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SLI Explained
Although adopting the similar name 'SLI' as used in the 3dfx era, the two really can not be compared. We are not going in-depth as you all understand the principles and ideas of utilizing two graphics cores. The technology behind that is fascinating yet not something most of you really care about, we'll handle a short explanation though. SLI is short for Scalable Link Interface.
Most GeForce 6600 and 6800 models have an internal architecture that allows the graphics core's to work in pairs. When the two GPU's work in pairs (parallel) they'll start load-balancing the workload. To explain it very simply I'll throw in a small explanation of one of the rendering methods. Each graphics core can render either the upper or lower half of the screen. The frame (single image) is divided in two, calculated and then merged back together.
You can understand that this requires some hefty CPU power so you better be sure to have a high-end processor in your PC. And with High-end I really mean a Pentium 4 3.4 GHz, Athlon 64 3200+ or equivalent to that which I consider to be the bare minimum.
Once you installed your drivers new SLI options become available.
When you have a SLI capable solution active (see above picture) rendering can take place in three modes. If your game is not supported automatically Single GPU rendering will be enabled. When the game does support SLI then the driver will select the best SLI mode, which is either Alternate (AFR) or Split Frame (SFR) rendering. Let's have a quick explanation on them.
Among the new options SLI rendering modes
SFR - Split frame rendering. Each graphics core is going to render a part of the screen, split horizontally. One card is responsible for the top section of the screen, the secondary card is responsible for the bottom. That situation is then load balanced as described above.
AFR - Alternate frame rendering. Each graphics core is rendering a frame at a time, odd and even. The full frame is prepared by the CPU and forwarded to the driver, which makes it render on each device to render a frame in turn. This is interleaving. First card A does a frame then card B the other.
As it seems AFR is NVIDIA's preferred method of rendering while using SLI. However SFR, I think, would be the way to go performance wise. But it boils all down to whether the game can handle it or not.
Something I dislike a bit is NVIDIA's approach to support for SLI. SLI is not something you get out of the box. The game must support it and if it does it'll be supported through NVIDIA's drivers. Now don't get me wrong, the majority of games are supported, especially the more popular and bigger titles, our tests will show you that. Next to that you'll need to consider this, people assume that SLI immediately doubles the speed of all games and that's just not the case. Several games do not support SLI. Looking from another point of view you can't expect a new technology to be perfect and fully supported by all games and drivers. NVIDIA has a big task ahead of them, SLI support standard out of the box.
One small thing, if a game is not tested and included in the NVIDIA drivers you can still force and enable SLI on it. If it'll work properly, that question remains to be seen. Suffice to say that all upcoming games will not have a problem with SLI.
I Demand Powerrrr
What you also need to realize is you will likely require a new higher-powered power supply, as twice the fun is twice your power bill.
The power draw was relatively low though. With an Athlon 64 4000+ based system with two HD's, a DVD-Rom, 512 MB memory in-game the PC pulled a maximum of 250-260 Watt. You need some reserve though. So don't go thinking that a 300 Watt PSU is sufficient. Btw and before you go thinking, yeah right how on earth does he know that. Simple we have a Wattage meter between the PC and the power outlet.
To your left the PC in idle, to your right 3DMark05 is running and power usage is maxing out.
