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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Shadowmaster625 - Tuesday, September 8, 2009 - link
Intel releases yet another new socket type, offering negligible performance enhancements vs socket 775. Soon they will obsolete another socket type still in use. And this is a good thing? I'm still dealing with the fallout from the socket 478...DJMiggy - Tuesday, September 8, 2009 - link
Thanks! Some good info! Now to decide what to do...Rabman - Tuesday, September 8, 2009 - link
Full disclusre -- I work for AMD, my comments are my own and do not reflect my employer, etc.A clarification on Windows 7's Core Parking feature -- it doesn't actually "[look] at the performance penalty from migrating a thread from one core to another". Rather, Core Parking was designed as a power saving feature for multi-core server machines, and is only enabled on Windows 7 client SKUs where HT is present (I won't get into specifics as to why this decision was made). The side benefit for processors with HT is that the hyperthreads can be parked so the Windows scheduler will spread threads across the "real" cores first, resulting in better performance characteristics.
rbbot - Tuesday, September 8, 2009 - link
That implies that it would have a negative effect on the chances of turbo mode engaging. On other OS, pure random chance would sometimes assign a waking thread to the hyper-core of the one already executing at full pelt. However, this means that on Windows 7, core parking prevents this happening and always wakes a 2nd core for the 2nd thread.puffpio - Tuesday, September 8, 2009 - link
If you disable turbo mode, will the individual cores still power down when unused?Take the 860 for example. With turbo mode enabled you get these overclocked speeds:
3C/4C Active: 3.54GHz
2C Active: 3.85GHz
1C Active: 4.00GHz
but with turbo mode disabled you get 3.99GHz at 1/2/3/4 cores active.
If the cores are still able to be powered down w/ turbo mode disabled, it would seem that would give you the best performance at any core activity level.
Comdrpopnfresh - Tuesday, September 8, 2009 - link
Specifically; power consumption, efficiency, and productivity/performance. On the consumer scale though- obviously with single-cpu boards benches geared towards commercial use would be droll.AFUMCBill - Tuesday, September 8, 2009 - link
Great Review.You mentioned the rising popularity of the uATX platform.
I would guess this is related to the rising popularity of laptops.
Except you can't find anything close to the performance of a Core i7 or i5 processor in a laptop form factor at anything remotely resembling a reasonable price - as in thousands and thousands of dollars extra. So people are headed to the uATX platform and the small(er) LAN party type boxes to get mobile performace. In my case I would like to be able to load high bitrate (25 Mbps and up) MPEG2 and MPEG4 footage into my video editor and have at it. My Q6600 handles the MPEG2 fine, but not the MPEG4 (AVCHD).
Found the Core i7 860 available at MicroCenter for $229.99 USD.
For me to make the buy, the only thing that is missing is USB 3.0.
Next year is looking good...and prices are likely to be even lower then :-)
Peroxyde - Tuesday, September 8, 2009 - link
Just checked at Newegg. Is there any error on the price? The newer and more performance i5 750 costs $209. The Q9550 cost $219. That sounds illogical.AFUMCBill - Tuesday, September 8, 2009 - link
I think it's called having old stock that was purchased before the new announcements. Obviously the folks they are going to be selling to are ones who are updating the processor in an older 775 socket motherboard based system - which with the new announcements are now rapidly receding into the past.C'DaleRider - Tuesday, September 8, 2009 - link
Sucks to have to depend on Newegg for buying, esp. considering what MicroCenter is doing. $199 for the i7 920 while Newegg gouges at $279, or the i5 750 for $179.Newegg long ago ceased being the place for the best prices.