PCI-E x16 Throttling

[jonsey18]

Take pity because I don't know anything about this aspect, hense me asking :)

I was told by someone that if you SLI the top/main x16 in your board then it throttles the second to x8 or something Like that. Can I get a rough breakdown of what happens to the slots when you use SLI. Do they slow to 8 if you use two or will 16 always stay 16?


 

[JamesDwho]

On the Z77 (1155) motherboards when you populate a PCI-E x16 Physical Lane with one PCI-E x16 video card that card will get full x16 bandwidth from the CPU, if you add another video card to your system the bandwidth gets split in half for both cards, so the first video card gets x8 bandwidth and the second get x8 as well.

 

That happens because the 1155 socket CPU's have a limit to the maximum PCI-E bandwidth it can provide. LGA 2011 CPUs have more PCI-E bandwidth and don't split to x8 and x8 with two video cards

[Hoppa]

Almost all standard motherboards will have at least one x16 pci-e slot

 

Some motherboards will have maybe 4 or so slots that are the x16 pci-e size, but not all will run at x16 speed such as the 1155 socket platform.

 

Usually the first pci-e x16 slot is 'wired' to be run at x16 speed. You can tell by looking at the pins in the connector.

Some of the Mid-Lower end motherboards won't have a second pci-e x16 slot that is wired to run at x16 speed, and instead a x16 slot wired to run at x8 speed.

You can tell if a x16 slot will run at x8 speed because of the little metal pins inside the plastic connector holes will stop at half way down the slot.

 

When using 2 graphics cards, they have to run at the same connection speed to the motherboard so if your board can't support x16 and x16 it will run at x8 and x8.

[jonsey18]

Perfect, Cheers :D

[kaneandtaker]

Will GPUs be bottlenecked at 8x/8x?

[Ddm5]

Will GPUs be bottlenecked at 8x/8x?

From what I've been told, current GPU's won't bottleneck on said setups, also it depends on what Gen PCI-e it's running on, 3.0 is basically 2.0 x16 (x2) slots bandwidth wise.

[R4STABAN]

To clear up:

 

16x / 8x etc refers to bandwidth. So saying 'will they slow to 8x' makes no sense.

 

It's like a 16 lane road versus an 8 lane road. One can handle more traffic.

 

What you have been told is partially true but also partially rubbish. If we take the example of an ivybridge processor, it has one 16x lane for graphics on the cpu. Thus you can run one card at 16x, two at 8x or even three at 8x 4x 4x (depending on board).

Some boards have additional chips to artificially supply more bandwidth to the pci-e slots. They don't improve performance but allow you to run (for example) 4 gpus on 1155 platform. (gimmick)

 

I run my 680s at pcie 2.0 8x/8x which is like pcie.30 4x/4x and I have no problems.

 

The cards don't need to interface with the same bandwidth.

[Glenwing]

Current cards do not saturate PCIe 1.1 x16/PCIe 2.0 x8/PCIe 3.0 x4

[Leonard]

That is only dependent on the motherboard's PCI-Ex16 connector specifications. On some boards that will happen because some motherboards may have 2 PCI-Ex16 slots but one will run at x16 and the other will run at x4 or x8 while other motherboards that have 2 or more PCI-Ex16 slots that run and x16, x16, x16 per slot. Take MSI's GD45 for example, it has 3 PCI-Ex16 slots that run x16 x16 x8 so if you put an 2 way SLI or Crossfire setup you will get x16 and x16 but if you go for 3 way SLI or Crossfire you will get x8 x8 x8 on the same board.

 

If you want to run an SLI or Crossfire set up just make sure the motherboard PCI-E can handle full x16 an you won't get any throttling down of you PCI-E speed. 

[R4STABAN]

That is only dependent on the motherboard's PCI-Ex16 connector specifications. On some boards that will happen because some motherboards may have 2 PCI-Ex16 slots but one will run at x16 and the other will run at x4 or x8 while other motherboards that have 2 or more PCI-Ex16 slots that run and x16, x16, x16 per slot. Take MSI's GD45 for example, it has 3 PCI-Ex16 slots that run x16 x16 x8 so if you put an 2 way SLI or Crossfire setup you will get x16 and x16 but if you go for 3 way SLI or Crossfire you will get x8 x8 x8 on the same board.

 

If you want to run an SLI or Crossfire set up just make sure the motherboard PCI-E can handle full x16 an you won't get any throttling down of you PCI-E speed. 

You're talking about the x79 gd45. socket 2011 processors have 2 16x lanes for expansion on the cpu.

[Leonard]

You're talking about the x79 gd45. socket 2011 processors have 2 16x lanes for expansion on the cpu.

Yes and in socket 1155 there is the Z77A-G45 or the P8Z77 WS which has 3 PCI-Ex16 slots each. I was just using them as a reference for jonsey18  to understand the GPU throttling reason and why it is dependent on the motherboard.

[Robi Dawn]

say i had a board with 3 pcie 2.0 slots at 16x will a pcie 3.0 graphics card work in it?

[mgsstar]

say i had a board with 3 pcie 2.0 slots at 16x will a pcie 3.0 graphics card work in it?

Yes it would work because it PCI-E 3.0 cards are backwards compatible with PCI-E 2.0.

[Art Vandelay]

On 1155, you only have 16 PCIe lanes, so you will run at dual x8, if you don't have a board with a PLX chip. If you have one of those, both cards will run at x16, but there will be increased latency compared to a true x16.

 

On the FX chips and socket 2011, you have 32 PCIe lanes, so you can do dual x16. The Asrock extreme11 can actually do quad x16 because it has PLX chips on it to split the 32 lanes into 64 lanes.

 

say i had a board with 3 pcie 2.0 slots at 16x will a pcie 3.0 graphics card work in it?

 

PCIe is backwards compatible, so a 3.0 card should work in a 2.0 motherboard, or the other way around.