Nvidia's Response to Early DIY Quad-SLI

"With Open GL Applications, 4-way AFR Works Effectively."

As Chris Daniel pointed out, the back buffer issues seen under Direct 3D applications do not seem to be a factor for OpenGL. Games built on the OpenGL application programming interface (API) should have much better scaling, right? Not entirely, as there are some limitations that are not due to the API.

Having more frames queued does not mean you will get better performance. More frames in the queue means there is more to organize and more tasks to complete. This can create latencies as the first frame in queue has to come out before the second - and so on. When you start shifting control to the CPU, you can end up with an overworked processor. We can already overload a CPU with single and dual graphics configurations; four processors can add even more to manage.

Well, you might say, "What if we have two frames that are split between four processors as in AFR , SFR or AFR of SFR?" Nvidia states that "Quad-SLI will default to "AFR of SFR" for many D3D applications. This mode works well in some D3D games, but may not be as effective with others." Under a two processor setup, the frame is split into two parts and each processor does its part. Each partial frame has to calculate all of the vertices but only has to use part of the data. This adds workload to both cards; workload that is inefficient, but necessary in order to render the frames correctly. To make sure all of the delegation of the workload is done correctly, someone has to order the steps and therefore can become another source of management overhead.

This could be the reason some games do not scale well. AFR of SFR works well with Far Cry and F.E.A.R. but not in others like Black and White 2 or Source Engine based games.

Nvidia commented that it tries AFR mode first, then AFR of SFR, then traditional SLI and if there is not an SLI profile then Quad will revert to single processor operation. Most OpenGL applications run in AFR mode, most Direct 3D applications run at AFR of SFR mode, a handful will run in two processor SLI and rare older games without an SLI profile will run in single mode. While the question of what games are running in which mode is still a bit of a mystery, we are looking to find out which settings are being used. There is an XML file in the driver folder which could hold the keys to the mystery.

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