4890's Crossfired at 8x/8x

[Dougx1317]

I have a board that can run crossfireX at 8x/8x. If I were to get a second 4890, would it be bottlenecked at anything above stock settings? If the below quote that I found is true than wouldn't it mean that overclocking the memory over 4GB/s wouldn't make a difference? I've read about people getting to 4.6-4.8GB/s and I've never thought about this as a problem. Is the below statement correct? Would it not make sense overclocking the memory to more than 4GB/s?

"PCI-E 2.0 8x runs one lane at 500 MB/s.
8 X 500 MB/s = 4GB/s
4890 uses 3.9GB/s at stock"

 

[jay2tall]

If it's PCI-E 2.0 you are fine. I saw a review somewhere for the 4870 some time ago and at 8x 8x there was virtually no difference in performance compared to a x16 x16.

 

[cjl]

As Jay said, at PCIE 2.0 8x/8x, you won't see any degradation at all.

 

[Dougx1317]

So, I should feel free to overclock as high as I want without that being a concern? Is there anyway for me to test how my single card would do at 8x? Is there a way to turn down a single slot from 16x to 8x?

 

[cjl]

Some motherboards allow you to manually set the number of PCI-E lanes. Honestly though, I wouldn't worry about it at all (including overclocking). You shouldn't notice any performance degradation at all.

 

[Dougx1317]

Thanks for the reassurance.

As a sidenote, should I even be overclocking with two cards in crossfire? Would they have to be overclocked to the same speed?

 

[JofaMang]

yes, as in Cf, both cards automatically clock down to the lowest one. If you OC one and not the other, you may never see it running the OC settings. I mean, there is a chance that in non-CF applications, you will see the OC settings, should you just OC the first card, but I haven't heard anything about that, and most major titles are CF enabled anyhow.

 

[Dougx1317]

If I overclocked both cards the same amount would they stick? And if both cards are clocked at the slowest speed, what happens to a 4870 and 4870x2? Does the 4870x2 go at half speed?

 

[JofaMang]

AFAIK, CF will allow both cards overclocked, as long as they are the same, else defaults both cards to lowest settings. As for 4870x2 + 4870, it acts like trifire 4870s, all GPUs run at same speed as the lowest one.

 

[Dougx1317]

Cool, thanks for all the information.