AI assistance

AI assistance was used for the final patch review and patch-equivalent rebase. No tests or linters were rerun during final merge review.

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ℹ️ Recent review info

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Run ID: 9621c279-8f59-4639-b78f-da51844fb5b0

📥 Commits

Reviewing files that changed from the base of the PR and between ad848fc and b6e8d9d.

📒 Files selected for processing (2)

• tests/v1/core/prefix_cache/test_partial_prefix_cache_hits.py
• vllm/v1/core/kv_cache_coordinator.py

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📝 Walkthrough

Walkthrough

The coordinator now permits partial hash hits for compatible DCP-sharded hybrid caches. New tests cover aligned Mamba-state reuse, per-group cache hits, copy behavior, and fallback when recurrent state is incomplete.

Changes

Hybrid prefix-cache coordination

Layer / File(s)	Summary
Partial hash-hit eligibility vllm/v1/core/kv_cache_coordinator.py	Partial hash hits now require aligned Mamba groups, fine-grained hash-capable groups, and compatible block sizes. DCP execution additionally requires hash-aligned Mamba state blocks.
DCP hybrid-cache validation tests/v1/core/prefix_cache/test_partial_prefix_cache_hits.py	Tests validate recurrent-state reuse across boundaries, expected group hit counts, attention-group copy behavior, and coarse lookup when recurrent state is incomplete.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to b6e8d

This localized change enables aligned hybrid prefix reuse while preserving the coarse fallback for partial recurrent state; no actionable merge-blocking risk remains after normal checks and review.

Possibly related PRs

• local-inference-lab/vllm#159: Related DCP-aware coordinator and mixed-cache allocation changes.
• local-inference-lab/vllm#179: Related partial-replication and DCP-aware prefix-cache behavior.
• local-inference-lab/vllm#393: Related Mamba alignment and partial-hit regression coverage.

Suggested reviewers: zjy0516, njhill

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: enabling hash-aligned DCP hybrid prefix-cache hits.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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