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Newbie SLI question - PCIe x16 + PCIe x4

jmp_78

jmp_78

jmp_78

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Hi guys, I have an Asus P7P55D motherboard which...

Supports ATI® Quad-GPU CrossFireX™ Technology
1 x PCIe 2.0 x16 (single at x16)
1 x PCIe 2.0 x16 slots (@x4 mode, 2.5GT/s)

If I were to get myself 2 x ATI video cards, would I be wasting valuable performance by not having 2 x full PCIe x16 slots?

As in... would it just be better to get 1 x more powerful card in the first place? I am only considering this as it's a cheaper option for me personally :)

Realised I should have entitled this post CrossFireX, not SLI - ooops!
 
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To awsner your question, if P7P55D is your mb then you can only support one card. Crossfire needs dual x8 lanes to work effectively.
 
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I'm pretty sure the 4x speed slot has been shown to not have that much of an impact on performance.
 
Nico_Joey said:
To awsner your question, if P7P55D is your mb then you can only support one card. Crossfire needs dual x8 lanes to work effectively.
Click to expand...

Hmmm, am confused then :confused:

The Asus website product page for the P7P55D says...

"Quad-GPU CrossFireX Support!"

What is this then?
 
image013.png


Thats basicly why. Your board could hold four cards and use them ( it has four x4 lanes, effective), but at 70% or less. And in dual crossfire it would still be 70% as the faster card would slow for the slower card. Which would result in less performance with two cards than one.
 
Nico_Joey said:
image013.png


Thats basicly why. Your board could hold four cards and use them ( it has four x4 lanes, effective), but at 70% or less. And in dual crossfire it would still be 70% as the faster card would slow for the slower card. Which would result in less performance with two cards than one.
Click to expand...

Wouldn't that be 70% of the 2 cards speed, so actually 140%?
Still less than you'd get with full speed sockets, but maybe a noticeable boost?

Also, you need to consider the graphic cards used. I think the fastest cards are only just maxing out the x8 speed slots. So, 2 480's would be heavily hit by using a x4 slot, but 2 460s or 5830s might not be as hard hit.
 
Doesn't matter, it would slow down by 30%, not more than that. It's still a pretty huge difference and I'd rather consider swapping a card over than buying another one for 4x crossfire. 8x/8x on the other hand losses only about 3% performance compared to X58 16x/16x Crossfire, which is by no means a biggie.
 
The specs are wrong if it's this board. As far as i know all P55 boards that have a pair of pci-e slots run the top slot at 16x with one card but if two cards are used then both slots run at 8x. The review i linked to confirms this. In which case using a pair of cards in crossfire will be fine.
 
pastymuncher said:
The specs are wrong if it's this board. As far as i know all P55 boards that have a pair of pci-e slots run the top slot at 16x with one card but if two cards are used then both slots run at 8x. The review i linked to confirms this. In which case using a pair of cards in crossfire will be fine.
Click to expand...

You're wrong, only higher-end P55 boards do symmetrical 8x/8x Crossfire/SLI. Asus P7P55D doesn't.

Expansion Slots


1 x PCIe 2.0 x16 (single at x16)
1 x PCI Express 2.0 x16 slots (@x4 mode, 2.5GT/s)
2 x PCI Express 2.0 x1 (2.5GT/s)
3 x PCI
Click to expand...
 
I most certainly did'nt. Did you even read it? Look here, 5th picture down and the paragraph under it. It states the following:-

"There are plenty of expansion slots available here with the P7P55D - The CPU itself provides the support for those two PCI Express 2.0 16x slots (which run in a dual 8x configuration in a multi-GPU configuration), while the P55 chipset itself gives us another two PCI Express 2.0 1x slots and three old-school PCI slots."

How can i misinterpret that?
 
Yeah second slot is definatly on 4x. I made this mistake when buying my new board as the description on the OC site wasnt clear. If I'd bothered to check the Asus site i would have realised. Needless to say I returned it...
 
It's quite a big performance hit, considering crossfire scaling is not always great anyway a further decrease would just make it a waste of time.

I remember trying 4850 CF on a 16x + 4x board and the performance was poor.
 
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