How does PCIe lane allocation work on Z390 motherboards?

[MrJim]

I've just bought a Gigabyte Z390 Designare motherboard, which I'm pairing with an 8700K CPU. Like a lot of motherboards, the Designare has the PCIe slots that are physically x16, but it can't provide x16 bandwidth on all three slots at the same time due to CPU limitations. The best it can offer is x8 x8 x4 with all three x16 slots populated. My question is this: because of the layout of the slots, it would actually be more convenient to put my single GPU card in the second slot, instead of the first. With the other two x16 slots left unpopulated, would that second slot run at x16 bandwidth (like it would in the top slot), or would it run at x8? I realise I could just contact Gigabyte support, but I'm guessing I'll get a faster response from the Hexus crew! Thanks!

[Jonj1611]

I believe the 2nd slot will always be 8x

[MrJim]

I believe the 2nd slot will always be 8x

Ah right, I suspected that would be the case. Thanks for the clarification

[AETAaAS]

I think Jon is right, from the Gigabyte manual on the Z390 Designare:

1 x PCI Express x16 slot, running at x16 (PCIEX16)
* For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16 slot.
Š 1 x PCI Express x16 slot, running at x8 (PCIEX8)
* The PCIEX8 slot shares bandwidth with the PCIEX16 slot. When the PCIEX8 slot is populated, the PCIEX16 slot operates at up to x8 mode.
Š 1 x PCI Express x16 slot, running at x4 (PCIEX4)
Š 2 x PCI Express x1 slots
(All of the PCI Express slots conform to PCI Express 3.0 standard.)
Š 1 x M.2 Socket 1 connector for an Intel® CNVi wireless module (CNVI)

The other question is, will it make any difference? If you are keeping the 1070 in your sidebar, probably not given a GTX1080 at 8 lanes is near enough to make no difference to 16 lanes (https://www.techpowerup.com/reviews/...press_Scaling/), and even 4 lanes is within spitting distance (one of the final charts shows x4 has 96% of the performance of the 16x).

edit: Was too slow, you already got your answer.

[MrJim]

I think Jon is right, from the Gigabyte manual on the Z390 Designare:

1 x PCI Express x16 slot, running at x16 (PCIEX16)
* For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16 slot.
Š 1 x PCI Express x16 slot, running at x8 (PCIEX8)
* The PCIEX8 slot shares bandwidth with the PCIEX16 slot. When the PCIEX8 slot is populated, the PCIEX16 slot operates at up to x8 mode.
Š 1 x PCI Express x16 slot, running at x4 (PCIEX4)
Š 2 x PCI Express x1 slots
(All of the PCI Express slots conform to PCI Express 3.0 standard.)
Š 1 x M.2 Socket 1 connector for an Intel® CNVi wireless module (CNVI)

The other question is, will it make any difference? If you are keeping the 1070 in your sidebar, probably not given a GTX1080 at 8 lanes is near enough to make no difference to 16 lanes (https://www.techpowerup.com/reviews/...press_Scaling/), and even 4 lanes is within spitting distance (one of the final charts shows x4 has 96% of the performance of the 16x).

edit: Was too slow, you already got your answer.

Thanks for the confirmation though You're right, I doubt it'd make a perceivable difference in gaming, but I do like the idea of using all the GPU bandwidth available to me.