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Operating Cores

By Brian Greskamp, Ulya R. Karpuzcu and Josep Torrellas


Many-core CMP scaling is about to crash into the power wall. Using data synthesized from the 2008 ITRS update, the figure above shows a growing gap between the number of Nehalem-like cores that fit on a die (about 100 cores by 2022) and the number of cores that can operate simultaneously under a 100W power budget (only 27 cores by 2022). In future designs, the majority of cores will necessarily be dormant at any given time in order to meet the power budget. Concerns over programmability and parallelism aside, the future literally provides more cores than we can possibly use. How can we exploit this surplus of cores to extract more performance? Solution: Expendable Cores Process variation in future CMPs will render some cores more power efficient than others. The most efficient cores are precious because parallel workloads can use them to run as many threads as possible under the power limit. Less-efficient cores can be used as “Bubble Wrap”: They accelerate the sequential phases, sacrificing themselves to defeat Amdahl’s Law by operating at higher-than-normal voltage and frequency. The elevated voltages and temperatures will quickly wear out or “pop ” BubbleWrap cores, but this is not a problem, as they can be replaced from a large pool of dormant spares. Meanwhile, the precious cores never run above nominal voltage, and so protected from the harsh conditions under which the BubbleWrap cores operate, lead long and efficient lives. The following figure illustrates our vision of a BubbleWrap CMP from year 2019. It includes 14 precious cores that operate only at nominal voltage and are guaranteed not to burn out over the service life of the chip. It also includes 35 BubbleWrap cores that are microarchitecturally identical to the precious ones but are destined to live fast and die young. Any cores not currently operating are power-gated. The figure shows a chip in mid-life, when some of the BubbleWrap cores are already popped (black). Precious, efficient cores BubbleWrap accelerato

Year: 2011
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