[iMMIQ]

Purpose

Background

We deploy DeepSeek V4 Flash Vision Exp with vLLM Ascend v0.27 in our internal network, with Simple CPU Offload enabled. During operation, we observed a cache miss rate exceeding 10%. Investigation showed that some reusable cache entries were not being retrieved due to the unimplemented TODO in vllm/v1/simple_kv_offload/manager.py: the existing API only matches a CPU suffix following the GPU prefix, rather than searching for a complete valid prefix across both pools.

After implementing joint lookup, we ported the change to the v0.27 branch in our fork and redeployed it in the same environment. The deployment has been running for five days, with normal operation and successful retrieval of cache entries previously affected by this limitation.

This PR targets upstream main. The deployment observations above come from the v0.27 backport in our fork.

Changes

Support valid prefixes composed of interleaved GPU and CPU cache blocks, replacing the limitation of matching a GPU prefix followed by a CPU suffix.

For example, given a GPU → CPU → GPU cache layout, reuse both GPU blocks directly and allocate and load a target block only for the middle CPU block. When both pools contain a required block, prefer the GPU copy.

The implementation:

• Provides a read-only joint lookup view for existing attention-specific matching algorithms, preserving hybrid-attention convergence and memoizing queries within each request.
• Records hit sources by group and block position, distinguishing GPU hits, CPU blocks requiring loads, and attention padding.
• Pins source blocks before allocation and allocates targets only for CPU hits. Targets are registered in the cache after loading completes, without re-registering interleaved GPU hits.
• Attributes GPU/CPU cache statistics and cache events to their actual sources.
• Preserves full-block CPU loads, copy-on-write for partial GPU hits, and existing watermark and speculative-decoding reservation rules. If capacity is insufficient, allocation of the entire match is deferred.

Relationship to related work

This PR implements joint lookup for interleaved GPU/CPU prefixes in SimpleCPUOffloadConnector. The related #52773 / #52774 address boundary backfill in OffloadingConnector, so the connector and implementation scope differ.

Test Plan

Cover the following behaviors:

• All-GPU, all-CPU, multiple interleavings, duplicate hits across pools, missing intermediate blocks, and different sources across groups.
• GPU block identity reuse, exact CPU transfer positions, and no publication of target cache entries before loading completes.
• SWA/Mamba, different block sizes, DCP/EAGLE matching boundaries, final-token recomputation, and copy-on-write for partial blocks.
• Insufficient capacity, speculative-decoding reservations, and reference cleanup after retries, cancellation, and reset.
• Bypassing separate GPU/CPU scans on the joint path, with no repeated probes for the same hash/group.
• GPU/CPU statistics and interleaved cache events.

Regression command after the latest code cleanup:

.venv/bin/python -m pytest \
  tests/v1/simple_kv_offload/test_scheduler.py \
  tests/v1/simple_kv_offload/test_kv_events.py \
  tests/v1/core/test_prefix_caching.py \
  tests/v1/core/prefix_cache/test_partial_prefix_cache_hits.py \
  tests/v1/kv_connector/unit/test_simple_cpu_offload_connector.py \
  -q

Run pre-commit on modified files, along with Python 3.10/3.12 type checks.

Local real-model validation uses OPT-125m, Qwen3-0.6B, and Qwen3.5-0.8B. GPU cache capacity is constrained and cache residency layouts are controlled to trigger actual CPU loads. Validation compares cold-cache and warm-cache outputs under deterministic generation settings and checks transfer positions and reference cleanup.

Test Result

After the October 30 merge with upstream main (dfe4d99e42):

• 628 cache/offload/connector regression tests passed using the regression command above.
• 440 additional scheduler, single-type manager, prefix-replay, GPU worker, and auxiliary-output tests passed. Three pipeline-parallel scheduler tests require 2-3 GPUs and fail identically on unmodified upstream main on this single-GPU host. Three streaming lookahead cases passed after downloading their missing Qwen3-Next model configuration.
• Qwen3-0.6B, bfloat16, eager MRV2: three exact cold/warm output comparisons passed. Each reused a 192-token prefix and loaded exactly three interleaved CPU blocks (48 tokens); transfer positions and released lookup/load state were checked.
• Full pre-commit on all 16 PR files passed, along with Python 3.11 and 3.12 mypy checks.
• Regression checks for joint eager CPU LRU refresh and lookahead event metadata fail when that compatibility handling is omitted and pass with it restored.

