What Is Micro Stuttering?

Occurrence

• Micro stuttering happens when the job of producing video frames is split between one or more Graphics Processing Units (GPUs), using a scheme called Alternate Frame Rendering, or AFR. In theory, two or more GPUs should be faster than one when it comes to cranking out frames. The problem comes in coordinating the output between the GPUs: each GPU has to finish the frame it's working on just after the previous frame has been completed by the other GPU. If it doesn't, the time between the frames you see on screen can slow down, even though you've got plenty of frame processing power at work. In fact, video benchmarking software may show you're getting, say, 60 frames a second, while the game looks like it's running at 30 frames a second.

Appearance

• Micro stuttering will cause a game to run slowly or in a jerky fashion. The motion may appear to freeze for a second. Often, periods of normally smooth motion will alternate with periods of stuttering. Scenes that put a lot of pressure on the GPUs, such as those with foliage or other naturalistic textures, can bring on a fit of micro stuttering.

What Systems are Affected?

• Micro stuttering has been seen in just about all video systems that use more than one GPU to render frames. Two of the more popular systems affected are nVidia SLi and ATI Crossfire. To use such video solutions, your computer needs to have a special motherboard that's configured to allow sharing of data among multiple GPUs.

  Not all games are affected, and not everybody using multiple video GPUs will see stuttering. Single-GPU video systems are not affected.

Possible Workarounds

• A number of workarounds have been developed for micro stuttering. One is to simply configure the system to use only one of the graphics cards--a solution that will definitely eliminate micro stuttering, but will also eliminate the performance benefits you were looking for when you bought the system. Another is to configure your game to run under Split Frame Rendering rather than AFR. This fix eliminates the problem of micro stuttering, but may create visual splits in the middle of the frame.

  A third approach is to configure the game to run a Frames-Per-Second (FPS) rate that is faster than the speed at which your monitor can refresh frames. So if your monitor has a refresh rate of 60Hz, you would set the game to run at 70 or 80 FPS. By over-driving the monitor in this way, you create a situation in which stutters can't be seen by the monitor.

  Often, updating video drivers can also eliminate micro stuttering for some games. Check with the manufacturer of your motherboard or multiple-GPU video card system to make sure you have the latest drivers.

Future Fixes

• Some of the more recent video cards have reduced, although not eliminated, the stuttering problem. Game developers are also more savvy at creating games for multi-GPU environments. As the demand for realistic 3-D games continues to grow, high-speed multi-GPU platforms will doubtless do a better job of making micro stuttering a rare occurrence.