How many PCIe lanes do I actually have and bottlenecks.

[UltraTin]

Hi all,

 

This just something I've been wondering. The mainstream intel CPUs, in this case the i5 4690k and i7 4790k, both have a maximum of 16 PCIe lanes. So if I put a single graphics card in a 16x PCIe lane, wouldn't I be using all the available lanes? If that is the case, won't SLI/Crossfire be impossible? I know this is not the case of course, but this question has been bothering me for a while. 

[WinNut]

When you put a second GPU in, the slots will share the 16 lanes with each other in an x8/x8 configuration. However, performance loss at an x8 connection is negligible at best.

 

These are not to be confused with the PCIe x1 slots, which get their lanes off the chipset.

[KWelz]

Nvidias cards in SLI run at 8x/8x. The performance difference is so minor it isn't worth mentioning. Crossfire can run down to 4x.

[MegaDave91]

Like @WinNut said - once you add another graphics card, the 16 PCIe lanes will be divided evenly between both cards giving them each 8 lanes. There will be no performance degradation despite the bandwidth literally halving; this is because there is still PLENTY. 

[UltraTin]

So what if I place 2 graphics card in SLI/Crossfire, which will split the 16x slots into 8x/2x, and I wish to add in an additional card like a wireless card (not really sure how many slots that take up), would it be further splitting the 16x slots or would it be taking additional slots from the chipset?

[WinNut]

So what if I place 2 graphics card in SLI/Crossfire, which will split the 16x slots into 8x/2x, and I wish to add in an additional card like a wireless card (not really sure how many slots that take up), would it be further splitting the 16x slots or would it be taking additional slots from the chipset?

This diagram is for Z77 but the concept is the same. 

 

So here's how it works:

 

From your CPU, you get 16 PCIe 3.0 lanes. These go to the x16 slots on your motherboard, and are used for driving the GPU because the graphics processor needs a high-speed link to the processor. These lanes will split up between the other full length slots but only when the other full-length slots are occupied.

 

The PCIe x1 slots get their lanes from the chipset, and are separate from the PCIe lanes that supply the GPU(s). So if you insert a wireless card into the x1 slot, it'll get the lane from the chipset and not from the CPU's lanes.

[MegaDave91]

So what if I place 2 graphics card in SLI/Crossfire, which will split the 16x slots into 8x/2x, and I wish to add in an additional card like a wireless card (not really sure how many slots that take up), would it be further splitting the 16x slots or would it be taking additional slots from the chipset?

SLI (Nvidia's variant for graphics cards working in tandem) will operate no lower than 8x/8x. If you have a motherboard whose chipset (eg Z97) supports PCIe 1x, then you have nothing to worry about.

[dragoon20005]

the chipset will be the one to support the expansion cards

 

but i depend on the mobo that you picked

 

this are reserved for more expensive models

[UltraTin]

Wow, thanks everyone! Really solved my confusion!