What difference does sharing secondary PCIe x16 with other PCIe lanes (x16 and x1s) make?

Question

I'm about to buy a motherboard and I have a difficulty understanding how sharing PCIe lanes works.

I have two motherboard options:

• Gigabyte GA-Z97-HD3
• Gigabyte GA-Z97X-SLI.

Lets call them MOBO#1 and MOBO#2, respectively, for convenience.

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According to product specifications MOBO#1 has

• 1x PCIe x16 slot running @ x16 speed.
• 1x PCIe x16 slot running @ x4 speed and bandwidth is shared with x1 slots (PCIe 2.0).

and MOBO#2 has

• 1x PCIe x16 slot running @ x16 speed.
• 1x PCIe x16 slot running @ x8 speed and bandwidth is shared with the primary x16 slot (PCIe 3.0).

If I were to install 2 GPUs, I would like to have my graphics processing power multiplied by two but I assume that due to lane sharing I will achieve lower efficiency when it comes to multiple GPUs (please correct me if my logic is flawed).

I've done some reading about PCI-Express and if I'm not mistaken the secondary PCIe x16 slot will have a bandwidth of 2 GB/s on MOBO#1 and 8 GB/s on MOBO#2.

But these results don't add up for me as MOBO#1 will provide 16 GB/s + 2 GB/s = 18 GB/s bandwidth and MOBO#2 will provide 8 GB/s + 8 GB/s = 16 GB/s bandwidth (most probably a flow in logic, I have, there). Does this mean MOBO#1 will yield a better performance in the case of multiple GPUs (assuming that the two GPUs are identical and will be installed on both MOBOs)?

How am I supposed to compare the total GPU performance if I were to install two GPUs on those motherboards?

How exactly is the performance affected by this sharing situation?

What should my though process be when evaluating motherboards considering this lane sharing variable?

Answer 1

There are many variables you are not considering that make impossible to objectively compare YOUR GPU performance for a dual GPU setup simply by comparing PCIe lanes on a motherboard.

1. The PCIe lanes are not used for processing, only for data transfer meaning while bandwith might be correlated to graphics performance, they are not directly related. Half the bandwidth may NOT mean half the performance.

2. Both boards are using the same chipset with only 8 PCIe lanes through the chipset. Intel Z97 Express Chipset - ark.intel.com/products/82012/Intel-DH82Z97-PCH. This means that access to CPU processing power and memory is the same on both boards which would affect GPU performance, but at an ukown amount.

3. Performance will also vary with application; that is what software you are using, and how the software uses the GPU, CPU, memory, etc. will affect real world performance

4. And finally, SLI and Crossfire add there own variables in that some applications do not take full advantage of both GPUs like on would hope hampering performance. Again, this is both application specific and hardware specific.

I suggest you take a leap of faith and just do your research, eliminate what variables you can, leave the variables you cannot, and hope for the best. Your GPU performance with a dual GPU setup is a variable you are not going to be able to nail down without real world testing and benchmarks on your hardware with your applications.