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[Bugfix] Avoid unused sparse decode split-KV workspace - #19
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LucasWilkinson merged 1 commit intoAug 26, 2026
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Co-authored-by: OpenAI Codex <codex@openai.com> Signed-off-by: Matthew Bonanni <mbonanni@redhat.com>
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vllm-project#19 allocated the split-KV accumulators only when the scheduler has more than one partition. The V4.1 sync (vllm-project#20) brought back the upstream interface, which allocates them whenever split-KV scheduling is enabled, and on SM90 that is always. With a single partition no request is split: the SM90 kernel writes `out` / `lse` directly and the combine has nothing to merge, so the (b + num_sm_parts) x s_q x h_q x d_v fp32 buffers are never read. In vLLM's mixed-batch layout (b=1, s_q=num_tokens) they grow to 8 GiB at 32k tokens and reintroduce the runtime OOM of vllm-project/vllm#53413 (vllm-project/vllm#59305). Skip the accumulators and the combine launch on SM90 when there is a single partition, and bring back the workspace test from vllm-project#19 for SM90. SM100 is left to vllm-project#42: its kernels in the new tree build the o_accum tensor map and offset its pointers unconditionally, so there split-KV has to be turned off before scheduling instead. Signed-off-by: Mikhail Kostryukov <mike@triptrack.net> Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Summary
Companion vLLM integration PR: vllm-project/vllm#53755.
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OpenAI Codex assisted with analysis, implementation, and validation. This PR is intentionally draft pending the human submitter's line-by-line review; the submitter will own and defend the change before marking it ready.