Dual 8x bottleneck

The Astroman

The Astroman

Active Member
I can't seem to find a clear answer to this question: will there be a bottleneck if you Crossfire two 4890s in dual8x?
My Asus P7P55D-E Evo has PCIe 2.0 single 16x dual 8x 8x, and I would know if I would gain a lot by adding a second 4890.
I can't believe I just ordered my build today (parts haven't even arrived yet) and I'm already thinking of an upgrade :P
I'm getting a Sapphire HD 4890 Vapor-X 2 GB, and plan to maybe get a second one in a couple of months when the price goes down, but want to know if it's worth it (if there would be a bottleneck).
 
hmm

I thought the first one runs at 16x and the second care is 8x

For my mobo I am told just to buy a powerful single board, Im sure someone here knows, Im just trying to keep your post fresh :)
 
The Astroman said:
I can't seem to find a clear answer to this question: will there be a bottleneck if you Crossfire two 4890s in dual8x?
My Asus P7P55D-E Evo has PCIe 2.0 single 16x dual 8x 8x, and I would know if I would gain a lot by adding a second 4890.
I can't believe I just ordered my build today (parts haven't even arrived yet) and I'm already thinking of an upgrade :P
I'm getting a Sapphire HD 4890 Vapor-X 2 GB, and plan to maybe get a second one in a couple of months when the price goes down, but want to know if it's worth it (if there would be a bottleneck).
Click to expand...


For 4890's, I'd say 1% to less than 1% performance loss over full 16x/16x. I saw benches of 5870's comparing 16x/16x, 8x/8x, 4x/4x, and even 1x/1x.

For 5870's, which are quite a bit more powerful than a 4890, there was 1-2% loss for 8X. There was only a 5-6% loss for 4x! But there was like 30 some percent for 1x. So even with 4x/4x you wouldn't see much loss with 4890's. This is why I laugh at people saying that P55's are not good for gaming because of only 8x/8x Xfire/SLI. Probably only 3-5% of gamers run two cards powerful enough to see any noticeable losses.

Oh and both cards have to run at same speeds. It's always 16x/16x or 8x/8x. If you only have a 4x second PCIe slot, the first slot will get downgraded to 4x as well in Xfire.
 
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87dtna, thank you very much. That is quite reassuring. It's a question of bandwidth, or rather that there is sufficient even in dual 8x for two 4890s, am I correct?
 
The Astroman said:
87dtna, thank you very much. That is quite reassuring. It's a question of bandwidth, or rather that there is sufficient even in dual 8x for two 4890s, am I correct?
Click to expand...

Correct! Cards are just now starting to go beyond 8x/8x capabilities. Won't be for quite awhile though that 8x/8x will be obsolete because two 4890's should play any current game at any resolution/settings. It's actually DX11 thats going to be the factor, but two 5850's would solve that and should lose less than .5% performance in 8x.
 
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