Today’s analysis began by questioning whether Intel's Lynnfield-based Core i5 and Core i7 processors, along with the P55 platform that complements them, is good enough for CrossFire, and the answer is probably “sometimes.”
Test results showed that a PCIe x8 slot provides 4% less performance when using the fastest possible single GPU, and those results did translate into a 4% performance deficit in CrossFire. That 4% loss isn’t horrific, and less powerful cards would likely show less performance difference. However, these results shouldn’t dissuade anyone from using an LGA 1156 processor to host only one dual-GPU card, since the card’s x16 interface limitations perfectly match those of LGA 1366-based processors.
Less surprising was the horrific performance of the x4 slot available on many P55 motherboards through the chipset’s PCIe hub. These lanes offer only PCIe 1.1-class bandwidth regardless of their PCIe 2.0 labeling, dropping 26% behind even the x4 slot available on similarly-priced X58 motherboards. A slot that slow might be useful for adding a graphics card, but only if that card is used for lower-performance applications, such as a dedicated PhysX processor or 2D desktop expansion.
Perhaps the biggest question answered here is whether we should have forgone the P55 solution in our most recent $2,500 PC build and the answer, of course, is yes. A 4% performance difference might not make or break a real-life build, but it’s a huge obstacle in any competition such as our SBM.
So what’s left? We still haven’t seen how an X58 motherboard with x16/x16/x4 slots compares to today’s x16/x8/x8 sample, whether or not a PCIe hub such as the NF200 can help the P55 to cope with additional cards, or how badly a P55 motherboard in x8/x8 mode can hurt a pair of dual-GPU cards, such as the Radeon HD 5970. We don’t have the parts in place to test all those variables, but will continue to consider your suggestions while we assemble components to put at least some of these questions to rest.