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`QSAKeyStateCache.get_kv_cache_spec` asserted that the ring capacity divides the attention block size. The safety property stated in the comment is one-sided: the ring must be at least `span` rows (a narrower ring lets a rejected draft row overwrite a committed key); a wider ring is slack. Asserting instead of widening made `num_speculative_tokens` 5..8 (and 13..16) unreachable on every power-of-two block size and on the hybrid LCM sizes 848/1616, with `compress_ratio` 4: those depths need a 12-row ring and the block sizes have no factor 3. Factor the computation into `qsa_ring_capacity()`, which returns the smallest whole-group ring >= span that divides the block size, and log once when it widens. Every previously legal depth keeps its ring size (no-op), and an impossible configuration now raises a ValueError with the numbers instead of an assert. Fixes vllm-project#54552 Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_011SuBgdp87NbfLbiigmzn1z Signed-off-by: Jürgen Schmied <juergenschmied70@gmail.com>
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…eferred Review follow-up: without a bound, num_speculative_tokens 13..16 (minimal ring 20 rows) would silently widen to 212 rows on block size 848 and 404 on 1616 (16 * 53, 16 * 101), and every request holds a ring block for its lifetime. Cap the widening at QSA_RING_MAX_WIDENING (2x) times the minimal size and raise a ValueError with the numbers beyond that. The intended band (5..8, 12 -> 16) is unaffected. Docstring: the divisor is preferred so that adding the circular-buffer group does not increase the scheduler block size (the LCM over all groups); the LCM itself does not require divisibility. Tests now state the widened and the refused cases explicitly. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_011SuBgdp87NbfLbiigmzn1z Signed-off-by: Jürgen Schmied <juergenschmied70@gmail.com>
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@bojiang3 — linking this to the issue you looked at, since the two threads never got connected: this PR is the fix for #54552, where you confirmed the arithmetic on 2026-09-02 and agreed that widening Two things have been added since: Runtime evidence. I applied the widening on a DGX Spark (GB10, sm_121, TP1) serving Qwen3.8-Flash-Next NVFP4 with The widening is bounded. A review follow-up caps it at 2x the minimal ring, because without a cap Also worth flagging for whoever picks this up: on the V1 runner this assert is not the only thing in the way. Clearing it lands immediately on One process question: CI here shows |
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Purpose
Fixes #54552.
QSAKeyStateCache.get_kv_cache_specasserts that the raw-key ring capacity dividesthe attention block size. The safety property in the comment above the assert is one-sided: the ring
must be at least
spanrows; wider is slack. Withcompress_ratio = 4,num_speculative_tokens5..8 (and 13..16) need a 12-row (20-row) ring, and neither the power-of-two block sizes nor the
hybrid LCM sizes 848/1616 have the factor, so those depths fail at engine init:
The change factors the arithmetic into
qsa_ring_capacity(), which returns the smallest whole-groupring
>= spanthat divides the block size, logs once when it widens, and raises aValueErrorwiththe numbers when no such size exists. Every previously legal depth keeps its ring size, so the change
is a no-op for existing configurations.
Test Plan
tests/models/qwen4_exp/test_config.py: for block sizes 848 and 1616 andnum_speculative_tokens0..16 the ring is>= span, a multiple ofcompress_ratio, divides theblock size, and equals the old value wherever the old value was legal; plus the concrete cases
(4, 5, 848) -> 16,(4, 8, 1616) -> 16,(4, 5, 16) -> 16, and(4, 13, 16)raises.num_speculative_tokens=5,attention block size 1616: previously a hard init failure; with this change the engine starts, logs
QSA ring widened from 12 to 16 rows ..., serves, and passes a needle-in-a-haystack retrievalcheck 5/5 at ~20k context. The same change has been running on my box for two days of benchmark
loops (MTP n=5, prefix caching on) without errors.
Test Result
Not verified: behaviour under draft rejection specifically with a widened ring, beyond the fact that
a wider ring cannot overwrite earlier than the minimal one (the property the assert protects).
@bojiang3 offered to reproduce on a GB10 on
main.Disclosure
Includes AI-assisted code (Claude Code); every line reviewed and the tests run by me.