PCI-E 16x slot running GPU at 4x speed

[anthony8989]

I have a GTX 1070 in the top slot of an Arock Extreme3 z77 motherboard. The motherboard and card support 16x 3.0. But the card runs at 4x speed according to GPUz. Even with the render test on.

Nothing else is in any of the other PCI-E slots. When I plug the GPU into the second slot, it's runs at 8x 3.0 speed as expected.

Any ideas why this would happen?

 

[fancarolina]

It has to do with the number of PCIe Lanes the CPU supports and how the sockets are electrically wired. A 3rd Gen i5-3570K only has 16 PCIe lanes from the CPU. On your board there are 3 physical x16 slots. But if you look at them from the side you will notice that the metal contacts only go all the way across on one slot the top one closest to the CPU. The second slot down the contacts go halfway, therefore, x8 the third slot goes a quarter of the way, therefore, x4. When you run multiple cards it will cut the bandwidth to the other slots so the cards will function but they will be bandwidth limited. If you want full x16 bandwidth then you should be running your GTX 1070 in the top slot without any other PCI cards in the other two x16 slots. As an additional note if you do a little research you will find that a GTX 1070 really only uses up to about x8 amount of data so it doesn't really hurt performance.

 

[anthony8989]

It has to do with the number of PCIe Lanes the CPU supports and how the sockets are electrically wired. A 3rd Gen i5-3570K only has 16 PCIe lanes from the CPU. On your board there are 3 physical x16 slots. But if you look at them from the side you will notice that the metal contacts only go all the way across on one slot the top one closest to the CPU. The second slot down the contacts go halfway, therefore, x8 the third slot goes a quarter of the way, therefore, x4. When you run multiple cards it will cut the bandwidth to the other slots so the cards will function but they will be bandwidth limited. If you want full x16 bandwidth then you should be running your GTX 1070 in the top slot without any other PCI cards in the other two x16 slots. As an additional note if you do a little research you will find that a GTX 1070 really only uses up to about x8 amount of data so it doesn't really hurt performance.

Thanks for your reply fancarolina. I appreciate your clarifying the PCI-e lanes.

But my issue is in running my GTX 1070 in the PCI e slot closest to the CPU (which should be 16x) but it's only operating at 4x speed.

 

[anthony8989]

In case anyone runs into the same issue, I've figured out the solution.

The issue was 2 slightly bent pins in the CPU socket of the motherboard. They were just bent enough to cause instability during extreme overclocking and evidently poor signalling to the main PCI-e 16x slot. I carefully bent them back into place with a plastic pick and everything works 100% now.