Turbo box

Explaining Intel's Turbo Boost technology
by Peter Glaskowsky
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Intel promotes the Turbo Boost technology in its new Core i7 Mobile processors as a way to adapt to the needs of the software and get more performance from the chip, but this isn't the real reason the technology exists.
The new "Clarksfield" Core i7 Mobile processorsintroduced at the Intel Developer Forum last week are certainly very impressive. They're huge high-performance quad-core chips with Hyper-Threading, support for two channels of DDR3-1333 DRAM, and an on-die PCI Express controller for the fastest possible connection to discrete graphics chips.

Intel VP Mooly Eden shows off the new Core i7 Mobile processor and its companion I/O controller at theIntel Developer Forum.
(Credit: Intel)
In his IDF session announcing these parts, Intel Vice President Mooly Eden said the best of these parts, the 2GHz Core i7-920XM Extreme Edition, is "the fastest quad-core processor, the fastest dual-core processor, and the fastest single-core processor"-- all in one chip.
The key to this dramatic claim is a feature called Turbo Boost technology. Basically,if the current application workload isn't keeping all four cores fully busy and pushing right up against the chip's TDP (Thermal Design Power) limit, Turbo Boost can increase the clock speed of each core individually to get more performance out of the chip.
It's easy to see how this works when just one or two cores are being actively used; whatever power the other two or three cores would haveconsumed can be redirected over to the active cores, allowing them to run at higher speeds.
The quad-core mode of Turbo Boost is a little more subtle; it works when the four cores aren't running a worst-case workload--for example, integer-heavy processing, since it's generally floating-point calculations that consume the most power--so they aren't bumping into the TDP limit. Turbo Boost canincrease the frequency of all four cores until they're running as fast as they can for the current workload.
Eden said that the Turbo Boost controller samples the current power consumption and chip temperature 200 times per second and makes whatever adjustments are necessary. Of course, if Windows isn't asking for more performance, Turbo Boost doesn't deliver it.
In the ideal case, where just one coreis running, Turbo Boost can increase the clock rate on that core from the chip's rated speed of 2GHz to 3.2GHz--that's like getting a chip eight speed grades faster than what you paid for. (Speed grades, or "bins" in the parlance of semiconductor manufacturing, usually go up in steps of around 10 percent to 20 percent. The Core 2 Mobile processor P series parts have speeds of 2.26, 2.4, 2.53,2.66, and 2.8GHz. The T series extends this range to 2.93 and 3.06GHz, so by this measurement, 3.2GHz would be about eight steps above 2GHz.)
That's how Intel wants everyone to think of Turbo Boost, but it isn't really the natural way. To explain why, I'll have to digress briefly and describe how chips are designed and built.
Any given microprocessor core architecture, like the Nehalem architectureunderlying these new parts, has a certain typical complexity expressed in terms of a number of equivalent gate delays. The clock period has to be long enough to accommodate all of these gate delays.
Any given process technology, like Intel's 45nm "P1266" technology, has its own characteristics. These can be tweaked somewhat to optimize for higher speed, higher yield, lower power consumption,higher transistor density, etc., but generally a company like Intel has just one recipe for high-performance microprocessors like the Core i7.
The combination of the gate delays in the logical design of a chip with the physical transistor and interconnect performance figures for a process determines a maximum clock speed for that chip on that process. As chips are manufactured, they're tested for...