Quad SLI: Nvidia GeForce 7900 GX2 review
Nvidia’s SLI technology has had two graphics cards running in tandem for a while, but Nvidia has now developed a way of getting four GPUs working in harmony. At over £4,500 for the Scan system we tested (the cards are only being sold as part of a system), Quad SLI clearly isn’t for the masses, and it’s only for what Nvidia calls Extra High Definition gaming, at massively high resolutions. Whereas standard dual-GPU rigs can comfortably play games at 1,600 x 1,200, you’ll need resolutions far in excess of this to even tax Quad SLI.
Given that dual-GPU cards have been around for a year, it might be a surprise to see what looks like four cards when you open up this system. There are in fact two twin cards. Although only two PCI Express 16x slots are needed on the motherboard, Nvidia has cut holes in the top of the piggybacked PCB to allow air in between the two 305mm long cards. Still, overheating obviously remains a concern: the 7900 GX2 GPUs and RAM run at 500MHz compared to 450MHz of the 7900 GT and 650MHz of the 7900 GTX. The 512MB of RAM for each 7900 GX2 GPU runs at the same speed as that of the 7900 GT (600MHz) rather than the 800MHz of the 7900 GTX, so each of the four cards therefore runs roughly 25% slower than a single 7900 GTX card.
Four cards should still give vastly superior performance over two 7900 GTXs, as long as the GPU interconnects and driver can move data around efficiently. Each Quad SLI twin-card has a custom 16x PCI Express link hiding between the two PCBs, with cards interlinked via conventional SLI bridge connectors. Logically, the GPUs are laid out at the four points of a square, which introduces a certain amount of latency. With, say, the top-left GPU in the square responsible for the final screen-out, the data from the bottom right GPU must pass through another GPU, or else over the motherboard chipset. The driver has its work cut out, splitting loads four ways and then arbitrating the complex inter-GPU communications.
But our 3D benchmark scores show that it does work, with impressive results across the board. At 1,600 x 1,200, the graphics processing power was such that the Athlon 64 FX-60 of our Scan machine became the limiting factor. Far Cry with HDR ran at a scorching 75fps while Call of Duty 2 raced to 40fps. But it’s at extremely high resolutions that Quad SLI makes sense: we ran a series of tests using a 30in Dell 3007WFP monitor with a 2,560 x 1,600 resolution and applied 4x AA and 8x AF. With scores around 40fps, it’s enough for jitter-free gaming. Slacking off on the AA gave us comfortably smooth gameplay in every game but the extremely punishing Elder Scrolls IV: Oblivion at maximum detail. And Oblivion only stutters momentarily from time to time.
Quad SLI also brings some new image-quality settings. With each GPU capable of 8x Supersampling AA, combining them gives Quad SLI a 32x AA mode. This will seriously affect your frame rate (around 15fps was the norm at 1,600 x 1,200), so it currently seems more about bragging rights over CrossFire’s 14x AA mode. There’s also a new rendering mode to join AFR (alternate frame rendering) and SFR (split frame rendering). AFR of SFR takes each frame and splits it for two GPUs to work on, while splitting the next frame for the other two GPUs to work on and so on. It gives greater flexibility to get the most out of Quad SLI in every game.
But there’s really only a point to doing all this with a 30in+ screen – a standard dual 7900 GTX SLI setup will give around 40fps in the toughest of current games at high settings on a typical 24in TFT running at 1,920 x 1,200. Only F.E.A.R. needed to have AA and AF removed to play smoothly using maximum detail settings. That’s unless you want to indulge in SLI Physics – the engine responsible for creating realistic fog, fluid and cloth effects, for simulating how heavy an object is and what sort of force is required to move it.