PCIE Lanes issue. GPU running at 4 x8 instead of x16

aidan364

Hi all,

New builder her and having issues with pcie lanes. I just noticed an issue with my GPU not running at the correct speed. It is running at 4 x8 instead of x16. I first became aware of the issue when I turned my PC on and I got a message to say my gpu was running at PCIE gen 3 speeds. I am not sure why the change happened but I was alerted to it on startup. I managed to manually change the generation in the bios for 4.0 and thst worked. However the speed is only x8 even under load. My troubleshooting to date is:

Reseat the gpu a d clean contacts and slot

Reseat the cpu

Clear Cmos

Update bios

Change motherboard

Remove all nvme drives except for boot drive in slot 1.

Disable integrated graphics on cpu (I thought they were automatically disabled upon detection of gpu but I needed to manually disable this)

Would anyone have any idea what's going on here? I've ruled out motherboard because I got a new one, and I doubt Asus would have given me two bad boards.

Thought it must be a configuration issue, but I have tired with only one nvme in slot 1 to act as boot drive and no change.

System

CPU: 7800x3d

Motherboard: Rog Strix b650e-e

RAM: corsair Vengeance 32gb 6000nhz

GPU 7900xtx red devil

Storage: crucial P5 500gb and crucial P3 2tb

Any help here would be great. I know it's not a massive performance decrease, but I have paid for the performance so would like to get it.

Spear

From a quick read of the manual it should work. The x16 slot shares bandwidth with m.2 slot 4, so if there's no drive in there, it should be fine. How does it behave if the remaining nvme drive is removed? You should be able to check the status in the BIOS, without the need for an OS.

Most of the other things I'd suggest, you've already done.

Do you have another machine to check the GPU in?

Is PCIe bifurcation disable in the BIOS? I'd expect to be by defult, but it could explain the issue if it was enabled.

aidan364

I haven't checked tried removing the first drive I will try that tomorrow and see how it behaves. Thanks for the suggestion.

I don't have another machine to check the gpu in. So trying all of the process of elimination things.

One thing I've noticed is, when the integrated graphics were enabled at the same time as the gpu they were running at 4 x16. I thought that was a little strange because with the integrated graphics running at x16 the gpu at x8 and the two nvme drives at x4 that would total 32 pcie lanes when the cpu only had 24 usable lanes.

This is what's reported in gpu-z

Spear

The iGPU doesn't consume PCIe lanes, it's internal to the CPU entirely. It uses the Infinity Fabric to communicate using the PCIe protocol, and doesn't rely on the external lanes. PCIe can run over lots of connections, SAS cables, OCuLink cables etc, it's not limited to PCIe slots.

GHOST MGG

1.Is your GPU in the Top Pcie slot nearest the processor?

2.how many sata ports are populated and do you have any pcie wireless cards installed?

3.I believe on that motherboard the gen 5 m.3 slot if populated will auto bifurcate to 8x8x

1. The difference in Pcie 8x and 16x lanes is 1-2% so its not a massive loss in performance.

aidan364

https://www.boards.ie/discussion/comment/121722044#Comment_121722044

The GPU is in the top slot

No sata ports are occupied and no pcie wireless cards installed -

Right now I have one nvme m.2_1 going straight to the cpu and the second one in m. 2_4 which routes through the chipset. Moved it there in my last round of configuration options to see if it resolved the issue.

There definitly doesn't seem to be too much of a difference in benchmarking tests with other people who have the same card. I'm just at a stage now where I want to figure out why it isn't running at x16.

Luck100

Did you click on the question mark icon to the right of the "Bus Interface" box in GPU-Z? You won't see the full speed of the link until you do that (it runs a simple test load to force the link into high performance state).

Spear

https://www.boards.ie/discussion/comment/121731822#Comment_121731822

It's a lane width issue, not a speed issue. The lane width doesn't change dynamically like speed does.

aidan364

https://www.boards.ie/discussion/comment/121731822#Comment_121731822

Yep did it indeed.

I've removed all NVME drives and gone into BIOS but strangely enough I can't see the graphics card in the BIOS. I obviously know it works because I get an image and its reported as being detected in device manager and Hardware Info but just can't see it in the BIOS. Doing this also didn't change it from x8 to x16.

My next option is to try another graphics card that can do x16 and see how that reports. I probably wont get the time for a week or two to try this, but will report back once I do.

cornholio509

@aidan364

Its entirely possible one of your drives is sapping away PCIE lanes . You need to check the documentation to see what drive is causing the issue . THe NVME nearest the CPU is dedicated 5.O speeds . One of the other drives could possibly be cutting the PCIE lanes in order to run in gen 5 mode . I know this happens on some b650 boards . SO try to move the other drive to PCIE gen 4 slot and see if that sorts the issue out .

