I7 4790K PCIe Lanes

[davidst95]

Hi, I have a I7 4790K and a Gigibyte Z97 Gaming GT with 4 PCI 3.0 x16 slots that are able to run all at the same time in x8 mode.   I'm kind of newbie with PCIe lanes.   How can the CPU with 16 lanes support 32 lanes plus any PCIe x1 slots.   Also, how can if it's using another chip-set to get additional lanes, how to I check to make sure that my GPUs are using the CPU lanes.

 

Thanks

[TheDeadGhost]

It cant. If you use all the pci lanes then they will be running at 4x

[davidst95]

It cant. If you use all the pci lanes then they will be running at 4x

 

Thanks for the reply.    Could you check the specs of my board:

 

http://www.gigabyte.com/products/product-page.aspx?pid=4969#sp

 

It claims it can run the PCIe x16 slots at 8 lanes each when all used.   It's designed for 4 way SLI and I think each GPU would need at least 8 lanes.   I might be reading it wrong.

 

David

[Lotus]

You have two PCIE 3.0 16x slots. When both are populated, they will be running at 8x each.

 

Those other two slots are PCIE 8x slots, and take lanes from the PCIE 16x slots. If you populate them all, you will be running at x4, x4, x4, and x4.

 

The i7-4790k only has 16 PCIE 3.0 lanes available. The Z97 chipset has an additional 8 PCIE 2.0 lanes, which is what those PCIE 1x slots use. For SLI GPUs, you're limited to two running at 8x and 8x max, unless you get into the whole PLX chip nonsense which I would never recommend. Crossfire allows GPUs to run in slots as slow as x4 though.

[davidst95]

You have two PCIE 3.0 16x slots. When both are populated, they will be running at 8x each.

 

The i7-4790k only has 16 PCIE 3.0 lanes available. The Z97 chipset has an additional 8 PCIE 2.0 lanes available, which is what those PCIE 1x slots use. For GPUs, you're limited to two running at 8x and 8x max, unless you get into the whole PLX chip nonsense which I would never recommend.

 

Ok thanks.   I just wanted to verify that something other was providing additional lanes.

[davidst95]

Ok thanks.   I just wanted to verify that something other was providing additional lanes.

 

Ok thanks.   I just wanted to verify that something other was providing additional lanes.

 

You have two PCIE 3.0 16x slots. When both are populated, they will be running at 8x each.

 

Those other two slots are PCIE 8x slots, and take lanes from the PCIE 16x slots. If you populate them all, you will be running at x4, x4, x4, and x4.

 

The i7-4790k only has 16 PCIE 3.0 lanes available. The Z97 chipset has an additional 8 PCIE 2.0 lanes, which is what those PCIE 1x slots use. For GPUs, you're limited to two running at 8x and 8x max, unless you get into the whole PLX chip nonsense which I would never recommend.

 

Would running a GTX 980 in x4 mode cause a performance hit on the GPU?

[Lotus]

Would running a GTX 980 in x4 mode cause a performance hit on the GPU?

A single one? It wouldn't be a problem. However NVidia will not allow SLI in any configuration less than x8. I can't imagine what you would plug in other than GPUs that would force it down to x4 though.

 

If you were trying for a 3 or 4 GPU setup, well, that's rather ridiculous anyway and you shouldn't be doing it in the first place.

[davidst95]

A single one? It wouldn't be a problem. However NVidia will not allow SLI in any configuration less than x8. I can't imagine what you would plug in other than GPUs that would force it down to x4 though.

 

If you were trying for a 3 or 4 GPU setup, well, that's rather ridiculous anyway and you shouldn't be doing it in the first place.

 

I knew NVidia required x8 lanes for their GPUs.   The issue is that I have 2 x GTX980 in SLI.    I also have a PCIe SSD that needs to go in another x16 lot (actually it only needs a x4 slot but the board does not have any of those).   I guess my concern is that if either the GPUs or the SSD are not using the real CPU PCIe lanes something is taking a performance hit.    Am I misunderstanding something?   Thanks.

 

David

[Lotus]

I knew NVidia required x8 lanes for their GPUs.   The issue is that I have 2 x GTX980 in SLI.    I also have a PCIe SSD that needs to go in another x16 lot (actually it only needs a x4 slot but the board does not have any of those).   I guess my concern is that if either the GPUs or the SSD are not using the real CPU PCIe lanes something is taking a performance hit.    Am I misunderstanding something?   Thanks.

