[Bugfix][Spec Decode] Cache the Mamba state at the block-grid position of EAGLE resume

[akshaver]

Purpose

Fixes the second of the two EAGLE + --mamba-cache-mode align prefix-cache defects pinned
by #52371.

Full attention hits at a position it holds a key for. EAGLE prunes one hash unit off that
candidate and drops it. The Mamba group materializes state only on its own block grid, so
nothing exists at the resulting position and the hit floors back to the previous block
boundary.

Commit 1 covers the case where the shared prefix runs to the end of the producer's prompt.
When it ends earlier — a system prompt followed by a per-request suffix, the deployed shape
— the producer's tail sits over tokens no sibling shares, so a check-point there is
unreachable. The sibling stops at the last shared boundary and EAGLE drops one hash unit
below it. That position is already recorded: request.shared_prefix_boundary, set by
get_computed_blocks to the longest single-group hit.

Commit 2 is two parts:

• Scheduler._mamba_block_aligned_split ends a chunk at the junction as observed rather
  than floored to the block grid, which rounds the resume point away. Past the prompt it
  falls back to the block-floored stop — the manager writes nothing there, but stock still
  check-points at the block boundary, so dropping the stop would lose a snapshot.
• MambaManager._cache_partial_tail_block accepts the junction in addition to the prompt
  tail, bounded to the prompt chunk being computed.

Commit 3 removes a mooncake connector change introduced in Commit 1, deferring solving
disag so a follow-on PR.

Details

The junction stop is the earliest mandatory stop, so it replaces the block-boundary
stop — a junction the manager then refuses leaves less cached than not splitting at all.
KVCacheManager therefore computes one predicate both sides read:

condition	why
EAGLE annotated on the model	the manager takes the drop per KV cache group, the scheduler flag is model-wide; derived separately they disagree on any annotated model
fine-grained partial hash hits on	requires --prefix-match-unit. Under align, _align_hybrid_block_size raises the attention block to the Mamba page and then sets mamba_block_size = block_size, so both groups share one block size and the GCD that feeds hash_block_size equals the scheduler block — nothing to look an entry up with
no multi-module MTP	cache_blocks then hands the manager num_computed - num_reprefillable, not the chunk end, so the position published is not the one held

Rejected alternative. Shifting the cacheable grid to k*block - hash. That poisons the
cache — slot p would hold state after (p+1)*block - hash tokens while cache_blocks
publishes it as (p+1)*block.

Opt-in, default off

--enable-mamba-fine-grained-prefix-cache (CacheConfig.enable_mamba_fine_grained_prefix_cache).
Off, the junction stays block-floored and only the prompt tail is registered — the behaviour
of commit 1 alone, pinned by test_disabled_by_default. The field is in compute_hash's
ignored factors, so it does not perturb the compiled-graph cache.

Test Plan

uv run pytest tests/v1/core/prefix_cache/ tests/v1/core/test_prefix_caching.py \
              tests/v1/core/test_mamba_align_chunk_split.py \
              tests/v1/kv_connector/unit/test_mooncake_store_coordinator.py -q
uv run pytest tests/v1/core/ -q     # baseline-differenced against main

Test Result

• Targeted suites: 229 passed, 0 failed.
• Full tests/v1/core/: identical failure set to main — 0 new, 0 fixed.
• Every condition in the new production code is mutation-covered:

mutation	tests that fail
manager stops accepting the junction	5
scheduler reverts to block-flooring the junction	4
flag propagated without the narrowing conditions	2
multi-module MTP exclusion dropped	2
past-the-prompt stop dropped instead of falling back	1
prompt bound dropped	1

• test_scheduler_never_stops_where_the_manager_refuses sweeps the armed
  (block, hash, prompt_len) space: without the manager's acceptance clause, 10 of 15 armed
  configs split at a junction the manager then refuses.

• Per edited tests test_prefix_caching.py::test_hybrid_local_kv_retention_mtp_reuses_latest_boundary and test_mamba_align_chunk_split.py::test_unaligned_resume_never_runs_past_its_block, the boundary changed which required adjusting the pinned boundary.

Reuse Pareto

The quantity this change moves is prefix-cache hit length; TTFT and throughput are
downstream of it. Measured exactly by driving the real Scheduler._mamba_block_aligned_split
and the real KVCacheManager, at the geometry align mode actually serves — attention
block equals mamba block, hash unit = prefix_match_unit. Figure and the 84-config
grid it plots are attached in a comment below.

• 0 of 84 configs where enabling the flag reduces reuse.

• The saving is a constant block − hash_unit tokens per request, independent of where
  the shared prefix ends. The ratio therefore tracks how much other uncached work a
  request has, and how large the served block is. At the ATHENA workload (57,600 shared,
  6,400 suffix, hash unit 64):

  served block	uncached prefill off → on	ratio
  2,048	8,704 → 6,720	1.30
  4,224	13,312 → 9,152	1.45
  8,448	21,760 → 13,376	1.63

  Across suffix lengths at block 4,224: 2.32x at 400 tokens, 1.96x at 1,600, 1.45x at
  6,400, 1.15x at 25,600. Read the served block_size off a server startup log to pick the
  right row — the shape holds for every block size, only the magnitude moves.

AI assistance

AI assistance was used (Claude Code), including adversarial review passes that removed a
dead code path and two silent cache-reuse regressions from earlier drafts of this change.
Every changed line was reviewed by me and the commands above were run locally.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[github-actions]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @akshaver.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[benchislett]

Over 1000 lines of tests for 200 lines of code is far too much. Please do a pass and reduce the testing to cover the critical use-cases, and strip out any redundant ones.

[akshaver]

Over 1000 lines of tests for 200 lines of code is far too much. Please do a pass and reduce the testing to cover the critical use-cases, and strip out any redundant ones.

Done. Removed extra tests that were present from this starting as a TDD effort. Left only critical use-cases.

[github-actions]

✅ Queued 2 failed job(s) for retry in Buildkite CI #87100.

[akshaver]

The failure on "CPU-Language Generation Shard 1" appears to be because it exceeds its 50m budget on cold-cache runners. 10/12 tests pass before the timeout, no failures. Also, build 87071 hit the same job on an unrelated PR.

[akshaver]

/ci retry

[github-actions]

✅ Queued 1 failed job(s) for retry in Buildkite CI #87100.

[mergify]

Documentation preview: https://vllm--53945.org.readthedocs.build/en/53945/

[coderabbitai]

Actionable comments posted: 1

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only still-valid issues, skip the rest with a brief reason, keep changes
minimal, and validate.

Inline comments:
In `@docs/features/automatic_prefix_caching.md`:
- Line 24: Update the automatic prefix caching documentation for
--prefix-match-unit to state that it must be smaller than the Mamba block size,
divide every prefix-cacheable group block size, and align with tokens_per_state
for each applicable Mamba state; alternatively, link to the runtime validation
rules.

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[akshaver]

/ci run

[github-actions]

✅ Triggered Buildkite CI #87157 for commit 4b92effe361d.