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SLI Scaling Summary

PCI Express And SLI Scaling: How Many Lanes Do You Need?
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Our list of SLI-based benchmarks showed excellent scaling, but only at the highest-tested 2560x1600 resolution. A serious CPU “bottleneck” is the most likely cause for decreased SLI scaling at lower resolutions. For most games, it doesn’t even make sense to test a pair of GeForce GTX 480 graphics cards at anything less than 2560x1600, and one benchmark was completely crippled by the performance of our 4.00 GHz CPU, even at 1920x1200. Let’s see what effect this CPU limit had on our overall scaling performance:

While two cards outperform a single card by up to 90% in most games, that only happened at our highest test resolution. Poor scaling at lower resolutions dropped our average gain to only 63%. Moreover, the one game that was most bandwidth-dependent in our single-card tests was the same game that became almost completely CPU-bound in SLI, obliterating the 8% performance difference previously noted in our single-card PCIe evaluation.

While the performance gain of SLI exceeded the increased power consumption of today’s system, we again note that it happened only at high resolutions. A net loss in SLI power efficiency can be attributed exclusively to the inclusion of 1680x1050 in today’s tests.

One other peculiarity of today’s test was that our x8/x8 SLI configuration required the card coolers to be adjacent to each other, while the x16/x16 configuration had one empty space between cards. Yet, we never saw a card overheat. How much of a problem did shoving the cards together create?

Nvidia puts a hole in the back of its GTX 480 graphics card, behind the fan, so that the fan can take air in from both sides. The result is that we didn’t see a big difference in temperature between cards that were placed closer together. We expect this design to be less effective for the center card in three-way configurations, and we plan to scale our tests to even greater heights in future articles.

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