 

David

Well, basically you're stuck with two options: run with only a single GTX 980 and use your SSD, or keep your SLI GTX 980s and put your SSD into a PCIE 2.0 1x slot and deal with the reduced performance.

 

edit: nevermind, you can't do option two since the PCIE 2.0 1x slots prevent you from plugging longer cards into them. Basically you're going to have to choose between your second GTX 980 and the SSD because NVidia is stupid and forces an arbitrary requirement on you that makes no sense. If these were AMD GPUs, you could have one of the GPUs running at 8x, the second running at 4x, and the SSD running at 4x, all at 3.0 speeds on that board.

[davidst95]

Well, basically you're stuck with two options: run with only a single GTX 980 and use your SSD, or keep your SLI GTX 980s and put your SSD into a PCIE 2.0 1x slot and deal with the reduced performance.

 

Yeah that's kind of the problem.   The PCIe SSD needs a x4 slot that's why I have it in one of my x16 slot.    I might look into the X99 platform.   Thanks for the info!

[Lotus]

The only other option for getting them all to work is to get a Z97 chipset board with a PLX chip to add on artifical (not-real) PCIE 3.0 lanes. It's a very clunky and expensive solution, but it exists.

[Lotus]

Huh, I could be wrong. Some stuff is saying your motherboard HAS a PLX chip and you'll be fine as-is, but the page you linked to makes no mention of one. It would be nice if they would accurately describe their products. I have no way of knowing for sure now, but according to PCPartPicker, Gigabyte is incorrectly describing their own board:

 

http://pcpartpicker.com/part/gigabyte-motherboard-gaz97xgaminggt

 

now I'm confused. I can't tell if it has a PLX chip or not.

[davidst95]

Huh, I could be wrong. Some stuff is saying your motherboard HAS a PLX chip and you'll be fine as-is, but the page you linked to makes no mention of one. It would be nice if they would accurately describe their products. I have no way of knowing for sure now, but according to PCPartPicker, Gigabyte is incorrectly describing their own board:

 

http://pcpartpicker.com/part/gigabyte-motherboard-gaz97xgaminggt

 

now I'm confused. I can't tell if it has a PLX chip or not.

 

Hi, I looked in the manual and it says it uses PLX PEX874 chip.   I was just concern because I read that PCIe SSD cards take a performance hit if they don't directly access the CPU PCIe lanes.

[Lotus]

Hi, I looked in the manual and it says it uses PLX PEX874 chip.   I was just concern because I read that PCIe SSD cards take a performance hit if they don't directly access the CPU PCIe lanes.

As I said, it's a clunky solution. It probably will take a performance hit, but it'll work. How much of one I'm not sure, but if it's noticeable it's probably a very minor hit. Just make sure you plug in the SSD into the second x8 slot so the primary GPU is running at x8 without relying on the PLX chip.

[davidst95]

As I said, it's a clunky solution. It probably will take a performance hit, but it'll work. Just make sure you plug in the SSD into the second x8 slot so the primary GPU is running at x8 without relying on the PLX chip.

 

Ok, sorry for another question.   Could I use the second GPU in the 3rd PCIe slot which I assume would be use the PLX chip?

[Lotus]

The configuration should be:

 

PCIe 3.0 16x #1: GPU

PCIe 3.0 8x #1: nothing

PCIe 3.0 16x #2: GPU

PCIe 3.0 8x #2: SSD

 

This may or may not translate into the actual layout on your board. Just get the slot numbers and types correct. The second GPU and SSD may be able to be switched if desired with no ill effect (not sure). You just want to have the primary GPU on its 8x full CPU PCIE 3.0 lanes without any PLX sharing involved.

[davidst95]

The configuration should be:

 

PCIe 3.0 16x #1: GPU

PCIe 3.0 8x #1: nothing

PCIe 3.0 16x #2: GPU

PCIe 3.0 8x #2: SSD

 

This may or may not translate into the actual layout on your board. Just get the slot numbers and types correct.

 

Thanks suggestion.   I'll try that configuration and see if it works.