Dual GPU question on X570 - mixing PCIe gen4 and gen3

A bit of a theoretical question, purely out of curiosity.

If I were to put a PCIe 4.0 GPU in the first x16 slot of a X570 board, with a Ryzen 3000 or 5000 series CPU, it would run at x16 PCIe 4.0.

If I now were to add another GPU in the second x16 slot, and this GPU would be a PCIe 3.0 card, both cards would get x8.

But would card 1 still be PCIe 4.0, or would the second PCIe 3.0 card pull it down to 3.0? Since the x16 lanes from the CPU are “split” (suggesting a connection between the two x8 rather than independant from each other)?

It would be motherboard dependent… and normally an extremely high end one at that.

Most manufacturers have the details on how their mb’s pci-e are split up in the manuals.

Pci-E 4 gpus is not really a thing yet.

I’d be very surprised if the PCIe controller was able to manage separate clock domains and all the other details necessary to make mixed versioning work that way on the same bus, but maybe a better question is: would you really care? I’m not aware of any GPUs, much less any consumer-land applications where you’d be able to easily tell the difference.

This would mean more if it was about storage…

Everything still gets split in the normal fashion unless something new comes along.

As an afterthought… maybe look into how nvme storage between the gens is split and controlled. Relationship is there.

Thanks all for your input. Human malware issues have delayed my response here.

It seems that there’s no standard answer to my question. I know that the NVME x4 bus from the CPU can run at 4.0 even with a GPU at 3.0. But is the x16 a single bus? Good point this will make a difference mostly with storage controllers.

Maybe someone with a Ryzen 3000 or in a few days 5000 series can chime in and let us know how it would look with 2 cards of different PCIe versions installed?

BTW what tool is recommended to determine at what version a card is running? Assuming the BIOS cannot answer this.

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