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Does Turbo Boost Help Or Hurt Core i5/i7's Power Efficiency?
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There’s no denying that Intel did a great job of optimizing its Nehalem architecture for the mainstream. System idle power went down considerably, and even though system peak power didn’t decrease much, you get a lot more responsiveness. Intel integrated the PCI Express interface into the processor and adjusted clock speed management for each of the four available cores via its second-generation Turbo Boost technology to maximize performance when needed.

As a result, the Core i5/i7’s efficiency (performance per watt) increased nicely. The new platforms can do more work at slightly reduced peak power consumption levels. You’d suspect that Turbo Boost deserves credits for this development, but this isn’t quite true, at least for the Core i7-870. Turbo Boost's massive clock speed bumps (up to 667 MHz) utilize the available thermal envelope more aggressively, which, in the end, actually decreases the power efficiency of the Core i7-870 system when compared to the same machine not running Turbo Boost. We don’t want to generalize these results, but they’re certainly the case when benchmarking PCMark Vantage.

In everyday life, Turbo Boost will probably kick in for smaller periods of time and we’d assume it typically does so for one or two cores. Scenarios in which three or all cores can be accelerated requires heavily threaded applications, and in such cases you probably want all the performance you can get. Efficiency obviously isn’t a concern at these times. Having one core running at 3.6 GHz will certainly not increase system power consumption enough to significantly impact efficiency.

We will follow up with more analysis soon.

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