20 Lanes off a i7-7700k?

[RavyuS]

Hi,

 

I'm currently testing a Triple Gaming VM set-up on my unRAID server 6. This is my current config-

 

CPU: i7-7700k Stock

Motherboard: Gigabyte Z270X-UD3

GPUs: GTX 750ti (Slot 1), 2x GTX 1070s (slot 2 and 3)

PSU: Corsair RM750 (750W)

 

So, I managed to run 3 VMs, with each VM taking 1c/2t from the CPU, with 1 GPU passed through to each. I tested Unigine simultaneously and was pretty impressed with the results, except for the VM running with the GTX 1070 in slot 3. That one was giving serious artifacting (both in-game and desktop) and I wondered if it was an issue of lack of lanes from the CPU. The i7 only has 16 lanes, so I assumed it would be a 1x8/2x4 setup. But that wouldn't make sense since the other two VMs were running flawlessly. I used GPU-Z to check the lane distribution, only to find this. Turns out, it was 2x8/1x4, with the first 2 GPUs using 8 lanes. I understand that the motherboard specifies x8 to the first two slots, and x4 to the last slot, but I'm clueless as to how this would work when the flagship CPU only has 16 lanes for PCIe. 

 

Curiously, if I swapped the GTX 1070 in Slot 3 with any of the other two GPUs, the VM would artifact too heavily for normal use. Just swapping the 1070s yield this problem as well. Any thoughts?

 

Thanks!

 

[John_M]

Presumably the third slot gets its PCIe lanes from the chipset via the DMI link to the CPU. So, yes, performance will be much worse than the two cards directly connected to the CPU in slots 1 and 2.

[JorgeB]

Presumably the third slot gets its PCIe lanes from the chipset via the DMI link to the CPU. So, yes, performance will be much worse than the two cards directly connected to the CPU in slots 1 and 2.

That's correct, interestingly Sandy/Ivy bridge server boards like the X9SCM can use 20 CPU pcie lanes, even on low end celeron and pentium CPUs, despite Intel reporting 16 max.

 

[SSD]

A test to confirm would be to pass through the GPU in slot3 to a different VM, and a GPU from slots 1 or 2 to the problem VM, and confirm the problem follows the slot.

 

You might be a bit underpowered for 3 gaming VMs depending on the game, but that is a fast chip and should work pretty well.

[RavyuS]

13 hours ago, John_M said:

Presumably the third slot gets its PCIe lanes from the chipset via the DMI link to the CPU. So, yes, performance will be much worse than the two cards directly connected to the CPU in slots 1 and 2.

Oh, that is interesting. I skipped over the idea that the motherboard could be doing some extension of sorts. I can't seem to find a way to disable this in my motherboard UEFI/BIOS. I want to try a 1x8/2x4 setup and see if there's some improvement, but the mobo settings are disgustingly basic.

 

13 hours ago, SSD said:

A test to confirm would be to pass through the GPU in slot3 to a different VM, and a GPU from slots 1 or 2 to the problem VM, and confirm the problem follows the slot.

 

You might be a bit underpowered for 3 gaming VMs depending on the game, but that is a fast chip and should work pretty well.

Yup, I've tried this and can confirm VMs using the slot 3 GPU artifact heavily. I've only ran Unigine Benchmarks on them so far, and that isn't particularly heavy on CPU so 1c/2t should suffice for each VM at the moment. Besides, I ran 2 VMs using 1c/2t and they worked flawlessly!

[John_M]

1 hour ago, RavyuS said:

I want to try a 1x8/2x4 setup and see if there's some improvement, but the mobo settings are disgustingly basic.

 

It can't be done. The first x16 PCIe slot will have all 16 lanes from the CPU, eight of which can be re-routed to the second x16 slot by a process known as bifurcation.The third x16 PCIe slot is not wired directly to the CPU. Its four lanes come from the Z270. The setting you are looking for isn't available in the BIOS, not because of an omission but because it isn't possible.