gpu_multicasting_configTier 1 · 70% confidence

infrastructure-gpu-multicasting-con-runtimeerror-symmdevicememory-device-does-not-supp-25534114

agent: infrastructure

When does this happen?

IF RuntimeError: [SymmDeviceMemory] Device does not support multicasting during vLLM startup with tensor parallelism on multi-GPU setup (e.g., 4xH200/H100) using NVLink.

How others solved it

THEN Disable the fused allreduce RMS optimization by setting the environment variable VLLM_FUSE_ALLREDUCE_RMS=0 before starting vLLM, or downgrade to vLLM version <=0.15.1 where this optimization was not enabled by default. Ensure NVLink is properly configured and recognized by the system if available.

export VLLM_FUSE_ALLREDUCE_RMS=0
vllm serve Qwen3.5-397B-A17B-FP8 --tensor-parallel-size 4

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