SLI Question

[DarkRanger]

So I've been looking for a good P67 (Socket 1155) board, and most of them say the following under specs:

2 x PCIe 2.0 x16 (x16 or dual x8)

Now, as far as I understand, dual x8 is as good as x16. Why does it go slower when you SLI? Am I missing something?

I feel so confused...

[RuadRauFlessa]

It is because there is only one PCIe buss on the motherboard. Putting two nozzles on the same pipe effectively halves the flow of water that will come out each of them.. Same thing.

[DarkRanger]

Ok but then whats the use of SLI?

[RuadRauFlessa]

The use of SLI is to have multiple GPU's processing different scenes at the same time. The idea is so that the scenes can be rendered in higher detail with more accuracy without suffering in performance. The data passed to the cards are still the same amount as that is controlled by the application. The application sending the rendering requests won't all of a sudden want to send twice the amount of data.

[SykomantiS]

Also, to add what RRF said, we still can't saturate the pci express bus at this point. Or at least, we couldn't last time I checked. So while single gfx cards run in x16 mode, they still only really need 8x mode for adequate operation.

Someone correct me if I'm wrong?

[StarBound]

Let's get the basics away first. If you want to use SLI your board MUST SUPPORT SLI. Even with 2 16x pci-e or 2 8x pci-e slots it doesn't mean your system supports nvidia multi gpu setups. Same with crossfire. It needs to be supported by the board.

How SLI/xfire works is each card generates a frame. So 1 card would generate even frames and the other would generate the odd frames. Toms hardware mentioned that when looking at an multi card setup always consider cards that can firstly handle the game decently. A pair of low end cards would be a waste due to lack of processing power to start with.

On SLI and 8x pci-e its just the rate that data is sent to your cards. 8x is about 10% slower than 16x when your doing a single card benchmark but your making that figure up again when your using the sheer brute force of 2 cards.

Also make sure your board comes with a SLI connector. Nvidia doesnt ship SLI connectors with their cards. Only AMD does.

[DarkRanger]

The board I'm Interested in is the ASUS P8P67M-PRO, which, for the price (R1300 odd incl vat at Suppliers) is a decent board which supports SLI @ 8x.

Then, I'll be using 2 x 550Ti's in SLI (I have 1 and it's already an AMAZING card, very happy with the way it performs). So it should be decent. I was just wondering, because 2x PCI-E 16x slots seem to be a rare thing.

[RuadRauFlessa]

You'd be more than fine with that.

[Slimshaedy]

You find 2x PCIE x16 on top of the range x58 mobos, like the ROG extreme boards. Sandybridge doesn't have enough PCI lanes to have 2x16 gpus running together...

[RuadRauFlessa]

And sandy does not need it in any case