NVIDIA SLI Physics
Currently, the new Havok FX engine will support SLI physics even for a single NVIDIA GPU. But NVIDIA states that an SLI configuration is still the preferred mode since it allows the second GPU to be dedicated for effects physics processing.
Although NVIDIA listed the minimum supported graphics card as the soon-to-be-released GeForce 7600 GS, they assured us that even older high-end cards like the GeForce 6800 Ultra and GeForec 6800 GT should be able to support SLI physics.
Now, coming to the actual mechanics itself, NVIDIA starts off by explaining traditional physics processing, which is done entirely by the CPU. This includes both gameplay objects (namely, you, the character) and effects objects like grenades, buildings, smoke, fog, etc.
But in games and graphics cards that support SLI physics, the GPU takes over the processing of effects physics. The CPU is left with only the job of processing the gameplay object physics.
Both game and graphics driver must support SLI physics. In Havok FX, the engine sends the effects physics data to the graphics driver through DirectX. If either game or graphics driver do not support SLI physics, then it won't be possible to offload effects physics to the GPU.
If you have an SLI configuration, the second GPU can be used for effects physics processing in games that support it. Currently, dynamic load balancing is not yet possible. So, the second GPU is either used entirely to process effects physics, or graphics. But NVIDIA is working on a load balancing model that will allow maximum use of both GPUs.
Now, just how much performance improvement can we see from using SLI physics? Let's find out!