Solved PCIe 2.0 x 16 + x 4 or x 8 + x 8

Custom built pc / N/A
February 24, 2014 at 18:10:31
Specs: Windows 8.1 64 Bit, AMD FX-8350 8 Core @ 4.0 GHz - 2x4 DDR3-1600
Hey Everyone,

My Motherboard (MSI 970A-G43) can support up to 20 PCI 2.0 Express Lanes (one x 16 and one x 4, according to their website).

I was possibly planning on upgrading my system to run crossfire with two Radeon 7870s.

According to some tests, when running on 4 2.0 Lanes, the card suffers a 6% performance loss. When it runs on 8 2.0 Lanes, it takes a 2% hit. When running a full 16 Lanes, it suffers no performance loss.

What do you guys think would be better, running two cards at 8 lanes (4% total loss) or one at 4 and one at 16 (6% loss). Yes, the first option should cause less total loss, but since they would be connected via a crossfire bridge, would the other GPU (connected via 4 lanes) be able to use any of the excess bandwidth provided by the 16 lanes? If so, would it give better performance (since now a total of 20 (16+4) lanes are being used, compared to just 16 (8+8))?


See More: PCIe 2.0 x 16 + x 4 or x 8 + x 8

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February 25, 2014 at 16:30:16
✔ Best Answer
You have no choice but to set it up as 16 + 4 because that's the way your board is setup. There is no 8 + 8 option. The following info is taken from your motherboard manual:

"PCI_E2 supports up to PCIe x16 speed
PCI_E4 supports up to PCIe x4 speed"

"1. Install two AMD Radeon™ HD graphics cards into the PCI_E2 & PCI_E4 expansion slots.

2. With the two cards installed, two CrossFire™ cable are required to connect the graphics cards. Attach one side of the cable on each of the cards by way of the metal contacts. Please note that although two graphics cards have been installed, only the display ports on the graphics card installed in the first PCIe x16 slot will work. All displays should be connected to this graphics card."

message edited by riider

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February 25, 2014 at 16:41:44
mmm...I see. Well, thanks anyway!

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February 25, 2014 at 16:46:41
And be aware that most LCD monitors have a refresh rate of 60Hz, that means anything above 60 FPS will not be detectable.

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