The Rendering Methods: SuperTiling, Scissor, AFR
There has been much speculation regarding the efficacy and compatibility of a type of multi-GPU load balancing called "SuperTiling" that ATi will be adding to the mix with their CrossFire products. Here are details on ATi's implementation of the technology, which shows promise for better efficiency and scalability.
The CrossFire Compositing engine, in combination with ATi software algorithms, allows each GPU to work on rendering loads for alternate tiles in a given frame, in a checkerboard grid style format. Each tile is made up of a 32 pixel square area, which does allow for good granularity of workload processing.
In the scene above, the potential benefits of SuperTiled load balancing are detailed. Here we see how each GPU can share the workload on a given grid area, the CrossFire enabled card taking the red colored tile areas and the standard card the blue tile areas. As you can see with a given scene's varying rendering load in certain areas, the GPUs are able to share processing resources in a more evenly distributed compute fashion. Scissor or split-screen rendering is probably the least efficient method of load balancing. However comparably a case could be made that SuperTiling is even more efficient that AFR (Alternate Frame Rendering), which could have potential for varying workloads from one frame to the next.
ATi also informed us that their new CrossFire technology in fact supports all three methods of load balancing.
CrossFire powered gaming for all game titles - Right out of the box
It was reported to us that ATi's Catalyst Control Center suite will adaptively select the method of render load balancing required for a given game title. If a game profile in the driver database calls for SuperTiling, that will be enabled. If AFR is listed as most efficient then AFR will be turned on... and so on. If the game title has no specification in ATi's driver database, AFR rendering will be enabled as a default mode for OpenGL based game titles and SuperTiling for Direct 3D games. The real kicker is that ALL GAME TITLES will benefit from ATi CrossFire technology, right "out of the box". In cases where a game engine is more CPU bound, as with older game titles for example, end users may simply then have more headroom for addition pixel processing with higher levels of Anti-Aliasing and Anisotropic Filtering.
In any case this, in our opinion, is a huge selling point for CrossFire. While NVIDIA's Driver Team is doing a fairly good job of enabling SLI for many current and new titles, the list of supported games isn't exhuastive, as it is with ATi's new multi-GPU technology. When a new game title hits the market, users supposedly will see performance benefits from CrossFire immediately. We'll have to see how this pans out when we get our hands on the hardware and as the technology matures in the weeks to come.