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Crossfiring on PCI-E (x16, x4)

post #1 of 16
Thread Starter 
Basically what I need to know is how much I would be limited if I were to sell my current 6950 (to a friend) and pick up two 6870s to crossfire on my current motherboard (which has one x16 slot and one x4 slot (GA-P67A-D3-B3)).

I had always been under the impression that crossfiring on a x4 slot would be a major performance barrier, however I saw this link just recently that made me wonder: http://www.hardocp.com/article/2010/08/25/gtx_480_sli_pcie_bandwidth_perf_x16x16_vs_x4x4

As I only play at 1920x1080, I would assume if these results are accurate I will be met with very little performance loss (I would consider just a couple frames lower on average only a minor loss) from crossfiring in x16/x4, but as I've only seen this result in one place (the above link) I wanted to ask some more experienced people to verify this information. Actually, if I wouldn't be limited at this resolution even if I crossfired 6950s, I would prefer to do that instead of changing to 6870s, so any feedback on whether or not I would be limited for dual 6950s would also be appreciated.


On a related note, as I would be buying 1GB versions of the 6870s to crossfire, keeping in mind that I only play at 1920x1080, will I ever notice any real issues with VRAM (compared to if I say, bought another 2GB 6950 to crossfire)? If I will, will it just be that on some games I need to keep my anti-aliasing down to x4 instead of x8? Maybe I'd need to forgo anti-aliasing altogether on some games? Or worse, though I doubt it, would I actually have issues in some games in the near future even without anti-aliasing?


tl;dr: Would running crossfire at x16/x4 limit 6870s in crossfire?; 6950s? Would I have any issues with only 1GB vram at 1920x1080? Thanks much for the help.
    
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post #2 of 16
Is that one of them boards that has the x4 running off the southbridge?

FYI bandwidth tests like that are using cards running equal/similar lane widths, not mismatched like you are wanting to do. Bad mojo when the mismatch is this large.
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post #3 of 16
You will get no real performance problems with that setup. X16/x4 is enough to run those cards. You may not quite hit the full potential of the Xfire, but you'll get 90% of it. 1GB VRAM is enough for ultra settings with a little AA turned on at that resolution on most games. It'll look great.
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post #4 of 16
Thread Starter 
Ahh, so the main problem with crossfiring on x16/x4 wouldn't be just that the bandwidth on the x4 is low, but that it's a big mismatch between x16 and x4?

Is that 90% number accurate for the 6870s only? Or also for the 6950s? I would probably be willing to go for it if I could get another 6950 right now and crossfire for 90% of the full crossfire performance and maybe upgrade my motherboard eventually so I can squeeze out the full performance, but I don't think I'd want to go to 90% performance on 6870s and drop to 1GB vram. In that case I'd probably rather wait a while and upgrade my motherboard.
    
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post #5 of 16
Quote:
Originally Posted by Aros View Post

Ahh, so the main problem with crossfiring on x16/x4 wouldn't be just that the bandwidth on the x4 is low, but that it's a big mismatch between x16 and x4?
Is that 90% number accurate for the 6870s only? Or also for the 6950s? I would probably be willing to go for it if I could get another 6950 right now and crossfire for 90% of the full crossfire performance and maybe upgrade my motherboard eventually so I can squeeze out the full performance, but I don't think I'd want to go to 90% performance on 6870s and drop to 1GB vram. In that case I'd probably rather wait a while and upgrade my motherboard.

Don't worry about the numbers, big issue for you is where that x4 is coming from? 1155 only has x16 and anymore than that is not coming from the cpu and is added to the southbridge. The bandwidth of the gpus is not shared with the accessories etc. Typically that x4 is tied to the accessories so you will have to have a plx pcie switch.
Edited by tsm106 - 2/5/12 at 3:05pm
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post #6 of 16
The exact number depends on numerous factors, such as which GPU, what GPU freq, your GPUs clock as well and etc. But I'm sure that at least on the test's I've seen you will experience at least 75% of crossfire performance.

However, I would make sure, before, that that x4 slot is really usable for crossfire. My mobo for example has a x16 slot, another x8 and a 3rd x4. I can crossfire at x8(x16)+x8 but the x4 slot is only usable if I put a physX card, a sound card or a video card for folding or extra monitors, but not for crossfire.

