Four Architectures, Four Chipsets, Tons Of Variables
Let’s break down our four hardware platforms for a better idea of what we’re about to see.
First up is our Core i5 setup—arguably the most controversial configuration because it isn’t representative of any actual CPU model Intel is launching. The engineering sample we're using runs at 2.66 GHz by default and supports a single bin of Turbo Boost acceleration, locking it in at 2.8 GHz. Hyper-Threading is not supported, making this a quad-core processor able to execute four threads concurrently. In essence, it's a Core i5-750 without the ability to hit 3.2 GHz in single-threaded situations.
|Intel Core i5 @ 2.8 GHz|
|Socket Interface||LGA 1156|
|PCI Express Configuration||1 x 16-lane, 2 x 8-lane|
|Core Configuration||Four physical cores, four threads (no HT)|
We ran our tests on an early version of Asus’ P7P55D Deluxe motherboard, which sports three PCI Express x16 slots, two of which are tied in to the Core i5’s on-die PCIe connectivity. With one graphics card installed, you get 16 lanes of second-gen PCI Express. With two, those lanes are divided into a pair of x8 links, thereby serving up to 8 GB/s per slot rather than 16.
Now, there should be some inherent performance advantages to putting connectivity on the processor die itself, rather than having graphics cards communicate through a northbridge and its interface to the CPU. Although GPU command streams are really the only data traveling over PCIe from GPU to CPU (and bandwidth consumption is purportedly negligible), texture data, video streams, and vertex data travel between system memory and the GPU. If i5 can do this faster than X58, P45, or 790GX, we might see a performance speed-up.
It'd be an easier comparison if we weren't swapping processor architectures every step of the way. We also have to consider that Core i5 gives up a single 64-bit DDR3 memory channel and Hyper-Threading, so LGA 1366 Core i7s could still technically be faster. The real question, though, is what happens when you adopt CrossFire or SLI, splitting PCIe between two high-end cards?
This isn’t an issue with Core i7—at least the LGA 1366-based models (Intel is launching LGA 1156-based i7s, too). The X58 chipset includes enough PCI Express 2.0 connectivity to give each card in a two-way CrossFire/SLI setup its own x16 link. Moreover, with a QPI link running at up to 6.4 GT/s (moving up to 25.6 GB/s of data) between X58 and Core i7, there are no perceptible bottlenecks negatively affecting graphics performance.
If there is a reason to buy LGA 1366-based Core i7 platforms over LGA 1156-based Core i5s, this architectural detail will be it.
|Intel Core i7 @ 2.8 GHz|
|Socket Interface||LGA 1366|
|PCI Express Configuration||1 x 16-lane, 2 x 16-lane, 1 x 16-lane/2 x 8-lane, 4 x 8-lane|
|Core Configuration||Four physical cores, eight threads (HT)|
We stuck to our reference platform for testing Core i7—Asus’ P6T motherboard. With one graphics card installed, it offers 16 lanes of PCIe 2.0. With two graphics cards installed, it serves up 16 lanes to both. The platform’s only real disadvantage is that Core i7 is officially rated to work with DDR3-1066 memory, while Core i5 ticks that up to DDR3-1333. It’s really a non-issue for most enthusiasts, though, since we’ve had our Core i7-975 Extreme running at up to DDR3-2133 speeds.
Finally, we did make one important concession in pitting Core i7 against Core i5. In order to match the clock frequency of the two platforms, we locked our i7-975 in at 2.8 GHz—similar to what you’d get from a stock i7-920 with Turbo Mode enabled. Again, we're analyzing PCIe performance here, and don't need processor clocks throwing off the results.