[r/LocalLLaMA] Multi-GPU? Check your PCI-E lanes! x570, Doubled my prompt proc. speed by switching 'primary' devices, on an asymmetrical x16 / x4 lane setup.

Summary

A user with a dual 3090 setup on an x570 motherboard, featuring asymmetrical PCI-E lanes (x16/x4), discovered that the default CUDA0 device was assigned to the slower x4 slot. By explicitly setting `CUDA_VISIBLE_DEVICES="1,0"`, they effectively made the x16 slot the primary device, which doubled their prompt processing speed in `llama.cpp`. This finding highlights a crucial optimization for multi-GPU users, particularly on x570 boards, who might be unknowingly bottlenecked by PCI-E lane allocation.

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