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

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

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Top Comments
  • 11 Hide
    megabuster , September 8, 2009 4:29 AM
    AMD better have something up its sleeves or it's instakill.
Other Comments
  • 11 Hide
    megabuster , September 8, 2009 4:29 AM
    AMD better have something up its sleeves or it's instakill.
  • -1 Hide
    rambo117 , September 8, 2009 6:20 AM
    AMD... your loosing your game...
  • 9 Hide
    dirtmountain , September 8, 2009 6:39 AM
    A PhenomII x4 920? ouch
  • 1 Hide
    radnor , September 8, 2009 8:50 AM
    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.
  • 0 Hide
    cangelini , September 8, 2009 9:02 AM
    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!
    http://www.tomshardware.com/reviews/intel-core-i5,2410.html
    and
    http://www.tomshardware.com/reviews/core-i5-gaming,2403.html
    Thanks for the feedback notes!
  • 3 Hide
    bounty , September 8, 2009 4:34 PM
    "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.
  • -2 Hide
    Shnur , September 8, 2009 5:21 PM
    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?
  • 5 Hide
    cangelini , September 8, 2009 5:36 PM
    Shunr,
    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!
    Chris
  • 2 Hide
    Shadow703793 , September 8, 2009 8:06 PM
    megabusterAMD better have something up its sleeves or it's instakill.

    lol! do you mean instagib?

    Joking aside, AMD needs something to counter this.
  • 4 Hide
    Alkapwn , September 8, 2009 9:14 PM
    There may be a slight flaw to your testing methodology on the i7 vs i5. You used an i7 975 against the i5 750. You clocked the CPUs to the same, 2.8 Ghz, but made no mention of the uncore speed. Uncore clock of the i7 975 runs @ 2.66 Ghz, whereas the i5 750 runs @ 2.16 Ghz. Since the PCI controller on the i5 is part of the uncore, answering the question "How Does Lynnfield's On-Die PCI Express Affect Gaming" against the i7 975 will only show skewed, or dare I say flawed, results.

    If you're going to make an *apples to apples* comparison, the i7 920 would have been a better choice, as its uncore is clocked the same as the i5, and both run their cores at the same stock speeds. This would have presented a level playing field in both processor speed as well as uncore speeds.

    Thank you.
  • -3 Hide
    CookYouAll , September 8, 2009 10:18 PM
    Sad, AMD losing again, again & again..
  • -1 Hide
    tkgclimb , September 8, 2009 11:10 PM
    Why did they use a 790gx platform not a 790fx, doesn't the FX have more pci-e bandwith, and does better supporting multi gpu configurations? or am I about to buy the wrong mobo.
  • 1 Hide
    rambo117 , September 8, 2009 11:21 PM
    tkgclimbWhy did they use a 790gx platform not a 790fx, doesn't the FX have more pci-e bandwith, and does better supporting multi gpu configurations? or am I about to buy the wrong mobo.

    did you read the whole article? its explained here:
    http://www.tomshardware.com/reviews/core-i5-lynnfield,2379-3.html
  • -2 Hide
    tkgclimb , September 8, 2009 11:47 PM
    rambo117did you read the whole article? its explained here: http://www.tomshardware.com/review [...] 379-3.html


    Yeah I read it, but it really doesn't make sense even though the i5 has only 16x bandwidth I don't think they should limit AMD to only 16x also.

    But good article,

    weird how the the PHenom scales so badly in crsfr still using the same amount of bandwidth as all the rest of the CPU's
  • 0 Hide
    spearhead , September 9, 2009 7:41 AM
    i dont find it strange that PHII seems to scale alot less here. PHII was clocked back to 2,8GHZ otherwise the results would have been much more even when benched vs a stock clocked i5 750. This benchmark brings out the clock advantage of the architectures. But it dousn't smell like real world preformance to me. never the less scores above 60-75 FPS are above noticeble. fewer then that would also not hurt tough since a few frames are always being prerenderd. That is why i cannot justify I7 920 any longer for anything less then a multiple solution. never the less i5 750 is going to be priced quite near the PHII 955 just a few tens more expansive, with some luck AMD considers dropping the price a bit more. Over all I5 750 is a good awnser VS the phenom 955 and 965. And while both offer there own advantages such as intels clock advantage it would not run as near its physical limits as a stock phenom II dous whe your into overclocking. But AMD on the other hand is still a bit cheaper and will bring out a 6 core istanbul based 32nm cpu somewhere next year which will work on every current decent AM3 board. core i5 still be the current winner when your aim is to overclock
  • -5 Hide
    Ciuy , September 9, 2009 8:26 AM
    so it sucks
  • -1 Hide
    suitaoli , September 9, 2009 12:08 PM
    what aboutthe Hyper threading in i7 Bloomfield benchmarks, disable or enable?
  • -2 Hide
    suitaoli , September 9, 2009 12:10 PM
    suitaoliwhat about the Hyper threading in i7 Bloomfield benchmarks, is it disable or enable?

  • -5 Hide
    Anonymous , September 9, 2009 12:22 PM
    so let me get this straight...you took two almost exactly the same architectures and ran tests where the extra bandwidth of the memory of one don't matter and where both had sufficient PCI bandwidth and you're shocked that they performed the same clock for clock? seriously? You guys are fucking geniuses!!! STOP THE FUCKING PRESSES TWO SIMILAR ARCHITECTURES PERFORM SIMILARLY should be the title of this article
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