Its also possible the that there is a PCIE setting in the bios that somehow got changed to x8 . ALso in the bios there should be an option for the primary display device . It might be on auto so change that to PCIE or external rather than onboard . SHould at least eliminate the onboard GPU from the equation .

shnoop

Hi guys, I have a similar question. I'm building a pc and want to use the Corsair T700 m.2 which is gen 5, I also want to use an asus board, however all the boards i'm looking at have just the one m.2 gen 5 slot which is beside the cpu and when this slot is occupied the GPU will be x8 and not x16?

** M.2_1 shares bandwidth with PCIEX16(G5)_2. When M.2_1 is enabled, PCIEX16(G5)_2 will be disabled and PCIEX16(G5)_1 will run x8 only.

This just sound backwards considering im building a pc with the latest and fastest components, thoughts anyone?

Spear

https://www.boards.ie/discussion/comment/121743560#Comment_121743560

There should normally be enough lanes for at least one m.2 drive along with x16 for the GPU. Got an example board in question?

shnoop

https://rog.asus.com/motherboards/rog-maximus/rog-maximus-z790-formula/

Spear

https://www.boards.ie/discussion/comment/121744189#Comment_121744189

Yeah, in that one, yes, there's 4 m.2 slots, but the only PCIe 5.0 one shares with the PCIe slots. It's either accept PCIe 4.0 speeds, or accept a very small hit to the GPU. There's no winning there.

shnoop

So are all new boards doing this?

Spear

https://www.boards.ie/discussion/comment/121744395#Comment_121744395

Anything consumer I suspect. Most CPUs still only have 24 to 28 lanes. If you're using 16 for the GPU, 4 to the chipset and 4 a direct m.2 slot, then there's not much leeway after that. In the case you've linked to it's worsened by the hybrid fudge between PCIe 4.0 and 5.0, where the CPU supports 5.0, but the chipset only has 4.0.

cornholio509

https://www.boards.ie/discussion/comment/121744395#Comment_121744395

It all depends on the boards specs from both intel and AMD . I KNow x670 E boards from AMD have a second I/O controller and that is required by AMD . However how that controller is used to split PCIE lanes up is down to the manufacturer . INtel also has boards that do the same as far as i know .

https://www.boards.ie/discussion/comment/121744491#Comment_121744491

100% true . This is our own fault as consumers . ITs bothered me for years . In the days of sli 40 PCIE lanes was the norm . WHen sli went the way of the dodo pcie lanes started getting chopped down . As consumers we allowed for it and look at us now . Screaming for more PCIE lanes but still paying for over bloated LED crap .

Cuddlesworth

Its ok where its at, more lanes = more cost. Every PCI-ex generation is a exponential increase on the last. Meanwhile, actual need/usage has not been exponential. 20 5.0 lanes is a already insane amount of bandwidth.

In the OPs case I don't think there is even a performance difference running 4.0 x8. The 4090 only really sees a 2-3% increase per generation and I suspect that's just a increase in signaling speed. Other cards, even less.

For most other things, the niche use cases for other components that actually use the lanes are getting less and less, or being replaced by external devices on usb/thunderbolt interfaces.

He should be getting x16 though.

cornholio509

Your PCIE lanes are about connectivity not bandwidth . Bandwidth is part of the standard per generation .

NVME drives work over PCIE . SO it requires 4 lanes per slot to run an NVME drive . THose lanes have to be take form somewhere . SInce data storage takes priority over the PCIE slot. Then you know where those lanes are going to be taken from .

SO yes we need more PCIE lanes from the CPUS controller . or have another proper onboard controller for storage that has a direct line to the CPU that runs at the same speeds .

Now on the ops PC he cant get the gpu to run x16 because some of the lanes are been used for one NVME drive . Because that board has a second controller some of the NVME slots run from the CPU's controller , and some from the secondary controller . He needs the NVME slots that are wired to the secondary controller , and move the drive to one of them . THat should return the GPU to x16 gen 4 .

Cuddlesworth

His board has three M2 slots, the slot that sits between 2 pci-ex slots M2_3 makes the primary x16 slot run in x8 mode and the op has specifically said he is not using that slot any more.

PCI-ex slot 16_1(top slot) should get x16,

M2_1 gets 5.0 x4 direct to CPU(up near the ram)

M2_2 gets 5.0 lanes from the top slot, dropping it to x8(between two x16 slots)

M2_3 + 4 are direct to the chipset 4.0 x4.(to the right of the board.

I'd backup the bios config, then clear cmos and see does it start assigning lanes correctly.