2x Radeon AI PRO R9700 + Qwen3.8-27B: 31 t/s on Windows → 113 t/s on Linux/vLLM. The fix was an M.2 riser, because the chipset slot doesn't do PCIe atomics.
If your second r9700 sits in a chipset-connected slot, it probably has no PCIe atomics. RCCL then fails, and tensor parallelism is effectively dead. Moving the card to CPU lanes with a cheap M.2-to-PCIe riser, plus switching from llama.cpp to vLLM, took me from ~34 to 72–113 t/s on Qwen3.8 27B. Har…
Read the full story at r/LocalLLM ↗