Everything You Need to Know About Nehalem's "Turbo Mode"

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How the world turns. Mention overclocking ten years ago at IDF and a Pinkerton would escort you off the show floor to a room where three Intel engineers would beat you with old Pentium Pro motherboards. Today, Intel is actually actively promoting overclocking, but big blue is calling it Turbo Mode.

Turbo Mode is just one of the several groundbreaking features in Nehalem, but it’s also certainly one of the most head-turning. But how exactly does it work and how do you control it? Walk with us as we decode Intel’s Turbo Mode, show you how you’ll set it up in the BIOS (with first photos), and tell you what you should expect from your next heatsink.

The elevator explanation for Turbo Mode is that it automatically overclocks the core (or cores) in use when it senses lightly threaded loads on the other cores. It is, of course, far more complicated than that. It’s the free performance in applications that won’t take full advantage of all eight threads that will be available in a Core i7 CPU (and part of the pragmatic view that went into the design of Core i7).  Even as Intel pushes for more and more cores, it knows that the majority of applications and games won’t see a linearly scaled performance boost from additional computing cores. So why not just overclock the one core that the program uses?

There’re a few key changes in the design of Core i7 that let Intel do Turbo Boost – power and thermal savings through new low-to-no power states and a PCU (Power Control Unit) that’s built right onto the die. The PCU is able to measure the thermals and power usage of the cores.

In practice, it works like this: If you’re playing a single-threaded game, the CPU will overclock from its 3.2GHz by, say, 266MHz. Turbo Mode will also pay dividends on multi-threaded apps if the PCU senses that even with all four cores supposedly working there’s still thermal headroom left.

How will it work in practice? We, umm, came across a BIOS snapshot of Turbo Mode during our Nehalem fondling last month that helps explain how Turbo Mode will be set up. This particular BIOS version is early and is likely to change by the time the CPU ships later this year, but it does give us a glimpse as to how much control you’ll get.

The shot tells us that you’ll be able to set each individual limit for Turbo Mode. So, if the game you’re playing is single-threaded and you have a lot of thermal headroom, you could go from the stock 22x multiplier to, say, 30x and take your 2.93GHz Core all the way to 4GHz. If you think your heatsink is up to it, you can then say, set your system to Turbo at 25x or 3.5GHz when running two cores are under a heavy load. The same is done all the way up to four cores.

One thing that will be handy is to know is the TDP or Thermal Design Power of your heat sink. Intel is encouraging vendors to test and label the heat sinks with 130 TDP or 150 TDP. With that info handy, you can actually punch in the TDP setting so the board has some idea of how good your cooling setup is.

If you still don’t understand what Turbo Mode is, we’ve put together a quick video presentation to help you decipher this new technology. Check it out here: http://www.youtube.com/watch?v=qcDztfFXPww&feature=related

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