Which motherboard: For 2-Way SLI/CF: x16/x4 vs x8/x8? For 3-Way SLI/CF: x16/x8/x4 vs x8/x8/x8?

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raisonjohn

raisonjohn

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Which motherboard would you choose (better) if its PCIe 3.0 x16 slots have the following modes:

For a 2-Way SLI/CF support: Mobo A: x16/x4 -OR- Mobo B: x8/x8
For a 3-Way SLI/CF support: Mobo C: x16/x8/x4 -OR- Mobo D: x8/x8/x8

Or does it even matter?
 
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basroil
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basroil

basroil

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Oct 19, 2013
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1) It doesn't matter
2) Never use 3+ way SLI/CF, it never works at anywhere near as well as the increased price
3) Avoid 2 way SLI/CF unless you already have the fastest card available (in the case of Nvidia, ignore the Titan line between the first model and maxwell version)
 
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Gam3r01

Gam3r01

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Well, Motherboard A would not support SLI at all, as you need an x8 lane at a minimum to enable SLI. In an Xfire setup it would be bottlenecked.
So in the first two, Motherboard B would be the better (and only) choice.

Same goes for Motherboard C. The x4 lane would not enable SLI, so it would only be 2way SLI.
In this case, Motherboard D is the only option.
 
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raisonjohn

raisonjohn

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Thanks guys for all your inputs. I think I got the basics. Now let's see...

From the data I have in the motherboard specs it says:

"2-Way SLI support; 2-Way CrossFireX support; and 3-Way CrossFireX support"

And from it's PCIex16 mode, the specs says:

"x16/x0/x4, x8/x8/x4"

This means (let me see if I got it right),

For any single GPU = runs at x16
For 2 Nvidia SLI GPU's = 1 runs at x8 and 1 runs at x8
For 2 AMD CF GPU's = 1 runs at x16 and 1 runs at x4 -- OR -- can it also be run in x8/x8 ????
For 3 Nvidia SLI GPU's = It's not supported, since only 2 PCIe slots can run at x8's - and SLI will not run in x16/x4.
For 3 AMD CF GPU's = Logically, 1 runs at x8, another at x8, and the third in x4.

Now, my question pertains to the 2 AMD CrossFire GPU setup... after plugging the 1st card at the 1st PCIe slot, where would you plug the other GPU? At the 3rd slot for a x16/x0/x4 mode -- OR -- at the next adjacent slot for a x8/x8/x_ mode?
 
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basroil

basroil

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First thing is first, pick a pair of GPUs you'll be using! If they are not 2x 1080 or 2x Fury X, you shouldn't be using multiple cards. If they are, the answer will be very easy simply because of mounting considerations.
 
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raisonjohn

raisonjohn

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I am already set to purchase 2 MSI GTX 1080 Gaming X GPU's for my 3x 1440p/60Hz (surround). I'll be getting the either the MSI X99A Godlike Gaming or the Asus Rampage V Edition 10 - with either a Haswell-E or Broadwell-E (most likely a 40-lane Haswell-E). So, no problems with mounting considerations and compatibility - unless such 2-way setup can't perform 50/60-ish fps... Aside from gaming, this rig is also to be used for 3d modeling/rendering.

The reason I am asking is I am very curious in the spec-for-spec differences of the other mobo's I was initially considering before (esp. the Z170's - when I was still considering a Skylake build). I was initially confused with all the x's and the /'s but was able to figure out how the spec notation works.

It'll be my 1st time to build a multi-GPU system.
 
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basroil

basroil

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X99 with a second grade chip should support 2 way in 16/16 and 3 way in 16x8x8 (and most will do 8x8x8x8 four way) unless they have PLX in which case you might see x16 all the way across the board (but effectively the same as the above), but not the 6800K and other bottom tier ones, they only have 28 lanes and therefore limited to 16/8 or 8x8x8.

In the case of standard Z level boards, the chip will be limited to 20 lanes, so you have either 8x8 or 16x4 (crossfire only) and no higher options.... UNLESS of course you have one of the PLX based boards in which case you could see 16x16 and 8x8x8 or higher (though most boards don't use it any more, too expensive to develop/build and far too few people buying them)
 
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raisonjohn

raisonjohn

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Ah, I missed this post of yours after I asked my follow question on 2-Way CF: x16/x0/x4 vs x8/x8/x_....

You already answered the question in your earlier post. Seems like the way to go is the x8/x8 (for either SLI or CF) as you said, bottlenecking will occur on a x16/x4.

Final question:
This time in 3-Way CF. Theoretically, suppose your 2 choices of mobo's are ONLY these - Mobo E: x8/x8/x4 - OR - Mobo F: x16/x8/x4 - which is slightly better/worse for 3-Way CF?
 
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basroil

basroil

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Impossible to answer, since it will depend on pretty much everything except the lane count. There are no AMD cards that come even close to maxing out x8 PCIe3, let alone do it in CF
 
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raisonjohn

raisonjohn

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Yup! I'm most likely getting either the 5930K or the 5960X (if budget permits) - both of which are 40-lane CPUs.

For the mobo's I'm considering, the 1) MSI X99A Godlike Gaming (which has 5 PCIex16 slots) can do: x16/x0/x0/x0/x0 (Single); x16/x0/x0/x16/x0 (2-Way); x16/x0/x0/x16/x8 (3-Way), and x8/x8/x0/x16/x8 (4-Way). While the 2) Asus Rampage V Edition 10 (which has 4 PCIex16 slots) can do: x16 (Single); x16/x16 (2-Way); x16/x8/x8 (3-Way), and either x16/x8/x8/x8 or x8/x8/x8/x8 (4-Way).
 
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