[Bugfix][MRV2] Reserve spec-decode lookahead blocks in V2 warmup by rchalamala · Pull Request #50531 · vllm-project/vllm

rchalamala · 2026-07-31T05:58:00Z

[Bugfix][MRV2] Reserve spec-decode lookahead blocks in V2 warmup

Purpose

Split out of #50488 at a maintainer's request, so each fix is reviewable on its
own.

V2 warmup hand-builds its SchedulerOutputs and sized each reservation at
cdiv(num_computed + num_scheduled, block_size). KVCacheManager.allocate_slots
reserves num_lookahead_tokens more, because the speculator writes KV for the
tokens it drafts. The drafter's slot mapping does not bound the column index by
the request's block count, so a position past the reservation reads an
untouched, zero-initialised column — the null block. Warmup therefore exercises
a degenerate KV layout and never crosses the block boundary that production
always crosses.

Hybrid models were affected asymmetrically: warmup added
MambaSpec.num_speculative_blocks only in align mode while MambaManager
adds them in every cache mode, so in the default none mode every speculative
state slot aliased the null block.

Rather than patch the local formula, this introduces
VllmConfig.num_lookahead_tokens as the single place the lookahead rule lives,
and has both the scheduler and both warmup entry points read it. The two had
already drifted; a shared property is what stops them drifting again.

This is not specific to any drafter or model. The reservation is keyed on the
speculative width, so any speculative method on the V2 runner reaches it.

Test Plan

tests/v1/worker/test_gpu_warmup_blocks.py drives both warmup entry points
with a stub runner and asserts each request holds the blocks allocate_slots
would give it. test_reserved_block_count_matches_real_kv_cache_manager checks
the same prediction against KVCacheManager.allocate_slots itself, on a hybrid
config carrying a full-attention group alongside one Mamba group per cache mode
(none, all, align), so the prediction and the allocator cannot drift apart
silently.

Test Result

pytest tests/v1/worker/test_gpu_warmup_blocks.py — 26 passed, run on an
H100 sandbox against this branch overlaid on the pinned nightly wheel
(0.26.1rc1.dev77+g6f91edf96).

Revert check, which is what makes the test meaningful: 11 of the 12
reservation cases fail before the change, the one that passes being align-mode
mamba, and all 12 pass after.

ruff check and ruff format --check are clean on every touched file.

Model evaluation

tests/evals/gsm8k/gsm8k_eval.py, 400 questions, 5-shot, greedy, on
Qwen/Qwen3-4B with the published z-lab/Qwen3-4B-DFlash-b16 drafter at
num_speculative_tokens=16, V2 model runner, single H100: accuracy 0.880,
0.000 invalid, 400 questions. This was measured on the combined set of fixes
before the split, so it covers this change together with the others rather than
in isolation. The V2 runner was confirmed live from the boot log rather than
assumed, since the engine banner reads the same on either runner.

