Fitting Four 32 GB Resizable Bars Behind a PCIe Switch on Linux (doug.sh)

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.

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.

Most CPUs can address 48 bits of physical memory (hundreds of terabytes), why is that even a problem ? Is it just some hardware limitation somewhere else or badly written firmware ?

The word or acronym BAR is mentioned 34 times but never defined.

It feels ridiculous that a person has to go to these kind of lengths to do what is in the end a trivial thing: assigning little pieces of a gigantic virtual address space to specific peripherals.

The only thing that's complex here is having to influence a bunch of hidden software through three layers of indirection. The software is all standing in the way of a person just writing some values to registers.