The GreenDroid mobile application processor is a 45-nm multicore research prototype that targets the Android mobile-phone software stack and can execute general-purpose mobile programs with 11 times less energy than today's most energy-efficient designs, at similar or better performance levels. It does this through the use of a hundred or so automatically generated, highly specialized, energy-reducing cores, called conservation cores. Our research attacks a key technological problem for

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... cessor architects, which we call the utilization wall. 1 The utilization wall says that, with each process generation, the percentage of transistors that a chip design can switch at full frequency drops exponentially because of power constraints. A direct consequence of this is dark silicon-large swaths of a chip's silicon area that must remain mostly passive to stay within the chip's power budget. Currently, only about 1 percent of a modest-sized 32-nm mobile chip can switch at full frequency within a 3-W power budget. With each process generation, dark silicon gets exponentially cheaper, whereas the power budget is becoming exponentially more valuable. Our research leverages two key insights. First, it makes sense to find architectural techniques that trade this cheap resource, dark silicon, for the more valuable resource, energy efficiency. Second, specialized logic can attain 10Â to 1,000Â better energy efficiency over general-purpose processors. Our approach is to fill a chip's dark silicon area with specialized cores to save energy on common applications. These cores are automatically generated from the code base that the processor is intended to run-that is, the Android mobile-phone software stack. The cores feature a focused reconfigurability so that they can remain useful even as the code they target evolves.