Engine core stalls on the first large chunked-prefill after boot (WSL2, KVarN + DFlash2)

## Platform **WSL2 (Ubuntu, kernel 6.18.33.2-microsoft-standard-WSL2, NVIDIA driver 616.56, single RTX 3090 24GB).** Everything in this report was observed on WSL2, and our leading suspicion involves the WSL2 GPU stack, so the platform is a first-class part of the bug report, not a footnote. If this cannot be reproduced on bare metal, WSL2 itself is a primary suspect. ## Summary With this repo's `SPEC=dflash2 CTX=huge PREFIX_CACHE=1` single-user config (KVarN 4/2-bit KV, DFlash2 7-token speculative decoding; vLLM 0.27.1 at commit 2ae239f), the vLLM engine core can **wedge on the first large chunked-prefill request after a fresh boot**: - the unconditional 10-second `Engine 000: … Running:` stats lines go silent; - the API process stays up — the port answers, requests are accepted, but none is ever scheduled; - in one production incident (2026-08-28) this persisted **~4.5h** with the GPU at 100% (~24.1/24 GB resident) producing nothing, until the core spontaneously unwedged; the operator then stopped the engine. - over the following ~36h the same failure recurred **5 times**, each minutes after a fresh boot, always during a large-prompt (10k+ token) request. A one-request reproduction harness, an A/B across three configurations, and a py-spy capture of the wedged core are below. We did not root-cause it to a single line, so this reports the phenomenon + evidence rather than a speculative fix, and ends with two questions for the maintainer. ## Environment - WSL2 ...