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Support, Issues, And Conclusion

Multi-GPU Setups: The Basics Of CrossFire And SLI
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AMD and Nvidia are both missing a reliable driver-based test to see whether the CrossFire or SLI modes are actually enabled and working. The check box in the drivers could be checked, even though the graphics cards are no longer communicating. Such situations are not uncommon, but they can easily be fixed by unchecking and re-checking the box and then rebooting. You might do this if you notice that the performance level of SLI or CrossFire has changed when playing games. A quick performance test would make sense for indicating whether full graphics power is available or not. Even as an experienced computer user, you have to start a benchmark and study the results to be sure that SLI or CrossFire is still working as expected. For a gamer with little interest in technology, but a need for the extra performance, this may be way too complicated and annoying.

The screenshot shows a scene from Battlefield: Bad Company 2. In some CrossFire configurations, grass is drawn where there shouldn’t be any. We're always surprised when bug fixes listed in new driver releases solve problems in games that we've been benchmarking for weeks without any issues. The PC market, with all its different hardware and software, is so complex these days that you're better off not drawing any conclusions regarding driver quality based on a single test system. The effects of using different platforms and OSes have too much of an impact.

We didn't evaluate support for different games, as there are simply too many titles and different opinions on which games are important. Basically, support is usually very good for the most popular games, while niche products can suffer from driver issues and weak SLI or CrossFire support for long periods of time. In addition to monthly driver updates, AMD offers separate profiles for different games in CrossFire. Unfortunately, the number of games is rather limited, so you have to regularly keep an eye at the README files to find out if better support for your favorite game has been added to the drivers. If no suitable profile exists, you can always rename the game's executable to 3dmark.exe, forcing the profile of this benchmark to be used for the game. This works really well, although using the wrong profile can cause problems with anti-aliasing.

Some CrossFire and SLI caveats should be mentioned. As soon as two or three cards are combined, the temperatures of the GPUs almost always increase, making fan speeds go up and the PC louder. The compact design looks great on a single card, but when putting several cards together as a CrossFire or SLI configuration, the closely spaced PCIe slots barely give the cards any room for sucking in cool air. With three cards, the middle card is heated from both sides. For stable operation, you should add an extra fan next to the cards.

Conclusion

Installing the drivers for Nvidia graphics cards usually goes flawlessly, and the cards are immediately recognized as an SLI configuration. Things can be a bit more problematic with AMD, especially if you try to do too many installation steps at once. Ideally, the installation works instantly, but in case of problems you have to install the cards one by one, and then enable CrossFire.

Our tests consistently show performance increases of 65% to 82% when using multiple graphics cards (we've shown even more impressive scaling using the latest GeForce GTX 400-series cards). The performance scaling is contingent on game support, resolution, and the GPUs used. AMD Radeon HD 5750 cards perform well in CrossFire, and two of these budget cards can compete with a Radeon HD 5850. If you want to take a closer look at the numbers we've generated for the GeForce GTX 480 in one-, two-, and three-card arrangements, take a look at The GeForce GTX 480 Update.

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