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PCI Express And SLI Scaling: How Many Lanes Do You Need?

The PCIe Bottleneck?

A mere seven months have passed since our most recent PCI Express scaling article showed modest performance differences between PCIe x8 and PCIe x16 slots. But it has been a very busy seven months!

The first salvo came when Nvidia’s much-delayed GeForce GTX 480 smoked AMD’s Radeon HD 5870 as the fastest single-GPU card on the market, and the mid-priced solution that followed showed the highest multi-GPU performance scaling we’ve ever seen.

Unfortunately, such an elevated degree of technological achievement is difficult to swallow for a motherboard reviewer, as it makes my earlier findings irrelevant to most users.

The focus of today’s question will center on you, the PC owner. Do you actually need an X58 platform to support the latest graphics technologies, or will something with fewer lanes suffice? MSI helped us to facilitate the answer with a single product, by producing an X58 motherboard that also has the x8 and x4 modes found on some P55 solutions.

We’ve already seen how X58 and P55 motherboards offer similar gaming performance when using a single x16 slot. And limiting ourselves to a single board allows us to focus exclusively on PCI Express lane width by eliminating every other variable. The name of that product is, of course, the Big Bang-XPower.

While it certainly doesn’t represent the P55 market’s moderate pricing, the XPower’s biggest liability becomes an asset for the purpose of today’s test. Its two PCIe 2.0 x16 slots are divided between up to three x16-length slots each, changing to x8-x0-x8-x8-x8-x0 modes when slots three and five are filled, and then to x8-x4-x4-x8-x4-x4 mode when slots two and six are filled. Thanks to MSI, we can now check x16, x8, and x4 transfer modes on a single motherboard, without using little fingers of tape to reduce the number of connections on the card itself.

Thomas Soderstrom
Thomas Soderstrom is a Senior Staff Editor at Tom's Hardware US. He tests and reviews cases, cooling, memory and motherboards.