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Daz Studios and GPUs 16x vs 8x slots?

GreatGonzoGreatGonzo Posts: 58
edited March 2018 in Technical Help (nuts n bolts)

I have a MSi X299 Tomahawk motherboard with a i7 7800X pro with 1x 1080ti and 1x980ti and I decided to add a third 980ti

The board has the following slots:
2x PCI-E 16 / 1x PCI-E 16 (8x) / 1x PCI-E 16 (4x) / 2x PCI-E 16 (1x)

I have the 1080ti and the 980ti plugged into the 2x PCI-E 16 slots and all runs fine.
(I get a nice 1:28 on the sickleyield render benchmark)

However,
When I plug the third 980ti into the 1x PCI-E 16 (8x) slot Daz sees it and uses it with no problem, but the render times are much much slower (about 3:50 on the same benchmark)

When I remove the third card the benchmarks goes back to normal, swapping the cards around makes no real difference.  So, I'm curious if using the 8x slot could be slowing down the two 16x slots?

It's not an overheating issue as all cards are running between 60c-65c while rendering

Anyone?
 

Post edited by GreatGonzo on

Comments

  • KitsumoKitsumo Posts: 1,216

    It shouldn't make a huge difference. There was a post in another thread where someone tested cards plugged into the PCI-E 1x slot and it was only a few seconds slower. There may be something about it in your motherboard manual. On my last motherboard, plugging a card into the 2nd slot did slow down the 1st slot. But no, I've never seen a slowdown that dramatic.

    Do you have your CPU selected for rendering? That could have an effect.

  • prixatprixat Posts: 1,590

    There are a few posts that suggest 'Optix' should be switched off with more than two GPUs.

  • GreatGonzoGreatGonzo Posts: 58

    I'll try OpitX as soon as the current render batch is finished, I know I had much better results with it on with two cards but definitely worth a try,

    The other thing that occurred to me is I was getting my monitor randomly go to sleep for about 5 seconds with three cards in the system, firstly I attributed this to the new motherboard, but that issue vanished last night when I removed the third 980ti card.

    Which could mean 'maybe' it's a bad connection on the third GPUs (it's in an open case so each GPU is connected via a 16x PCI-E riser)  I have a spare riser, so I'll swap out the riser in the 8x slot and see if it makes a difference? The risers and cards worked great on my old X99a board that had four 16x PCI-E slots - which is why I assumed it may be the 8x slot causing the issues.

  • DrNewcensteinDrNewcenstein Posts: 816
    edited March 2018
    • 4 x PCIe 3.0 x16 slots
      • Support x16/ x41/ x16/ x8 mode with the 44-lane CPU.
      • Support x16/ x41/ x8/ x4 modes with the 28-lane CPU.
      • Support x8/ x41/ x8/ x0, x8/ x41/ x4/ x4 modes with the 16-lane CPU.
    • 2 x PCIe 3.0 x1 slots
    1. The PCI_E3 lanes are supported by PCH.

    I'm not entirely sure, but it looks like Slot 0 and Slot 1 are cut down to x8 speed when Slot 2 is populated, and Slot 2 runs at x4.

    Post edited by DrNewcenstein on
  • GreatGonzoGreatGonzo Posts: 58

    Ok, I solved this (I think). It appears it was a dodgy 16x riser in the 8x slot connected tio the third 980ti and when I replaced it everything seems to work better.

    Benchmark:
    2 cards: 01:19
    3 cards: 1:02

    In an actual real render:
    2 cards: 24:15
    3 cards: 19:25

    HOWEVER: SLOT WEIRDNESS - part two!
    I did find some 'weirdness' that if I plug the faster 1080ti card into the 8x slot while a slower 980ti card is in the 16x slot - Windows refuses to recognise anything but the card in slot one. I have no idea why, but it does.

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