In Theory: How Does Lynnfield's On-Die PCI Express Affect Gaming?

We've had Intel's Core i5 in the lab now for almost three months, benchmarking, overclocking, and fiddling with a bundle of pre-production motherboards. Along the way, we've raised questions and sought answers. One of those questions was largely theoretical, but we wanted defensible backup anyway: does the on-die PCI Express connectivity integrated into Intel's Core i5 and Core i7 CPUs for LGA 1156 have any affect on gaming performance?

All of these benchmarks were actually run before we even received a CPU sample from Intel. But because we knew we'd be running tests on the new i5 and i7 processors (and before we started interpreting data from those processors or their P55-based platform), it was important to figure out how much our gaming results would be affected by the CPUs themselves, with Turbo Boost and Hyper-Threading already serving as significant variables.

Asking The Right Questions

Knowing that these two new mainstream processor families incorporate 16 lanes of second-gen PCI Express on-die, yet tout CrossFire and SLI support, what happens to your frame rates when you run a single card in such a low-latency x16 interface? How about the effect of splitting that link into a pair of x8 connections? Will Core i5 handicap you right out of the gate with multi-card configurations?

Just for a bit of perspective, remember that Intel's P45 chipset also divides 16 lanes into a pair of x8 links when two AMD-based cards are installed. Thus, we're comparing Core i5 to P45 here, in addition to X58 and AMD's 790GX (the former serving up two x16 links while running in CrossFire, and the latter delivering a x8/x8 split).

Up until today, we’ve been all about Core i7-920 overclocked to somewhere around 4 GHz on an X58 motherboard with 6 GB of memory in a triple-channel arrangement. That’s a nearly-$600 proposition between the CPU, motherboard, and memory. If Core i5 can bring processor and motherboard prices down far enough, we’re guesstimating a price tag of $400-$425 with a drop to 4 GB of memory. Should Intel's new mainstream lineup proves fast enough, the ~$170 savings could translate to a “free” GeForce GTX 260 or Radeon HD 4870…so long as gaming performance is at least competitive with an LGA 1366-based Core i7, that is.

On-Die PCIe: A New Design Cue

As you likely already know, the Lynnfield design (on which the Core i5 and certain Core i7 processors center) includes 16 lanes of second-gen PCI Express. Why? Because the processor attaches to Intel’s P55 chipset via DMI (as opposed to Core i7’s single QPI link); there isn’t enough throughput between the two components for single-, much less dual-GPU communication.

Of course, this opens the door to some interesting performance-oriented questions. Does the on-die PCI Express link reduce latencies enough to improve performance with a single card installed versus Core i7? Does splitting the 16-lane connection into a pair of x8s make enough of a difference to manifest itself in gaming benchmarks with two cards installed?

How about the folks upgrading from Core 2 Quad machines, or those considering Phenom II as they wait to see what Core i5 will do?

We’ve put together the hardware needed to answer those questions from a fairly high level. Remember, these benchmarks were run on pre-production hardware for the purpose of answering a largely academic question. Turbo Boost was disabled on the LGA 1366 platform, as were all of the power-saving processor technologies that might otherwise skew our look at frame rates.