Sakweid
1 Copper

Aurora R7, set up GTX 1080 SLI, PCIe speed problem

Hi, I have an Aurora R7 with i7-8700K, GTX 1080, 16GB 2933MHz

The desktop has two PCIe x16 slots and two PCIe x4 slots. The original GTX 1080 occupied one x16 and one x4 slot. I added up an NVMe M.2 SSD onto the other x4 slot (with M.2 to PCIe adapter).

Today I added the second GTX 1080 onto the second PCIe x16 slot to set up an SLI system. It works but on GPU-z it shows my second 1080 card only runs at PCIe x4 1.1 speed (bus bandwidth) while my original 1080 runs at PCIe x 8 3.0.

I know that Aurora R7 x16 slots run at x8 in reality. But don't know why my second graphics card doesn't run at full bandwidth. Note that the two cards have same specs (base and boost frequency). And because of this, I don't really see any better performance during usage, might because the SLI system downgraded them to both run at PCIe x4 1.1.

Can anyone please help me? Thank you very much!

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5 Replies
r72019
3 Zinc

Re: Aurora R7, set up GTX 1080 SLI, PCIe speed problem

Sounds like you have too many PCiE lanes in use so your speed is getting slowed (splitting bandwidth with the m2).  Pull out the m2 adapter and you should be all good.  It's going to get baked sandwiched between two 1080s anyway.

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Sakweid
1 Copper

Re: Aurora R7, set up GTX 1080 SLI, PCIe speed problem

Thanks! That might be the cause. I am thinking that the 8700k has 40 pcie lanes. Why does two graphics cards and one pcie x4 item max out them? What is the point to have sli setup on R7 with one more slot available but actually cannot be used? Is the motherboard doing something here? Thanks a lot.

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r72019
3 Zinc

Re: Aurora R7, set up GTX 1080 SLI, PCIe speed problem

The 8700K has 16 PCiE lanes, the motherboard z370 chipset has 24.  So total of 40.  But on the motherboard, there's lots of other items that eat up available chipset lanes; basically everything connected to it that sends data, for example, the usb headers on the top/back of your case, your M2 PCIE NVME boot drive, any SATA based drives, etc., connected to the motherboard.  The items connected to the motherboard don't all run at PCIe 3.0 speeds, or necessarily use the 16 direct to CPU PCiE lanes, but they do eat up chipset lanes.  I don't know what the allocation is on your system or what all you have connected.  But if your speed is being choked, with a SLI setup, 2x M2 NVME drives, etc., I think that is the most likely cause. 

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Sakweid
1 Copper

Re: Aurora R7, set up GTX 1080 SLI, PCIe speed problem

Thank you very much for the very detailed information! It is very helpful!

Now I face a new situation. I pulled out the original dell gtx 1080 card and the m2 to pcie adapted ssd and inserted my second PNY gtx 1080 card in the first pcie slot to find if it would work. However, the second card now runs at pcie x4 @ 3.0 while the original 1080 was running at pcie x8 @ 3.0 solely. I'm just lost. I don't know if there is any Alienware root settings that only allow dell graphics cards to run at full speed.

Thank you very much for your kind help!

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r72019
3 Zinc

Re: Aurora R7, set up GTX 1080 SLI, PCIe speed problem

Humm, since it's carrying over, that may be an energy saving feature built into the GPU. Click the ? mark next to the "bus interface" readout in GPUz, and then click "start render test."   It should update the readings. If not, see what the power settings are in MSI afterburner. 

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