GPU Running in PCIe x8 mode?

[Jarsky]

Hey all,

 

Just seeing why this would be happening, my understanding was that my CPU has 24 PCIe lanes (20 available to devices), and my X570 board should have an additional 8 lanes for PCIe & SATA devices, giving me a total of 28 usable lanes?

 

I have the below:
Ryzen 9 5950x

Asus ROG Crosshair VIII Hero Motherboard

EVGA RTX 3080Ti FTW3

Crucial P1 NVMe (Gen 3)

ASUS PCE-AC88 Wifi Card

Elgato 4K60 Pro Mk II Capture Card

4TB SATA WD Black

 

Going through the manual, I have the GPU in X16_1 , Capture card in X16_2, and Wifi card in X16_3 . I only have the 1 NVMe drive which is in M.2_1 (in PCIe mode), and I have a SATA drive in SATA6G_6 

 

 

Do I just have too many devices trying to use the PCIe lanes which is why its getting dropped to x8?

I'm using my old Asus Wifi card because my current home wifi only has 802.11ac, and not 802.11ax (Wifi6), so with the 4x4 MU-MIMO compared to the 2x2 on the motherboard I get far greater wifi speed.

[Nick980]

Wouldn't worry about it, PCI4.0 8x is the same as PCIE3.0 16x, don't think any graphics card on the market can fully use that bandwidth, from previous experience its probably a 1-2% performance hit

[Eigenvektor]

Try to put some load on the GPU, see if it changes. It's possible it uses less lanes for power saving when not needed.

[whm1974]

30 minutes ago, Nick980 said:

Wouldn't worry about it, PCI4.0 8x is the same as PCIE3.0 16x, don't think any graphics card on the market can fully use that bandwidth, from previous experience its probably a 1-2% performance hit

If I'm not mistaking dGPUs are still fine with PCIe Ver 3 8x?

[mariushm]

52 minutes ago, Jarsky said:

Just seeing why this would be happening, my understanding was that my CPU has 24 PCIe lanes (20 available to devices), and my X570 board should have an additional 8 lanes for PCIe & SATA devices, giving me a total of 28 usable lanes?

CPU has 24 lanes : 4 to chipset, 4 to m.2 connector,  16 to video cards (can be arranged in 1 x16 slot  or 2 x16 slots but each card running at x8 speed)

The chipset creates 8 pci-e lanes, which go to additional pci-e x1 or x4 slots (can be a x16 physically/visually but only 4 lanes in it), and other onboard stuff (more m.2 connectors, sata, usb)

 

Your card is in a x16 slot, and if the motherboard doesn't split the 16 lanes into 2 slots it will get all 16 lanes. 

The video card may be in a power saving mode - if there's need to consume power and use all 16 lanes, it will use only 8. In your picture, it actually reduced itself to x4 (four lanes) to save power.

52 minutes ago, Jarsky said:

Going through the manual, I have the GPU in X16_1 , Capture card in X16_2, and Wifi card in X16_3 . I only have the 1 NVMe drive which is in M.2_1 (in PCIe mode), and I have a SATA drive in SATA6G_6 

  Ok, so you DO have a card in the second pci-e x16 slot, which means the motherboard is forced to split the 16 lanes from the cpu into  2 groups of 8 lanes and put 8 lanes into each slot 

You're in this scenario (page 24 in the manual) : 

 

 

edit: If you want x16 to the video card (which would be pointless, really, you wouldn't notice the performance difference)  you would have to leave the 2nd pci-e x16 empty, put the capture card into the bottom x16 that has 4 lanes coming from chipset, and put the wireless card into the pci-e x1 slot (if needed using a riser cable / adapter).

 

 

[Jarsky]

35 minutes ago, Eigenvektor said:

Try to put some load on the GPU, see if it changes. It's possible it uses less lanes for power saving when not needed.

It goes to PCIe8@1.1 when idle, but to PCIe8@4.0 with load. 

