Intel's Core i7 870 & i5 750, Lynnfield: Harder, Better, Faster Stronger
by Anand Lal Shimpi on September 8, 2009 12:00 AM EST- Posted in
- CPUs
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.
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strikeback03 - Tuesday, September 8, 2009 - link
How would you have graphics then? You would be limited to the 4xPCIe off the P55 on motherboards which support it, as there are no integrated graphics (yet)MX5RX7 - Tuesday, September 8, 2009 - link
I'm not sure that CPU/GPU integration is a good thing, from a consumer standpoint. At least in the short term.For example, in the article you mention how the majority of modern games are GPU, not CPU limited. The current model allows us to purchase a very capable processor and pair it with a very capable GPU. Then, when the ultra competitive GPU market has provided us with a choice of parts that easily eclipse the performance of the previous generation, we either swap graphics cards for the newer model, or purchase a second now cheaper identical card and (hopefully) double our game performance with SLI or Crossfire. All without having to upgrade the rest of the platform.
With the current model, a new graphics API requires a new graphics card. With Larrabee, it might very well require a whole new platform.
Ben90 - Tuesday, September 8, 2009 - link
Yea, im really excited for Larrabee, who knows if it will be good or not... but with intel kicking ass in everything else, it will at least be interestingWith overclocking performance seemingly being limited by the PCI-E controller, it seems like an unlocked 1156 would be pretty sweet
All in all i gotta admit i was kinda bitter with this whole 1156 thing because i jumped on the 1336 bandwagon and it seemed that Intel was mostly just jacking off with the new socket... but this processor seems to bring a lot more innovation than i expected (just not in raw performance, still great performance though)
chizow - Tuesday, September 8, 2009 - link
Was worried no one was going to properly address one of the main differences between P55 and X58, thanks for giving it a dedicated comparison. Although I would've like to have seen more games tested, it clearly indicates PCIE bandwidth becoming an issue with current generation GPUs. This will only get worst with the impending launch of RV8x0 and GT300.Anand Lal Shimpi - Tuesday, September 8, 2009 - link
PCIe bandwidth on Lynnfield is only an issue with two GPUs, with one you get the same 16 lanes as you would on X58 or AMD 790FX.If I had more time I would've done more games, I just wanted to focus on those that I knew scaled the best to see what the worst case scenario would be for Lynnfield.
In the end 2 GPUs are passable (although not always ideal on Lynnfield), but 4 GPUs are out of the question.
Take care,
Anand
JumpingJack - Thursday, September 10, 2009 - link
Anand, a few other sites have attempted SLI/Xfire work ... on in particular shows 4 GPUs having no impact at all on gaming performance in general -- well, 3 or 4 FPS, but nothing more than a few percentages over norm.Could your configuration with beta or just bad first release drivers be an issue?
Jack
JonnyDough - Tuesday, September 8, 2009 - link
Would it be possible to incorporate two GPU controllers onto a die instead of one or is that what they'll be doing with future procs? I would think that two controllers with a communication hub might supply the needed bandwidth of x16 + x16.Comdrpopnfresh - Tuesday, September 8, 2009 - link
with two gpu's being passable- do you foresee that applying to both two independent gpus, as well as the single dual-card gpus?Ryan Smith - Tuesday, September 8, 2009 - link
Yes. The only difference between the two is where the PCIe bridge chip is. In the former it's on the mobo, in the latter it's on the card itself.Eeqmcsq - Tuesday, September 8, 2009 - link
Talk about bringing a bazooka to a knife fight. AMD better be throwing all their innovation ideas and the kitchen sink into Bulldozer, because Intel is thoroughly out-innovating AMD right now.