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.

Chris Angelini
Chris Angelini is an Editor Emeritus at Tom's Hardware US. He edits hardware reviews and covers high-profile CPU and GPU launches.
  • megabuster
    AMD better have something up its sleeves or it's instakill.
  • rambo117
    AMD... your loosing your game...
  • dirtmountain
    A PhenomII x4 920? ouch
  • bucifer
    I do not agree with the choices made in this article. You don't buy 2*4870x2 and the you slam a x4 920. The choices do not make sense.

    You should have used the best cpu(ex i7 920 oc@4GHz) to try to eliminate all bottlenecks and truly emphasize the limitations of x8/x16 pci-e lanes.

    The rest of the testing was done to include the new i5 which is not bad but not relevant for the bottleneck. I know many people would like to see how i5+p55 handles the gpu power but it's a highly unlikely scenario that someone would actually but such powerful and expensive cards on pair them with a cheaper cpu and a limited platform.

    I just think you should have tested things separately in different articles.
  • radnor
    I know you used a 2.8Ghz Deneb for Clock-per-clock comparisons. MAkes sense. But a 2.8 Ghz Deneb is something really no unlocked. Ussually unlock versions go 3.5Ghz on stock VID, non BE PArts can reach 3.3Ghz safely.

    A 2.8 Deneb/Lynnfield/Bloomfield have completely diferent prices. You are comparing a R6 vs a R1. I7 is the Busa trouting everybody else. Of course the prices are very diferent.
  • cangelini
    Gents, if you want to see the non-academic comparisons, I have the 965 BE compared in two other pieces for more real-world comparisons!,2410.html
    Thanks for the feedback notes!
  • bounty
    "Will Core i5 handicap you right out of the gate with multi-card configurations? The aforementioned gains evaporated in real-world games, where Core i7’s trended slightly higher, perhaps as a result of Hyper-Threading or its additional memory channel"

    Well you answered will i5 handicap you without hyperthreading, x8 by x8 and dual channel. It will by 5-10% If you wanted to narrow it down to memory channels, hyperthreading or the x8 by x8 you could have pice the game with the biggest spread and enabled each of those options selectively. Would have been kinda interesting to see which had the biggest impact.
  • Shnur
    Great article! But then again... I don't see why a 955 wasn't used in this scenario... since the 920 is thing that nobody uses. Already that we know that i7 is superior to AMD flagship in multi-GPU configurations you're taking a crappy AMD CPU, buying a 790GX doesn't mean you're going to cut on the chip... and you're talking about who's performing better in 8x lanes... from my point of view it's a bad comparison, and there should have been a chip that'll be actually able to take a difference between 1 card and two and the from 16x and 8x.
    And thanks for the other linked reviews, but I'm not talking about comparing the chips themselves, I'm trying to figure out is 8x still good enough or I need to pay more for 16x?
  • cangelini
    Thanks much for the feedback--again, this wasn't meant to be about the CPUs, but the PCI Express links. If you want to know about the processors themselves at retail clocks, check out the gaming story, which does reflect x16/x16 and x8/x8 in the LGA 1366 and LGA 1156 configs.
    Hope that helps!
  • Shadow703793
    megabusterAMD better have something up its sleeves or it's! do you mean instagib?

    Joking aside, AMD needs something to counter this.