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 · 2026-07-31T06:08:41Z

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The V2 warmup hand-builds its SchedulerOutputs and sized each request's reservation at cdiv(num_computed + num_scheduled, block_size). The scheduler reserves more: KVCacheManager.allocate_slots asks for num_computed + num_scheduled + num_lookahead_tokens slots, because the speculator writes KV for the tokens it drafts and those sit past the range the target model was scheduled for. Mechanism: the drafter builds its slot mapping straight from the persistent block table without bounding the column index by the request's block count (_compute_slot_mappings_kernel in gpu/block_table.py; DFlash's _prepare_dflash_inputs_kernel clamps to the row width, not to the allocation). A position past the reservation therefore reads an untouched column, which holds zero because both block tables are zero-initialised - that is the null block. The drafter's K/V for that position is written into, and read back from, block 0 instead of a block the request owns, so warmup silently exercises a degenerate KV layout and never crosses the block boundary that production always crosses. Nothing bounds-checks the column, so this stays benign only by virtue of the zero-initialisation. Trigger: any speculator that writes KV (eagle, eagle3, mtp, draft_model, dflash, dspark), on any warmup step whose token count lands close enough to a block boundary. Deterministic on the first drafting pass for DFlash with num_speculative_tokens=7 and block_size=16: warmup reserves one block for the 9 prompt tokens, then the drafter queries positions 9..16 and position 16 falls in block index 1. Likewise for eagle with num_speculative_tokens=8, whose draft decode steps walk positions 10..16. Further in, e.g. eagle with num_speculative_tokens=3, the third decode step holds 1 block for 13 computed + 1 scheduled tokens where the scheduler would hold 2, and whether the drafter reaches into the missing block depends on how many draft tokens were accepted. Hybrid models were affected asymmetrically: warmup added MambaSpec.num_speculative_blocks only in "align" mode, while MambaManager adds them in every cache mode (single_type_kv_cache_manager.py). In the default "none" mode the mamba kernels are handed a block table of width 1 + num_speculative_blocks per request (mamba_get_block_table_tensor) and warmup filled exactly one column of it, so every speculative state slot aliased the null block. Align mode was the only group warmup got right. Rather than patch the local formula, the reservation now derives from the policy the scheduler owns: VllmConfig.num_lookahead_tokens holds the rule Scheduler.__init__ used to spell out, and warmup reads the same property. Both warmup entry points - warmup_kernels and run_mixed_prefill_decode_warmup (used by the FlashInfer sparse MLA warmup) - now go through one helper that mirrors allocate_slots: token range plus lookahead capped at max_model_len, plus the mamba speculative tail, with the lookahead dropped in align mode exactly as MambaManager drops it to keep the allocation block-aligned. One behaviour change beyond the reservation itself. Routing `run_mixed_prefill_decode_warmup` through the shared helper also gives it the cross-attention special case that `warmup_kernels` already had: a `CrossAttentionSpec` group is sized to the encoder length rather than to the decoder token count, because the drafter does not extend the encoder sequence. That path's only in-tree caller is the FlashInfer sparse-MLA warmup, which is decoder-only, so nothing in tree changes; the two entry points now agree. The align-mode branch mirrors `MambaManager` exactly, including the cap: `allocate_slots` applies `max_model_len` to `num_tokens_need_slot`, the lookahead-extended range, and not to `num_tokens_main_model`, which is what sizes an align-mode group. Warmup token counts stay far below `max_model_len`, so this is latent either way. Larger reservations also make the mixed prefill+decode warmup's `num_blocks <= required_blocks` guard bite sooner, so a very small KV cache pool that previously ran that step may now skip it with the existing warning. That is the same trade as the `num_reqs` cap in `warmup_kernels`. Deliberately unchanged: the scheduler's reservation policy and every runtime allocation are identical, cross-attention groups still size to the encoder length, the drafter kernels still trust the block-table row (warmup is made to obey the contract rather than hardening the kernels against violations of it), and warmup's existing per-request block cap is untouched - the extra blocks shrink num_reqs through the cap already there. Tested: tests/v1/worker/test_gpu_warmup_blocks.py drives both warmup entry points with a stub runner and asserts each request holds the blocks allocate_slots would give it; 11 of the 12 cases fail before this change (the passing one is align-mode mamba) and all 12 pass after. `test_reserved_block_count_matches_real_kv_cache_manager` checks the same prediction against `KVCacheManager.allocate_slots` itself rather than a stub, on a hybrid config carrying a full-attention group alongside one Mamba group per cache mode - "none", "all" and "align" - so the held-block count is confirmed against the real allocator for every mode the spec can carry, not only the two that take distinct branches today. tests/v1/core (including test_scheduler.py) reports an identical set of environment-related failures before and after, covering the scheduler-side refactor. The mechanism above is traced from source: no drafter checkpoint was available in the test environment to observe the null-block write on device. AI assistance was used for this change. Every claim above is checkable from the repository or from the commands in the test notes; anything taken from a bug report rather than observed here is marked REPORTED. Co-authored-by: Janelle Cai <janelle.cai@modal.com> Signed-off-by: Rahul Chalamala <22563365+rchalamala@users.noreply.github.com>

njhill

Thanks @rchalamala, looks good to me!

Only ask is to trim down the comments a bit. I've done this in another commit here njhill@4fe8612 but don't have permission to push to your branch.

I wonder if you could cherry pick that and rebase? and then we can run the CI

njhill · 2026-08-04T22:10:38Z

        return 0

+    @property
+    def num_lookahead_tokens(self) -> int:


Thanks, this change is nice

…roject#48244/vllm-project#48944)

njhill · 2026-08-07T19:47:54Z

@rchalamala we would like to get this merged asap so I've opened another PR with a copy of your branch + the cleanup: #51438. You remain as co-author!

njhill · 2026-08-09T00:05:05Z

@rchalamala I have now merged this via #51438, thank you for this fix! And I am working through some of your other PRs.

rchalamala requested review from ApostaC, ProExpertProg, WoosukKwon, alexm-redhat, heheda12345, houseroad, ivanium, mgoin, njhill, orozery, robertgshaw2-redhat, tlrmchlsmth, yewentao256, youkaichao and ywang96 as code owners July 31, 2026 05:58

claude Bot reviewed Jul 31, 2026

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rchalamala mentioned this pull request Jul 31, 2026

[Bugfix][Spec Decode] Capture the widest uniform decode batch by default #50488

Merged

mergify Bot added mrv2 Model Runner V2 specific bug Something isn't working labels Jul 31, 2026

rchalamala force-pushed the rahul/k3-pr5-warmup-lookahead branch from bd6d36e to 401e495 Compare July 31, 2026 20:00

rchalamala force-pushed the rahul/k3-pr5-warmup-lookahead branch from 401e495 to ed54199 Compare August 4, 2026 00:12

njhill reviewed Aug 4, 2026

View reviewed changes

Comment thread vllm/config/vllm.py

return 0

@property

def num_lookahead_tokens(self) -> int:

njhill Aug 4, 2026

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Thanks, this change is nice

justtestingthingsx pushed a commit to meandmyboiclaude/vllm that referenced this pull request Aug 7, 2026

pick vllm-project#50531 (3way + semantic-mechanical resolve vs vllm-p…

911ed2e

…roject#48244/vllm-project#48944)

njhill mentioned this pull request Aug 7, 2026

[Bugfix][MRV2] Reserve spec-decode lookahead blocks in V2 warmup #51438

Merged

njhill closed this Aug 9, 2026

rchalamala deleted the rahul/k3-pr5-warmup-lookahead branch August 11, 2026 18:01

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[Bugfix][MRV2] Reserve spec-decode lookahead blocks in V2 warmup - #50531

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rchalamala commented Jul 31, 2026

[Bugfix][MRV2] Reserve spec-decode lookahead blocks in V2 warmup

Purpose

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