[Bugfix] Skip non-prefix-cacheable KV groups in the offloading divisibility assert - #55037
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Follow-up to the offloading divisibility-assert fix (vllm-project#55037): with a group whose tokens_per_chunk < tokens_per_hash (e.g. a CircularBufferSpec scratch ring: 4-token blocks vs a 1152-token hash) actually scheduled, two later points crash, all rooted in hashes_per_chunk == 0 for that group: 1. update_offload_keys(): islice(step=0) raises ValueError on the first scheduled request. 2. _build_store_jobs(): offload_keys for the zero-hash group are never appended while offload_block_ids positions advance via the shared hybrid allocator -> assert len(offload_keys) == len(offload_block_ids). Zero-hash groups carry no hash-addressable offload blocks by construction; skip them rather than crash. The len(offload_keys)/len(offload_block_ids) assert for participating groups is kept — zero-hash groups never reach it. Regression tests included: hashes_per_chunk==0 derivation for a CircularBuffer group, update_offload_keys skip behavior, and idempotency across repeated scheduler steps. Signed-off-by: The Anh Nguyen <ntheanh201@gmail.com>
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…ty assert build_offloading_config() asserts that every KV cache group block size is divisible by tokens_per_hash. Groups that opt out of prefix caching (e.g. CircularBufferSpec, a one-block-per-request ring whose block size is unrelated to the hash unit) are never addressed by block hashes, so the constraint is meaningless for them and makes native KV offloading unbootable on any hybrid model that carries one: AssertionError: tokens_per_block=4 not divisible by tokens_per_hash=1152. Hybrid models (e.g. Mamba+Attention) need --enable-prefix-caching to align block sizes. Restrict the assert to prefix-cacheable groups, falling back to every group when none participate. This mirrors the filter, fallback included, that resolve_kv_cache_block_sizes() already applies when deriving tokens_per_hash. Signed-off-by: The Anh Nguyen <ntheanh201@gmail.com>
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Closing as a duplicate of #54743, which predates this PR (opened 2026-09-01) and addresses the same bug more thoroughly. Same root cause, same failure: The approaches differ in the right direction for #54743: this PR exempts non-prefix-cacheable groups from the assert, whereas #54743 scopes the offload group list itself to prefix-cacheable groups at construction, so those groups are never keyed, stored, or looked up anywhere downstream. Removing the cause is better than widening the assert, and it also obsoletes the follow-up scheduler guards I had proposed in #55038 (now closed). My mistake was not running the AGENTS.md duplicate-work check before opening this. Sorry for the extra review load. AI assistance was used in preparing this PR. |
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Summary
Native KV offloading (
OffloadingConnector) cannot boot on a hybrid model that carries a KV cache group opting out of prefix caching — e.g. aCircularBufferSpecscratch group, a small per-request ring buffer whose block size (4 tokens) is unrelated to the hash granularity.build_offloading_config()asserts divisibility over every group:Such a group is exempt by construction:
CircularBufferSpec.prefix_cacheableisFalse, so its blocks are never addressed by block hashes and constraining them bytokens_per_hashis meaningless. The assert fires anyway and the engine dies at init.resolve_kv_cache_block_sizes()invllm/v1/core/kv_cache_utils.py— the very function that producestokens_per_hash— already filters on exactly this attribute, with anor-fallback for when no group participates:This patch applies the same idiom in the consumer of that value, so the assert is checked against the same set of groups the hash unit was derived from.
Who this affects upstream. The only producer of
CircularBufferSpecin the tree is Qwen4-exp's QSA cache (vllm/models/qwen4_exp/common/qsa_cache.py), which pairs the ring with anMLAAttentionSpecgroup — exactly the hybrid shape the tests below model. QSA deliberately sizes the ring so it divides the attention block size:That keeps the scheduler's LCM well-formed, but it says nothing about the hash unit:
tokens_per_hashis the GCD over prefix-cacheable groups, i.e. the attention block size, which the strictly smaller ring never divides. So the assert fires and native offloading cannot start. Any future model carrying a compression or scratch ring lands in the same place.Changes
build_offloading_config(): restrict the divisibility assert to groups whose spec reportsprefix_cacheable, falling back to all groups when none do (mirroring the upstream idiom above).tests/v1/kv_connector/unit/offloading_connector/test_build_offloading_config.py:FullAttentionSpec(block 1152) +CircularBufferSpec(block 4) group set now builds an offloading config, where stock code raises the assert above;alignmode pins the hash unit to the LCM (48) of the group block sizes, which the attention group's own block size (16) does not divide, and the assert still fires;The tests reuse
_make_vllm_config()and_full_attention_spec()from the neighbouringtest_config.pyrather than hand-rolling fixtures.Test plan
tests/v1/kv_connector/unit/offloading_connector/, alongside the existingtest_config.pyCircularBufferSpec— 4-token blocks against a 1152-token hash unit: the engine boots with KV offloading enabled where stock code crashes at init (3/3 attempts).Follow-up: #55038 fixes three later crashes in the offloading scheduler that only surface once this lands and such a group is actually scheduled. That branch is this one plus exactly one commit.