As for the 1gb ram, it really depends on what games are you running. The way I think with 5870s is that if a game needs more than 1gb to max it out then I probably also lack the GPU power to max it. Your case is different, since you have more powerful GPUs.



edit: looks like you card can run crossfire:
Quote:
GA-P67A-D3-B3 (rev. 1.0)
Multi-display support with 2 way CrossFireX
Flexible graphics capabilities - Up to 2 VGA cards are supported for 2 way CrossFireX™, delivering the ultimate in graphics performance for gaming enthusiasts who demand the highest frame rates without compromising on resolution.
source
Edited by EduFurtado - 2/5/12 at 3:02pm
post #7 of 16
Thread Starter 
OK, thank you all for the help. I will probably end up waiting a while and then selling my current motherboard to my friend instead of my 6950, then upgrading to a motherboard capable of crossfiring 6950s in 2 x16 slots, but the info on the bandwidth was still very useful because my friend will probably get a 6870 for now and crossfire it eventually and he's fine with that amount of performance loss due to bandwidth limitations.

+rep to you all

P.S. Actually one more question now that I think about it. When looking for a new motherboard should I make sure it has two x16 slots for crossfiring the 6950, or would one x16 and one x8 slot be fine bandwidth-wise for crossfiring 6950s? I really wouldn't want any performance drop at all in the cards if I'm getting a brand new motherboard though, so if there's any loss from bandwidth I'd just rather pay a little more for 2 x16 slots.
    
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post #8 of 16
Quote:
Originally Posted by Aros View Post

OK, thank you all for the help. I will probably end up waiting a while and then selling my current motherboard to my friend instead of my 6950, then upgrading to a motherboard capable of crossfiring 6950s in 2 x16 slots, but the info on the bandwidth was still very useful because my friend will probably get a 6870 for now and crossfire it eventually and he's fine with that amount of performance loss due to bandwidth limitations.
+rep to you all
P.S. Actually one more question now that I think about it. When looking for a new motherboard should I make sure it has two x16 slots for crossfiring the 6950, or would one x16 and one x8 slot be fine bandwidth-wise for crossfiring 6950s? I really wouldn't want any performance drop at all in the cards if I'm getting a brand new motherboard though, so if there's any loss from bandwidth I'd just rather pay a little more for 2 x16 slots.

Only the most expensive platforms, like x58 with tripple channel and the i7s can do x16 and x16. They can also do x16+x16+x8.
People like me with the p55 can only do x8+x8. And the answer is, there is absolutely no difference.

Of course this is for my 5870s, but I'm positive that it shouldn't hurt your 6950s as well, because there isn't a big difference between them.

Only when you go down to x4 you can find some noticeable difference.
post #9 of 16
Quote:
Originally Posted by Aros View Post

P.S. Actually one more question now that I think about it. When looking for a new motherboard should I make sure it has two x16 slots for crossfiring the 6950, or would one x16 and one x8 slot be fine bandwidth-wise for crossfiring 6950s? I really wouldn't want any performance drop at all in the cards if I'm getting a brand new motherboard though, so if there's any loss from bandwidth I'd just rather pay a little more for 2 x16 slots.

x8/x8 will not bottleneck any current video card. There may be a slight score difference in 3DMark but that's about it.

Plus, the boards that can do x16/x16 have a NF200 chip which adds latency and heat. Not to mention those boards are more expensive and not worth the extra cost unless you were wanting to go Tri-SLI (whole other story).

BTW, I'm referring to LGA 1155 boards.
post #10 of 16
Quote:
Originally Posted by Aros View Post

OK, thank you all for the help. I will probably end up waiting a while and then selling my current motherboard to my friend instead of my 6950, then upgrading to a motherboard capable of crossfiring 6950s in 2 x16 slots, but the info on the bandwidth was still very useful because my friend will probably get a 6870 for now and crossfire it eventually and he's fine with that amount of performance loss due to bandwidth limitations.
+rep to you all
P.S. Actually one more question now that I think about it. When looking for a new motherboard should I make sure it has two x16 slots for crossfiring the 6950, or would one x16 and one x8 slot be fine bandwidth-wise for crossfiring 6950s? I really wouldn't want any performance drop at all in the cards if I'm getting a brand new motherboard though, so if there's any loss from bandwidth I'd just rather pay a little more for 2 x16 slots.

If yer thinking about a new board, look at the ECS x79 Black Edition. It pwns for 1/2 the cost of the Rampage. But if you plan to stay on 1155, get a ws revo, it's only 225 and will rock your socks off.
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