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[sglang-miles] Add SGLANG_DISABLE_MULTIMEM_AG to force the NCCL all-gather path - #36110
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Motivation
On GB300 NVL72 running 8 colocated tp8 engines (miles RL, DeepSeek-V4-Flash-0731), the multimem all-gather's symmetric-memory rendezvous is unreliable when many engines initialize concurrently: some ranks time out after 600s ("multimem all-gather disabled (wait timeout ...)") and others deadlock inside the rendezvous right after "Registering N cuda graph addresses", which blocks engine init forever (the ray init handle never resolves and the whole colocate bring-up hangs).
Modifications
Add an
SGLANG_DISABLE_MULTIMEM_AGenv (default off, no behavior change) that forcesMultimemAllGathererto the NCCL fallback path, skipping the symm-mem rendezvous entirely. Correctness is unaffected — the NCCL path is the existing fallback the gatherer already uses whenever multimem is unavailable.Validated on the 8-node GB300 bringup: with the gate set, all 8 engines initialize reliably and training runs (6+ consecutive rollout/train cycles).
CI States
Latest PR Test (Base): ❌ Run #32698829006
Latest PR Test (Extra): ❌ Run #32698828336
Latest PR Test (AMD ROCm 7.2): ➖ No AMD PR run found for this commit.