Question about buying new mobo.

[supmahnugga]

Ok, so I plan on buying a new mobo that will allow me to SLI in the future. If it helps, here it is:

ASRock > Z77 Extreme4

It has 2 x PCI Express 3.0 x16 slots, but it also says

'(PCIE2/PCIE3: single at x16 (PCIE2) / x8 (PCIE3) or dual at x8/x8 mode)'
I'm not entirely sure what ^this means and I was hoping you guys could help me out. Thanks!

 

[PP Mguire]

It just means that when in SLI or Crossfire both cards will run in 8x mode. That is 100% fine for current cards.
That's also a good board.

 

[supmahnugga]

Is that standard with most boards?

 

[PP Mguire]

Yes, and has been this way on mainstream boards since the first iteration of SLI across all PCI-E gens. Having extra PCI-E lanes to drive full bandwidth on most platforms requires a PCI-E bridge chip which is expensive. So you'll only find dual 16x mode on 3 and 4 way boards or specific boards made for dual 16x.
But like I said, it's perfectly fine for current cards. PCI-E 3 has plenty of bandwidth that a card won't saturate an 8x slot.

 

[supmahnugga]

So just to confirm, SLI 8/8 mode on two pcie 3.0 is better than on two pcie 2.0 right? Because if i'd be getting the same performance, then obviously I'd just downgrade to a cheaper board with two pcie 2.0 slots.

Also what do you mean by 'saturate' the slot?

 

[PP Mguire]

You won't find a cheaper board with 2 PCI-E 2 slots for that platform. Z77/IB supports PCI-E 3 as a standard.

You have to realize how it works. For PCI-E bandwidth, during each generation the theoretical bandwidth is literally doubled each time. So, PCI-E 2.0 16x = PCI-E 3.0 8x, while 3.0 16x is double that. All cards currently run perfectly fine on the slots in question without bandwidth limitations.
Saturating a slot means that the card is pushing enough information both ways to be bottlenecked by the bandwidth of the bus. Much like how SATA3 has a max theoretical bandwidth limit of 600MB/s, and any SSD that could go over that amount would be held back by the bus itself. This is not the case for video cards running on 2.0 16x or 3.0 8x (they are the same like I said before).

And actually, a lot of cards on the market today are even fine with a 2.0 8x slot besides the higher end ones.

 

[supmahnugga]

Ahh it all makes sense now! Your help is much appreciated.