PCIe x16 switching to x8 even if no 2nd GPU connected?

[molnart]

Hi,

i have a tiny question on PCIe lanes. My Mobo has 2 PCIe x16 slots. The manual says that both of these ports operare in x16 modes, but if two gpus are connected they are scaled down to 8x modes.

However, it seems that the lanes are scaled down if ANYTHING is connected to the 2nd PCI x16 port,not only a GPU. My first PCIe x16 port holds a Gedorce GTX560 and the second a tiny PCIe x1 wifi card. However according to GPU-Z, the GPU is in x8 mode. Is that because of the x1 card in the other slot? Is this behaviour normal and expected?

I know that x16 vs x8 probably wont affect GPU performance significantly, i am just curious...

Thanks

 

[theonedub]

My understanding is that the 16 PCIe lanes supplied by the CPU can only be setup as (1) 16x or (2) 8x. So any card installed in the second slot will result in an 8x/8x split.

If you are using a PCIe Wifi card you should install it in the 1x PCIe slot.

 

[Aquinus]

My understanding is that the 16 PCIe lanes supplied by the CPU can only be setup as (1) 16x or (2) 8x. So any card installed in the second slot will result in an 8x/8x split.

If you are using a PCIe Wifi card you should install it in the 1x PCIe slot.

Bingo, all the second slot has to be is populated to drop to 8x. Also there could be nothing in the first PCI-E 16x slot and this would still be the expected behavior. Try to save 16x slots for 4x cards and larger. If you have a PCI-E 1x card, put it in a 1x slot, it won't do anything weird like this if you do.

 

[Tatty_Two]

Affirmative to that ^^^ the wireless card is still using a PCI-E lane(s) and therefore the slots would switch to x8 mode.

 

[Aquinus]

Affirmative to that ^^^ the wireless card is still using a PCI-E lane(s) and therefore the slots would switch to x8 mode.

I suspect that the second slot is only wired to 8x unless it is a 990FX, X79, or X58 motherboard. It is most likely just a PCI-E lane switch from the first slot.

If it is the motherboard in your system specs, ASUS' most recent motherboards follow this scheme: white slots denote 8x wired where blue denote 16x wired, as well as black being 4x wired.

I take it that you can't use the PCI-E 1x slot right next to the PCI-E 16x (blue) slot because your video card uses two slots.

 

[Tatty_Two]

I suspect that the second slot is only wired to 8x unless it is a 990FX, X79, or X58 motherboard. It is most likely just a PCI-E lane switch from the first slot.

Yes.....The chipset has only 16 lanes, as soon as his wifi in the second slot takes one lane then the first slot has to revert to x8 as there are then only 15 lanes available.... thats at least the way I understand it.

 

[Aquinus]

Yes.....The chipset has only 16 lanes, as soon as his wifi in the second slot takes one lane then the first slot has to revert to x8 as there are then only 15 lanes available.... thats at least the way I understand it.

I think it is more simple as that. I think it just switches all 8 lanes if a card is simply detected in the second slot. How many lanes that card uses doesn't matter, so as far as the controller is concerned 8 are used for the GPU on the first slot, 8 are switched and available to the 2nd slot, then I bet you that if he added another PCI-E card regardless of size to the 3rd black slot, it would move 4 lanes from that 2nd slot to do 8x/4x/4x. It's just switching based on card population.

 

[molnart]

I take it that you can't use the PCI-E 1x slot right next to the PCI-E 16x (blue) slot because your video card uses two slots.

exactly.

I bet you that if he added another PCI-E card regardless of size to the 3rd black slot, it would move 4 lanes from that 2nd slot to do 8x/4x/4x. It's just switching based on card population.

it seems that is not true. just yesterday i have added Xonar DX card to the last slot, and according to HWiNFO my PCIe slots are operating in 8x/8x/4x mode (GPU/Wifi/Sound card).

 

[Tatty_Two]

^^^ Thats as maybe but at the end of the day the chipset has only 16 lanes available and thats all your gonna get, so you will not "actually" ever get 8 x 8 x 4.

 

[Bo$$]

Don't worry about it, 16x-8x barely looses any FPS. shouldn't be too bad

 

[Aquinus]

^^^ Thats as maybe but at the end of the day the chipset has only 16 lanes available and thats all your gonna get, so you will not "actually" ever get 8 x 8 x 4.

I thought that the z77 chipset offered 4 PCI-E lanes off the chipset using DMI. That last slot might eat up your PCH's bandwidth instead which could impact disk I/O and anything connected to your PCH if you run that last slot at bandwidth. So really it is 8x/8x/4x, but that last 4x performance may vary depending on what subsystem on the PCH takes priority when it comes to DMI bandwidth. This is half of the reason why I got SB-E and a X79 motherboard. You don't run out of lanes when you have 40 lanes of PCI-E goodness, but that is just me and I was willing to pay a bit more for the platform. Everyone has their priorities.

 

[Tatty_Two]

I thought that the z77 chipset offered 4 PCI-E lanes off the chipset using DMI. That last slot might eat up your PCH's bandwidth instead which could impact disk I/O and anything connected to your PCH if you run that last slot at bandwidth. So really it is 8x/8x/4x, but that last 4x performance may vary depending on what subsystem on the PCH takes priority when it comes to DMI bandwidth. This is half of the reason why I got SB-E and a X79 motherboard. You don't run out of lanes when you have 40 lanes of PCI-E goodness, but that is just me and I was willing to pay a bit more for the platform. Everyone has their priorities.

Your right about the x4 off Dmi, I was just under the impression though that on this board if you had it populated then all you would get is 8x4x4.... maybe I got it wrong.

 

[manofthem]

Have you tried: Going into the bios and forcing the black pcie slot to x1, and then putting the wifi card there? My asus p8p67pro allows for that in the bios so I'd assume yours does too. My sound card is in the bottom slot and my gpu works at x16 when I stalled together.

 

[newtekie1]

Your right about the x4 off Dmi, I was just under the impression though that on this board if you had it populated then all you would get is 8x4x4.... maybe I got it wrong.

The black slot on his board is indeed run from the chipset, so it has a constant x4 link, though at the expense of some latency. Though the z77 chipset is capable of x8/x4/x4 operation, this board just happens to not use it.

 

[molnart]

Have you tried: Going into the bios and forcing the black pcie slot to x1, and then putting the wifi card there? My asus p8p67pro allows for that in the bios so I'd assume yours does too. My sound card is in the bottom slot and my gpu works at x16 when I stalled together.

that would be awesome. unfortunately i cannot seem to find such option in my bios... (however it rings a bell and maybe i've seen it.... could it be gone with a bios upgrade ??? )

 

[cadaveca]

Though the z77 chipset is capable of x8/x4/x4 operation, this board just happens to not use it.

It has nothing to do with the chipset...it's all about the CPU, that houses the main PCIe controller.

The Ivybridge CPUs can address three devices, the SB CPUs, only 2. So how the lanes are split is not dependant just on the chipset..the CPU matters too. There are quite a few boards where the third slot only works if you have a IvyBridge chip installed.


P67/Z68, and Z77 PCH all offer 8 lanes of PCIe 2.0, so some boards will use those lanes for the third slot, but it depends on how many added controllers they use as to how they decide to do the third slot. Using these for a PCie x16 slot comes at very little penalty, if any, as all lanes are full-speed PCIe 2.0. P67/Z68 changed that, where X58 and P55 had PCIe 2.0, but it wasn't actually full PCIe 2.0(was really PCIe 1.1 with PCIe 2.0 encoding), so some devices connected via the chipset took a performance penalty.