Overclocking Sandy Bridge-E
There is a silver lining, though. Sandy Bridge-E is expected to be overclockable in ways Sandy Bridge is not.
Two of the three SKUs purportedly planned for launch will be multiplier-unlocked, simplifying overclocking quite a bit. Both of those models should support ratios as high as 57x (just like the Sandy Bridge-based K-series SKUs were), easily taking care of the air-cooling community. The third, Core i7-3820, will get a number of accessible bins on top of the highest Turbo Boost multiplier, similar to Core i5-2500 and Core i7-2600.
But whereas P67 and Z68 employed internal clock generators, X79’s BCLK comes from a CK505 embedded clock fed through a buffer (a chip that follows Intel’s DB1200GS Differential Buffer Specification) responsible for “gearing” the frequency.
With Sandy Bridge, if you didn’t have an unlocked K-series SKU, or a partially unlocked Core i5/i7, then you were pretty much stuck. Changing the BCLK frequency directly affected other buses, quickly affecting stability. We’ve been able to push single-digit increases, but anything more than 9 or 10 MHz is asking for trouble. Of course, Intel's intention there wasn't nefarious. By integrating the clock, it cut power use and cost on the 25 and 14 MHz crystals. Inflexible scaling just turned out to be a side-effect.
Sandy Bridge-E should alleviate this somewhat by using the buffer chip to apply one of three different ratios to the BCLK. These will modify the PCI Express bus and DMI, creating a greater range of viable frequency settings. I emphasize the word should because I wasn’t able to get the mechanism working on our lab system. Increasing the BCLK and dropping our -3960X’s multiplier simply kept the platform from POSTing, no matter what combination of settings I used.
At least at first, the question to answer is going to be: who cares, anyway? When you consider the Core i7-2600K is completely multiplier-unlocked, who is going to want to fight a partially-unlocked quad-core -3820 on an expensive new X79-based motherboard? The extra 2 MB of L3 cache, two additional memory channels, and PCI Express-rich processor probably won’t inspire many upgrades. The real candidates for greatness are the six-core models, both of which Intel thankfully leaves unlocked. Specifically, I’m excited to see what the Core i7-3930K can do.