Just doesn't want to go to PCIe16@4.0; when I have some time ill try experimenting with using my onboard wifi and move the capture card to the bottom slot instead, i thought the x8/x8 was only if they were 2 high speed devices (the Elgato is an x4)

 

I thought with the 28 total lanes between the CPU & X570 chipset; 16 lanes for GPU, 4 lanes for the Capture card, 1 for the wifi, 4 for the nvme (and maybe one for the sata controller?) that it should have been able to do full x16

[Jarsky]

2 minutes ago, mariushm said:

CPU has 24 lanes : 4 to chipset, 4 to m.2 connector,  16 to video cards (can be arranged in 1 x16 slot  or 2 x16 slots but each card running at x8 speed)

The chipset creates 8 pci-e lanes, which go to additional pci-e x1 or x4 slots (can be a x16 physically/visually but only 4 lanes in it), and other onboard stuff (more m.2 connectors, sata, usb)

 

Your card is in a x16 slot, and if the motherboard doesn't split the 16 lanes into 2 slots it will get all 16 lanes. 

The video card may be in a power saving mode - if there's need to consume power and use all 16 lanes, it will use only 8. In your picture, it actually reduced itself to x4 (four lanes) to save power.

  Ok, so you DO have a card in the second pci-e x16 slot, which means the motherboard is forced to split the 16 lanes from the cpu into  2 groups of 8 lanes and put 8 lanes into each slot 

You're in this scenario (page 24 in the manual) : 

 

 

Right, so my misunderstanding of it, is that its any PCIe card in the second slot will force it to split? So putting my capture card in the x16_3 slot, and leaving x16_2 free should get my card to x16 speed? I do have a spare x1 slot I can try my wifi card in (I had issues getting this to work on my B450-F Strix so I just put it straight in an x16 slot)

[mariushm]

Yeah, I just added a comment at the bottom of my previous post. Leaving the 2nd slot empty should get your video card running at x16 mode. 

 

re your other board : on some boards, pci-e x1 are disabled when m.2 pci-e ssds are inserted in second m.2 connectors, because there's not enough pci-e lanes in total. B450 for example only creates 6 pci-e 2.0 lanes, 4 go to a 3rd pci-e x16 slot, and you're left with 2 lanes that can be shared between a m.2 connector and pci-e x1 slots.

 

[Montana One-Six]

No you don't have 28 lanes. You have 24 lanes which 4 of them go to the chipset which means you have 20 left for other stuff.

 

[Jarsky]

4 minutes ago, mariushm said:

Yeah, I just added a comment at the bottom of my previous post. Leaving the 2nd slot empty should get your video card running at x16 mode. 

Chees for that, ill have a tinker with it tomorrow (1am here) and confirm back

[Jarsky]

2 minutes ago, Montana16 said:

No you don't have 28 lanes. You have 24 lanes which 4 of them go to the chipset which means you have 20 left for other stuff.

 

My understanding was that the CPU had 24 lanes (4 dedicated to the chipset) but that the X570 board provided another 8 lanes, like a PLX bridge? 

[mariushm]

Just now, Jarsky said:

My understanding was that the CPU had 24 lanes (4 dedicated to the chipset) but that the X570 board provided another 8 lanes, like a PLX bridge? 

No, those "pci-e switches"  exist but they're rarely used now, because of the price (also pci-e 4.0 switchers were even more expensive than pci-e 3.0 chips)

From cpu, you get 16 lanes for general purpose (either 1x16 or 2x8)  plus m.2 connector, 1x4 

The chipset does the rest, as can be seen in the picture above... up to motherboard manufacturer how they configure it. 

[Montana One-Six]

6 minutes ago, Jarsky said:

My understanding was that the CPU had 24 lanes (4 dedicated to the chipset) but that the X570 board provided another 8 lanes, like a PLX bridge? 

Yes you could use more than 4 lanes over the chipset but those have to share the 4x bandwidth to the cpu with which means they wont run at full speed.

Either way you have 20 lanes available form your cpu directly. Which means 1x16 plus 1x4 or 2x8 plus 1x4. As soon as you get over that 20 your x16 will be dropped to x8.

But 4.0x8 is about the same as 3.0x16 which is enough for GPUs. As far as I know GPUs haven't reached the point yet where they can fully saturate 3.0x16 so you should be fine.

[Jarsky]

6 hours ago, mariushm said:

edit: If you want x16 to the video card (which would be pointless, really, you wouldn't notice the performance difference)  you would have to leave the 2nd pci-e x16 empty, put the capture card into the bottom x16 that has 4 lanes coming from chipset, and put the wireless card into the pci-e x1 slot (if needed using a riser cable / adapter).

 

Swapped them around, and now getting full PCIe x16 4.0 to the GPU. Thanks for the help @mariushm