The bottleneck with AI and hardware is really pushing things.
Even with six capable GPUs that have plenty of VRAM with enough PCIe bandwidth, you then discover that memory address allocation is preventing the hardware from doing what it's physically capable of.
This is my post. Basically, I needed P2P between four R9700s for tensor parallel serving, which means a 32 GB BAR on each card. With all six GPUs in, the BIOS couldn't map them, and Linux sized the switch's window too small, so only two cards got their BARs. The module in the post works around that. With P2P off, prefill was about half as fast.
Even with six capable GPUs that have plenty of VRAM with enough PCIe bandwidth, you then discover that memory address allocation is preventing the hardware from doing what it's physically capable of.