[Asmodian]

You probably have something else plugged into the motherboard stealing lanes from the GPU. It could also be a bad mount of the CPU, missing pin(s) causing dropped PCIe lanes.

It is NOT the GPU getting x8 of PCIe 5.0 bandwidth. It cannot work that way, the 4090 does not support PCIe 5.0 and cannot use PCIe 5.0 lanes at all (it would get 0 MB/s bandwidth for a PCIe 5.0 lane). Each PCIe lane is its own wire with very different signals running on them, it is not something abstract that could combine/split up like that.

 

[Asmodian]

Even when GEN 5 drives are available would it be worth to to sacrifice the GPU lanes for minimal gains?

How much performance is lost on a 4090 when running PCIe 4.0 x8?

 

[Asmodian]

I have both M.2_1 and PCIeX16_1 set to gen 4 mode in the bios. According to the user manual, M.2_1 shares bandwidth/lanes with PCIe X16 _ 2. I don't have anything in that slot, so im not sure why X16_1 would be affected

It is the two facts combined that indicates this is the expected behavior.

1) PCIe X16_2 shares bandwidth with PCIe X16_1
2) M.2_1 shares bandwidth with PCIe X16_2.

This means M.2_1 shares bandwidth with PCIe X16_1. If A = B and B = C then A must equal C too.

There are only 16 lanes shared between PCIe X16_1, PCIe X16_2, and M.2_1. In this context gen 4 or 5 doesn't matter, if the slot supports gen5 then it must be connected to the pins for the gen5 lanes on the CPU.

 

[Asmodian]

I have the same problem on my ROG Maximus z690 Extreme. I have an RTX 3090 and the MB does not use the full slot bandwidth. In the BIOS it shows which bandwidth is being used. I tested taking all m2 and it still shows x8. There must be some error in the BIOS.

I notice your BIOS does not show the status of PCIEX16(G5)_2. You are sure you do not have anything plugged into that slot?

That seems a lot more likely than a BIOS bug that prevents it from using full bandwidth. Missing pins is also a real possibility; I have had a bad CPU mount drop my GPU to x8 before, while otherwise working well.

 

[Asmodian]

I found a very interesting article about PCIe. And I finally understood how it works. I recommend reading.

NVIDIA GeForce RTX 4090 PCI-Express Scaling

The new NVIDIA GeForce RTX 4090 is a graphics card powerhouse, but what happens when you run it on a PCI-Express 4.0 x8 bus? In our mini-review we've also tested various PCI-Express 3.0, 2.0 and 1.1 configs to get a feel for how FPS scales with bandwidth.

www.techpowerup.com

See ya.

What does that have to do with whether you have a BIOS error causing you to be running 4.0 x8 instead of x16? That article is about the performance hit from running 4.0 x8.

Did you have something plugged into PCIEX16(G5)_2?

 

[Asmodian]

Read the article! More precisely the conclusion.

Well, it shows you don't really need to worry about it, but we already knew that. It doesn't explain why it is x8 instead of x16 for that extra tiny bit of performance.