Sandy Bridge-E’s Efficiency Suffers Significantly Overclocked
The results speak for themselves. While Intel’s Core i7-3960X is certainly the fastest processor money can buy, and although it offers plenty of overclocking headroom, its efficiency suffers tremendously when you push it beyond its stock specifications. Given its complexity and target market, there's simply no way for the LGA 2011-based chips to compete against the fast, yet relatively lower-power LGA 1155-based models like Core i5-2500K and -i7-2600K.
Is that a problem? For the folks who need the performance of an LGA 2011-based configuration, probably not. The Sandy Bridge-E die incorporates a four-channel memory controller, 40 lanes of third-gen PCI Express, tons of last-level cache, and as many as six enabled cores. Despite the mature 32 nm process on which it's manufactured, it still takes a lot of power to drive such a complex SoC.
Because power consumption is affected by both frequency and voltage, even turning up the clock rate without manipulating voltage impacts efficiency. The effect isn't pronounced under 4.5 GHz. However, each step of the way, efficiency does drop, as power use goes up faster than performance. Things get worse once voltage needs to be added, and our index tanks even faster.
So, you can get a decent efficiency boost from Sandy Bridge's four cores through overclocking, but the same can't be said for Sandy Bridge-E. If you're tuning for absolute performance, rather than getting more done per watt of power use, we'd recommend using a 42x Turbo Boost multiplier for five/six active cores, a 43x multiplier for three/four active cores, and a 45x ratio for one or two active cores because this shows a significant performance improvement with reasonable increases in power consumption.
In the meantime, we'll eager await Intel's upcoming 22 nm-based Ivy Bridge chips, which promise to improve efficiency even further.