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Crossfire Bad FPS PCI Express bottleneck?

HI guys,

So I've been struggling to get a R9 280x CF to work properly, but I am about to give up. I think that my problem is that my motherboard has a PCI Express 3.0 at 16x but the other one functions at 2.0 4x when crossfiring.

I think that is why I'm not seeing the two 280x perform as they should. I game at 1080p, mostly BF4 and get 70-110 FPS on medium settings... :(

What do you guys think, it the PCI at 4x bottlenecking my 2nd GPU so bad that I get these results? If I upgrade the MOBO will I get better FPS?

Rest of my specs:

Core I5 2500k
12 GB RAM
120 GB SSD
Gigabyte h67m-ud2h-b3 Motherboard

I appreciate if you can help me out here!
5 answers Last reply Best Answer
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  1. Best answer
    Yeah PCI-E 2.0 x4 isn't nearly enough bandwidth for a 280x. You need a motherboard that has a PCI-E lane splitter that allows the PCI-E lanes from the CPU to be split into an x8 x8 configuration for Crossfire to work properly, your second card won't work well running off the chipset's PCI-E lanes. The H series chipset motherboards from Intel don't have a PCI-E lane splitter, that is feature that these days is restricted to Z series chipsets, though for Sandy Bridge some of the P67 boards do also have a PCI-E lane splitter.

    Also you will only have PCI-E 2.0 speeds with your current CPU, PCI-E 3.0 wasn't introduced until Ivy Bridge (Core i5/7 3xxx). 8 lanes of PCI-E 2.0 for each card should not present a significant bottleneck for your 280x cards though.
  2. Supernova1138 said:
    Yeah PCI-E 2.0 x4 isn't nearly enough bandwidth for a 280x. You need a motherboard that has a PCI-E lane splitter that allows the PCI-E lanes from the CPU to be split into an x8 x8 configuration for Crossfire to work properly, your second card won't work well running off the chipset's PCI-E lanes. The H series chipset motherboards from Intel don't have a PCI-E lane splitter, that is feature that these days is restricted to Z series chipsets, though for Sandy Bridge some of the P67 boards do also have a PCI-E lane splitter.

    Also you will only have PCI-E 2.0 speeds with your current CPU, PCI-E 3.0 wasn't introduced until Ivy Bridge (Core i5/7 3xxx). 8 lanes of PCI-E 2.0 for each card should not present a significant bottleneck for your 280x cards though.


    Thanks for the reply! In terms of FPS is this noticeable? Because I get almost the same FPS with one GPU than using crossfire. I mean, could this problem make me get like 50/60 FPS less??
  3. It could definitely make a big impact as your second card can't communicate with the CPU fast enough to get data to really contribute because it only has 4 lanes to work with, that aren't directly connected to the CPU, slowing things down even more. If you're hardly seeing any improvement with Crossfire on, then the lack of PCI-E lanes on the second card is probably the cause of that problem.
  4. Supernova1138 said:
    It could definitely make a big impact as your second card can't communicate with the CPU fast enough to get data to really contribute because it only has 4 lanes to work with, that aren't directly connected to the CPU, slowing things down even more. If you're hardly seeing any improvement with Crossfire on, then the lack of PCI-E lanes on the second card is probably the cause of that problem.


    Ok, get it. Now that I think about it, this could also explain why with Mantle I get slighly better results than D3D, because Mantle takes some load of the CPU. Am I wrong?
  5. Mantle improves performance in BF4 regardless of what hardware you're running, mostly by reducing CPU overhead. You'd see a performance boost, even if you were running with just one card.
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