Rog Z790 Extreme and non working PCIE 5.0 16x

[MuppetOwl]

I just got a Astral RTX 5090 and trying to figure out why PCIE 5.0 16x is not available. Only running 5.0 8x  (I know its only 1-2% difference) but want it to work on 16x to be sure everything is optimal 🙂 

Before I had a 4090 running 4.0 16x without issues.
Only using PCIE 4.0 SSD using M.2_2 and M2_3 and not M.2_1 port or any other PCIE port sharing the 16x CPU lanes.
Motherboard is ROG Z790 Extreme with latest bios 2801 and I9 13900KS.

Think I have tested all PCIE related bios settings without success. Starting to think it may be a bios bug or possible PCIE port issue?  

[Nate152]

Hello Muppet0wl

It shows in the manual that M.2_2 runs off the cpu as well.

Try removing your ssd from the M.2_2 slot.

[MuppetOwl]

Thanks for the response. My understanding from the Intel Z790 blockdiagram is the CPU has one PCIE 5.0 16 lane and one PCIE 4.0 4x that should be possible to use at once? So it should be possible to have a SSD in M.2_2 slot and still get PCIE 5.0 x16. Or have I misunderstood? 

[inge70]

Correct.

The CPU has PCIe 5.0 lanes (16) and also PCIe 4.0 lanes.
The PCIe 5.0 lanes are limited to the GPU slot and the M2_1 slot.
PCIe 4.0 lanes are restricted to the M2_2 and can only be used there.
The problem here is that the PCIe 5.0 x16 slot shares the lanes with the M2_1 slot. It doesn't matter whether an NVME with PCIe 4.0 or an NVME PCIe 5.0 is used there.
The lanes are still divided because there are only 16 PCIe 5.0 lanes via the CPU and that also limits the use of a PCIe 4.0 NVME. PCIe 5.0 is backwards compatible and therefore the division also applies to the NVME you use.

Hence the advice to install the NVME in the M2_2 or M2_3 slot so that the GPU can use the full PCIe 5.0 x16 bandwidth.

The M2_3 slot is connected to the chipset, as stated in the manual.

[MuppetOwl]

Understood and thanks for a good answer 🙂
Just to specify, the 2 NVME drives is already only using M2_2 and M2_3. Nothing in M2_1 slot (the one above the GPU where the screen is. 

I will try the DIMM.2 slot module that was in the motherboard box and test to move the NVME using M2_2 to that one. But still think using M2_2 should be ok as that is a PCIE 4.0 x4 lane to CPU. Manual only mentioned M2_1 is the one sharing bandwidth.   

It may also be possible it goes down to x8 if GPU is not seated 100% into the slot or CPU pin issue? Did read about another user with bent CPU pin having the same problem. But guess since my previous 4090 got 4.0 x16, bent CPU pin is hopefully not the case for me. 

[arabulie]

Try switching between bios chips and check again . I have the same issue when i set the original bios the pcie runs at x16 but in the bios updated to the latest version it downs to x8.

They insist me this is not a bios issue but this happens in my case.

[MuppetOwl]

Tried to switch bios now. Still only 8x for me. Do you remeber the Bios version you had it working on? 

[crashworldtk]

Under Linux, I use a tool called “NVTop” to monitor the GPU usage. The reported PCI Generation and lane usage will fluctuate depending on GPU process load. For example, at idle the tool reports PCIE Gen 1 x 1, and under load reports Gen 4 x 4/8/16, depending on the lane availability and needs.

I recently installed a 5090 into my z790 build with a 5.0 x4 nvme installed into M2_1. I would suggest the OS drive be installed in the slot physically closest to the CPU/RAM to minimize latency. Even with lane sharing, the m2_2 is much further away from the CPU and that too will impact performance.

[Jiaszzz_ROG]

Hello @MuppetOwl 

Sorry if any information was missed during the discussion.
Regarding your concern, may we ask if you have checked whether the CPU pins are bent?
If you set PCIe Bandwidth Bifurcation Configuration to [Auto] in the BIOS and adjust PCIEX16(G5)_1 to [Auto] or [Gen 5], does it make any difference?
When installing the RTX 5090 in PCIEX16(G5)_2 for testing, what bus interface is displayed in GPU-Z?
If the above settings do not resolve the issue, could you kindly provide the following information to assist us in further investigation?
- share an image of BIOS main page, and confirm what manual adjustments have been made in the BIOS? 
If you clear CMOS and boot with default settings (without any manual configuration), does the issue persist?
- The PSU, RAM model and part number, and two .txt files generated in your Windows system via the steps below:
1) Press WIN + R on your keyboard to open the Run dialog, and type dxdiag → click OK.
2) Click Save All Information and select a save location → click Save.
3) Repeat the same steps with msinfo32 instead of dxdiag to generate the second file.
You may upload the files to cloud storage, grant access, and share a link with us.

Thank you.

[MRBULL]

im having same issue but on this motherboard 

ROG STRIX Z790-E GAMING WIFI

im using B-C-E  3x nvme so why is my 4080 runing at x8 i've set the bios setting in pcie to auto