Additional regression command:

HF_HUB_OFFLINE=1 OMP_NUM_THREADS=2 .venv/bin/python -m pytest \
  tests/v1/core/test_scheduler.py \
  tests/v1/core/test_single_type_kv_cache_manager.py \
  tests/v1/core/test_prefix_replay.py \
  tests/v1/simple_kv_offload/test_worker.py \
  tests/distributed/aux_output_connector/test_store.py -q --tb=short

The three missing-config streaming cases were rerun after fetching Qwen/Qwen3-Next-80B-A3B-Instruct/config.json. The three hardware-limited cases were reproduced in a separate worktree at upstream main. The real-model smoke test used GCC because the machine's default Clang is unsupported by the installed CUDA compiler.

Earlier, 22 model/configuration combinations passed with 9,146 exact cold/warm comparisons; that full matrix has not been rerun after this merge. Some earlier OPT/MTP cold/warm differences also reproduced on the legacy lookup path and are excluded from the passing results.

AI assistance

GPT-6 Astra assisted with the implementation, tests, and this description. The code has undergone human review.

Codex also assisted with the October 30 merge resolution and validation.

[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.

[mergify]

Documentation preview: https://vllm--57325.org.readthedocs.build/en/57325/

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[mergify]

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

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

[mergify]

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

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

[vMaroon]

Event-side notes only; I didn't review the lookup itself.

The store emitted after a joint load doesn't match the dense shape consumers expect for full-attention groups. cache_blocks masks out the interleaved GPU hits via _joint_cache_reused_blocks, but cache_full_blocks still spans token_ids from num_cached_block to num_full_blocks, so for gcgg the event is block_hashes=[h1] with 48 tokens, and for gcgc it is [h1, h3] with blocks 1-3's tokens, so h3 lines up with block 2. This is in the default incremental mode, so every subscriber sees it: llm-d skips it as non_dense_block_span (the loaded GPU copy is never indexed), and Dynamo's create_stored_blocks walks tokens positionally and indexes h3 under block 2's tokens. Wonder if we could emit one BlockStored per contiguous run of newly cached blocks (parent = hash before the run, token_ids = the run), either by splitting in cache_full_blocks when a mask hides cached positions or by caching each CPU run separately on the joint path. That keeps each event dense and lines up with the dense-run direction in #44488 / #58121.

On full reports, one event per contiguous GPU run with parent_block_hash set is self-consistent, and it's the same per-run shape suggested on #57358, so I think we should converge the two PRs on it (both change the emit_cached_block_events signature, start_block here vs cached_blocks / num_cached_tokens there). It does change the documented contract though: #60616 says a full report starts at the first block with a nil parent, and with a CPU block 0 the joint path emits no nil-parent event at all. Whichever lands second should update docs/features/kv_events.md and tests/v1/core/test_kv_events_contract.py.

Two smaller consistency points:

• The joint path emits the full report in allocate_slots after admission, while the non-joint path emits it in get_computed_blocks on every lookup. The admission timing seems better; might be worth stating which one we intend.
• emit_joint_cached_block_events uses manager.block_size (DCP-scaled) while emit_cached_block_events still uses kv_cache_spec.block_size, so under DCP the two paths disagree until #50118 lands.

[vMaroon]

The mask keeps the reused GPU positions out of the hash map, but cache_full_blocks still sets token_ids to [num_cached_blocks, num_full_blocks), so the resulting BlockStored has fewer hashes than token blocks, e.g. [h1, h3] over blocks 1-3 for gcgc, so h3 pairs with block 2's tokens. llm-d drops it as non_dense_block_span and Dynamo indexes h3 under the wrong tokens. I think we want one event per contiguous run of newly cached blocks here, so each event's parent, hashes and tokens stay aligned.

[vMaroon]

One event per contiguous GPU run with a real parent looks right to me, and matches what was suggested on #57358 for null-padded hits. Worth noting it changes the full-mode shape from "starts at block 0 with a nil parent": a CPU block 0 gives no nil-parent event at all, and later runs name a parent whose GPU store only arrives after the load. Might be worth a line in the description and a contract-doc update once #60616 lands.

[vMaroon]

nit: with start_block the docstring's "blocks reused from prefix cache" now also covers mid-prefix runs, and if a run can end on a partial (copy-on-write) block, block_hashes[i] / end_block * block_size hit the same partial-tail issue #57358 is fixing for the main path. Might be worth an assert-free guard or a test with hash_block_size < block_size.

[vMaroon]

nit: this only checks block_hashes, so the 48-token span on the post-load event isn't caught. Might be worth asserting token_ids length and parent for each event, and adding a gcgc layout plus an incremental-mode case, since that's the shape every consumer sees.

[mergify]

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

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

[mergify]

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

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