Overclocking Lynnfield at Stock Voltage: We're PCIe Limited

Remember the on-die PCIe controller? Yep. It's to blame.

Lynnfield is Intel's first attempt at an on-die PCIe controller and it actually works surprisingly well. There are no performance or compatibility issues.



The on-die PCIe controller needs more voltage as you overclock Lynnfield, limiting Lynnfield's stock vt overclocking potential.

Unfortunately the PCIe controller on Lynnfield is tied to the BCLK. Increase the BCLK to overclock your CPU and you're also increasing the PCIe controller frequency. This doesn't play well with most PCIe cards, so the first rule of thumb is to try and stay at 133MHz multiples when increasing your BCLK.

The second issue is the bigger one. As you increase the BCLK you increase the frequency of the transistors that communicate to the GPU(s) on the PCIe bus. Those transistors have to send data very far (relatively speaking) and very quickly. When you overclock, you're asking even more of them.

We know that Bloomfield can easily hit higher frequencies without increasing the core voltage, so there's no reason to assume that Lynnfield's core cannot (in fact, we know it can). The issue is the PCIe controller; at higher frequencies those "outside facing" transistors need more juice to operate. Unfortunately on Lynnfield rev 1 there doesn't appear to be a way to selectively give the PCIe transistors more voltage, instead you have to up the voltage to the entire processor.

Intel knows the solution to Lynnfield's voltage requirement for overclocking, unfortunately it's not something that can be applied retroactively. Intel could decouple the PCIe controller from BCLK by introducing more PLLs into the chip or, alternatively, tweak the transistors used for the PCIe interface. Either way we can expect this to change in some later rev of the processor. Whether that means we'll see it in the 45nm generation or we'll have to wait until 32nm remains to be seen.

The good news is that Lynnfield can still overclock well. The bad news is that unlike Bloomfield (and Phenom II) you can't just leave the Vcore untouched to get serious increases in frequency.

Overclocking: Great When Overvolted, Otherwise... Final Words
POST A COMMENT

343 Comments

View All Comments

  • Ann3x - Wednesday, September 9, 2009 - link

    1/ There is no chance that any of these chip can run safely with no fan.
    2/ You dont get huge temperature increases if you dont overvolt, just clock changes result in very small temperature changes. look at the article you linked. The 3 new chips use EXACTLY the same power despite their differing clock speeds.
    3/ New energy saving technology works with overclock just like it does with stock clock ed CPUs (eg energy states, my i7 is overclocked to 4ghz on stock volts, when its not needed it clock down - same end effect as these new chips (albeit slightly less elegant)).

    This whole fuss about turbo mode is just marketing gumph and yet people are totally sucked in by the hype.
    Reply
  • coconutboy - Wednesday, September 9, 2009 - link

    1- I never said anything about running the CPU w/o a fan. Fanless watercooling is an option, as is quiet low rpm fans.

    2- Clocking w/o increasing voltage does increase heat and whether or not you consider that to be a significant amount depends on the cooling solution you use. I made no claim that OC'd temps would increase as much as overvolting.

    Your opinion about the validity of turbo mode is just that, you opinion. You and I can agree to disagree.
    Reply
  • titanium001 - Wednesday, September 9, 2009 - link

    I was excited to see the article, but was left wondering and scratching my head when gaming performance was evaluated. I didn't see any 1920 x 1200 or 2560 x 1600 comparisons anywhere. Do the i7 800 series take a significant performance hit in these settings. I guess everything can't be delivered until a full in depth review. Have to just wait. I'll reserve my judgment about the 800 series until then, for now, it's just another proc. Thanks for the initial preview Anandtech.com. Reply
  • Anand Lal Shimpi - Wednesday, September 9, 2009 - link

    Have a look at the SLI/CF Multi-GPU Gaming page, I include some GPU limited tests at the bottom of that page.

    At higher resolutions P55, X58 and even Phenom II/790FX all perform the same if you're GPU limited. The PCIe limitations of P55/Lynnfield only come into play when you're running in multi-GPU mode because the x16 interface gets broken up into a pair of x8s.

    Take care,
    Anand
    Reply
  • NoobyDoo - Tuesday, September 8, 2009 - link

    ... remember when C2D was released ? Reply
  • coconutboy - Tuesday, September 8, 2009 - link

    After thoroughly reading a lot of the articles at Toms, Anand, xbit etc, Lynnfield looks great and has been worth the wait. However, it's not an absolute sell as a gamer box IMO. My g/f and I have been waiting to build a pair of new gaming computers, but wanted to see what i5 had to offer first. Now that NDAs are down, the $30-70 savings for p55 versus comparable x58 mobos is great, but some things about i5 still make me want an i7 920 instead.

    My g/f and I plan on buying a pair of GTX 275s, one for each computer. Then later on as our systems age, we'll put both 275s in one box and buy a newer vid card for the other system. We also moderately overclock our CPUs (3.2-3.4 would be what I expect for a i7 920) to boost performance w/o shortening the lifespan too much because our gamer boxes usually end up moving down the line in our home network to become servers or some such.

    Taking into account everything I've been reading at hardware sites thus far, we'll likely build one Lynnfield and one i7 for our gaming rigs. I expect one of the current gamer computers we build will migrate to become a VMware machine later on which means an i7 920 w/ (eventually) 24GBs of RAM is very attractive. The ability to use a 6 core CPU later on counts for a lot as well.
    Reply
  • coconutboy - Wednesday, September 9, 2009 - link

    edit- I meant to be comparing an mildly overclocked i7 920 @ stock voltage and 3.3-3.4GHz versus an i7 860/870, not an i5. I'd often prefer the 920 (but not always of course) for my uses. Reply
  • thebeastie - Tuesday, September 8, 2009 - link

    Such a good complete review, EXCEPT there is no information of how much power the CPU used with it was hard overclocked to its 4.2Ghz mark.
    With its intergrated PCIe 100million transistor count controller inside the CPU this would of been really interesting info.
    Reply
  • Gary Key - Wednesday, September 9, 2009 - link

    The system power utilized (measured at the wall) was 301W for the 4.2GHz overclock on the 870 under an eight thread 100% load test on the board. Reply
  • justme2009 - Tuesday, September 8, 2009 - link

    Why are you overhyping this garbage? I'm waiting for Clarkdale. I'm still ticked off that Intel caved to the manufacturers and held off on releasing it, we were supposed to have it in the 4th quarter of this year, now it will be first half of 2010.
    This new nehalem (even if it's for desktops) will be nothing compared to the mobile nehalem next year.
    My only other question is, why the hell has Clarkdale/Arrendale information been buried? There hasn't been a peep from anyone about it since February.
    Reply

Log in

Don't have an account? Sign up now