Which games/engines do you notice micro stuttering in? I added a second GTX 480 to my system a few months ago and didn’t notice anything like this.

The good news on the hardware side is not just that PCIe 3.0 is coming, but also that Sandy Bridge E will bring 40 lanes of PCIe bus with it – so 16x/16x will eventually be mainstream and not exclusive to X58/LGA1366.

All in all, these are exciting days – it’s really great to know that we are starting to get the most of the cards that we have in our machines. Thanks Ben and LR for the amazing progress!

My view is: if we have to run scissors to run SLI, there’s no point in doing it at all – X-Plane needs bus bandwidth, and asking a user to get an expensive 16×16 mobo to have the honor of making two insanely expensive GPUs both work is diminishing returns. At some point you’re better off with two cheaper computers.

This is why scissor tends to be the rendering mode of choice for apps not optimized for this type of technology – it means that each card processes every frame (to avoid some of the problems mentioned by Ben), but in the end will render only half of each. It is not efficient since all cards have to process essentially everything including geometry, and the bandwidth of the PCIe bus can be easily saturated because of all the data traffic. Because of that, the best results will be achieved by cards running on motherboards that support a 16x/16x configuration (instead of 16x/8x).

This is more of an issue with AFR and happens when you have a 2-card setup. Tests have shown that a 3-card setup eliminates the issue – at the cost of making the solution prohibitive 😉

In order to cleanly split the rendering between two cards, the app has to avoid operations that are legal for OpenGL but very inefficient for two GPUs in a Crossfire/SLI setup. The ones I know about include:
* Any type of inter-frame render-to-texture dependency.
* Any read-backs from card (or any other potential time-stamp stalling operation) that are delayed only one frame and not two frames.
These operations have basically no penalty on a single GPU but chew bandwidth or limit what the second card can do on a dual-GPU system. So we have to make sure we have none of these cases in X-Plane. I know off-hand that we have at least two of them.