Question 2080 TI locked at x8 PCI-E lanes

[Amorphiss]

Hello,
As mentioned in the title, my GPU (2080 TI) is locked at 8x PCI-E lanes, and I don't know exactly why. I have a theory but I need confirmation.
My build :
Mobo : Asrock x570 Taichi
CPU : 3900x
GPU : MSI 2080 TI Gaming X Trio
SSD : Corsair Force MP600 PCI Express 4.0 x4 M.2
RAM : 32GB G.Skill 3600 Mhz CL16

The GPU is installed in the top PCI-E slot and I have no other PCI-E cards. (this is supposed to use 16 CPU PCIe lanes)
The SSD that uses 4 lanes is mounted in the top M.2 slot (straight to CPU, uses 4 lanes).
I also mention that I know the CPU has 24 lanes (16 for GPU, 4 for Nvme and 4 for chipset).

While using multiple utility tools to check the number of lanes used by GPU, they all post "@x8 current usage". I used GPU-Z, Cpu-Z, Speccy and HWInfo.

My theory :
While using HWinfo, the tool shows the pcie slot max bandwidth and the current status. For example : Pci-e x16 8GT/s @ x8 8GT/s. Thing is that my motherboard has PCI-e 4.0 support, while the GPU only knows Pci-e 3.0. I know Pci-e 4.0 is backward compatible with 3.0, and as far as I know, x8 Pcie 4.0 = x16 Pcie 3.0 regarding bandwidth.
In my testing, I noticed that the max bandwidth for PCI-E 3.0 x16 is 8GT/s (is this correct?). My GPU, even though the software shows that it runs at x8 has the same bandwidth as PCI-E 3.0 x16 : 8GT/s.

Can anyone explain this ? Why is this happening ? Is x8 = x16 in my case ?

Thank you

 

[dotas1]

https://en.wikipedia.org/wiki/PCI_Express
It seems your assumption is correct.

 

[Amorphiss]

https://en.wikipedia.org/wiki/PCI_Express
It seems your assumption is correct.

3.0	2010	128b/130b	8.0 GT/s	984.6 MB/s	1.969 GB/s	3.94 GB/s	7.88 GB/s	15.75 GB/s
4.0	2017	128b/130b	16.0 GT/s	1969 MB/s	3.938 GB/s	7.88 GB/s	15.75 GB/s	31.51 GB/s

Consulting the wikipedia page, I can indeed confirm that the Transfer Rate for Pcie 3.0 is 8 GT/s and for Pcie 4.0 is 16 GT/s. But depending on the number of lanes used, the troughput differs.
This is the confusing part for me... is the utility software reporting :
1. x8 as Pcie 4.0 that means 17.75 GB/s ? This is also equivalent to x16 Pcie 3.0. (this is good)
or
2. x8 as Pcie 3.0 that means 7.88 GB/s ?

Thanks

 

[dotas1]

in GPU-z click the question mark and start the render test. Does the x8 change?

Post a screenshot of GPU-z please.

Speccy has some bugs reporting things, so I stopped using it as I don't trust it anymore.

 

[Amorphiss]

in GPU-z click the question mark and start the render test. Does the x8 change?

Post a screenshot of GPU-z please.

Speccy has some bugs reporting things, so I stopped using it as I don't trust it anymore.

I tried doing the render test with GPU-Z, it remains at x8. I also ran a demanding game to make sure the GPU has load, still x8 lanes were used.

Will post a screenshot soon for confirmation.

 

[Amorphiss]

in GPU-z click the question mark and start the render test. Does the x8 change?

Post a screenshot of GPU-z please.

Speccy has some bugs reporting things, so I stopped using it as I don't trust it anymore.

Here are the screenshots. I also posted one from HWinfo.

 

[dotas1]

It's very confusing. How about BIOS settings? What does it say? Do you have option to run it in x16 or x8?

 

[Amorphiss]

It's very confusing. How about BIOS settings? What does it say? Do you have option to run it in x16 or x8?

It has a PCI-E x16 Switch option that contains "4x4" "2x4 and 1x8" and "1x8 and 2x4". So basically no pure x16 option, so I left it on auto. If I try any of those options the gpu is either x4 or x8.
Even stranger thing is that SOMETIMES the system boots and showx x16 for the GPU, but this happens very rarely.
I also tried to move the SSD on other slots, reinstalled CPU, booted without USBs activated, changed Pcie power cables (btw, I use 850W PSU that is more than enough).

 

[dotas1]

One thing to try if you can, is putting your GPU in another system, if it does show x16 every time after multiple restarts then your motherboard might have an issue. If it stays x8 even once, then the GPU is faulty.

You can also try another GPU in your system as another way.

BTW which PSU are you using?

 

[Amorphiss]

Corsair RMx Series White RM850x 2018, 80+ Gold, 850W

Yes, already planned to do that with a friend next weekend, swapping GPUs, using the same logic.
Still I wonder if maybe this is actually normal, I mean 8x mobo lanes that are pcie 4.0 should be perfect. I just cannot confirm it.
Any way to tag MSI or Asrock here ?

 

[Amorphiss]

Update :
So this weekend I found the cause that was causing this issue, and it's insane !
So apparently, If I set "PWM" mode for the FAN Header in BIOS, the system is unstable + x8 on PCI-E.
If I set it to "Auto" or "DC" the system in stable and the PCI-E is always x16 !

I find this very strange...
My case is a FD Define R6 - has a fan controller. All 6 of my case fans are connected to the controller, and the controller is connected on CHA_1 fan header on motherboard. None of my fans are PWM.
Is there any explenation for all this ?