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After a load that ends in a partial tail, update_state_after_alloc sets next_stored_chunk_idx to cdiv(num_cached_tokens, tokens_per_chunk). This skips the chunk that holds the partial tail. The request then computes the rest of that chunk, but the connector never stores it. The next lookup of the same prefix stops at this chunk, finds the same partial tail again, and the request computes all tokens after it again. For a full-attention group, start the store at the chunk of the partial tail. Sliding-window and Mamba groups keep the skip: their block for that chunk can hold the tail state, which is not the state at the chunk end. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: Giannis Ntovas <ntovas.gi@gmail.com>
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This pull request has merge conflicts that must be resolved before it can be |
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Rebased locally on 971378f466: scheduler.py merges cleanly; the only conflict is in test_scheduler.py, where main added test_sync_partial_tail_load_targets_hashed_blocks and test_sync_load_budget_is_shared_per_step at the same spot, and keeping both sides resolves it. On main the new test still fails (assert 2 == 1); after the rebase it passes, and tests/v1/kv_connector/unit/offloading_connector/ gives 679 passed, 2 skipped.
The test asserts next_stored_chunk_idx, not the store. With the 30-token helper request each group has one key, so chunk 1 can't be stored in that geometry. A 40-token variant that loads the partial tail, steps to token 40, and asserts the full-attention group stores its chunk-1 key from block 32 fails on main and passes here, also with eagle=True and with a sync load. I'd assert the store that way.
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Encountered the same Issue with Qwen3.8-27B (GDN hybrid) on vLLM 0.31.0, CPU offloading + 7 parallel conversations, 40k-60k context, 77 turns: 62 turns stuck and 551k tokens recomputed without the fix, 0 stuck and 581 tokens with it. |
Overview
After a load that ends in a partial tail, the OffloadingConnector scheduler never stores the chunk that holds the tail. Each later lookup of the same prefix stops at the same partial tail, and the request computes all tokens after it again. This PR starts the store of a full-attention group at the chunk of the partial tail.
Related: #61019 (draft) changes the copy-on-write hand-off store of partial tails with EAGLE/MTP groups. It does not change
next_stored_chunk_idx, and its head (3276fdd) merges with this PR without a conflict.Duplicate check: I searched the open and closed issues and PRs for
next_stored_chunk_idx, "partial tail" and "offload". #56795, #52771 and #51761 report other defects of the store index. No issue or PR reports this case.AI assistance was used for this PR (Claude Code): the root-cause analysis, the patch, the test and this description. I reviewed every changed line and the test results.
Claims
Validation
Unit test (new):
test_partial_tail_load_keeps_tail_chunk_storableloads 28 tokens with 16-token chunks. Chunk 0 is a full hit, and chunk 1 holds the partial tail. The test checks that the full-attention group starts its store at chunk 1, and that the Mamba group keeps the skip.ruff check,ruff format --check,typosandtools/pre_commit/mypy.py 3.11pass on the changed files. CI runs these tests in thev1/kv_connector/unitstep.End to end, on an older base: nightly 21d93d0 (2026-10-05). The lines that this PR changes are the same in that base and in main. Both runs have the same other local patches.
align), W4A16 checkpoint with BF16 activations, 2× H200, TP=2, no expert parallelism.disable_eagle_block_drop.--prefix-match-unit 64. The attention block is 2,304 tokens, so most loads end in a partial tail.Without the fix, the CPU-tier hit of a session at 32 sessions stays at one boundary for several turns. For example, it stays at 114,752 tokens for prompts of 138K, 139K, 144K and 145K tokens, and each of these turns computes 24K–30K tokens again. At 16 sessions the computed prefill does not change (6,561 and 6,603 tokens per request).
Details
Root cause:
update_state_after_allocsetsnum_chunks = cdiv(num_cached_tokens, tokens_per_chunk). This count includes the chunk of a partial tail. For the chunk-level policy,next_stored_chunk_idx = num_chunksthen treats that chunk as a hit chunk. The load fills the chunk only up to the tail boundary, and the request computes the rest of it. The store starts at the next chunk, so the full chunk is never stored, and its key never becomes a hit. Example: a load to 114,752 tokens with 2,304-token chunks givesnum_chunks = 50, and the store skips chunk 49.Fix: for a full-attention group (
sliding_window_size_in_chunks is None), the store starts one chunk earlier when the load ends in a partial tail. Sliding-window, chunked-local and Mamba groups keep the skip: their block for that chunk can hold the tail state, which is not the state at the chunk end. The Mamba groups store their boundary states through the copy-on-write path (requires_cow_source).Scope: the change has an effect only when
supports_partial_tailis true (a Mamba group in align mode, one block per chunk) and a load ends in a partial tail. In all other casesnext_stored_chunk_idxis unchanged.Cost: one more chunk store per load that ends in a partial tail. The CPU tier did not have that chunk before.
Pull Request Checklist
I used vLLM's
/pr-checklistskill. (Mandatory for agents, optional for humans).AI assistance was used during the creation of this PR.
Design Fit: Minimizes impact on core components, reuses existing functionality, and justifies added complexity.
Testing and Validation: Validates the change and ensures any added tests are meaningful and reliable, with CI coverage or documented CI resource constraints and validation performed outside CI.
Code Quality and Style: Keeps code and comments clear and concise, and updates relevant documentation and examples.
Pull Request Contents: Includes a brief summary and relevant links, supports claims with evidence, explains root causes and implementation trade-offs, and follows the contributing guide.
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