of increasing Vth. The balance occurs when the static leakage power is roughly 30 percent of the dynamic power dissipation.

This leakage-constrained scaling began at roughly the 130-nm technology node, and today both Vsupply and Vth scaling have dramatically slowed; this has also changed how gate energy and speed scale with technology. The energy required to switch a gate is C multiplied by Vsupply2, which scales only as 1/κ if Vsupply is not scaling. That means that technology scaling reduces the power by κ only if the scaled circuit is run at the same frequency. That is, if gate speed continued to increase, half the die (the size of the scaled circuit) would dissipate the same power as the full die in the previous generation and would operate κ times, that is 1.4 times, faster, much less than the three-fold performance increase we have come to expect. Clearly, that is not optimal, so many designers are scaling Vdd slightly to increase the energy savings. That works but lowers the gate speeds, so some parallelism is needed just to recover from the slowing single-thread performance. The poor scaling will eventually limit the performance of CMPs.

Combining the lessons of the last several sections of this chapter, the committee concluded that neither CMOS nor chip multiprocessors can overcome the power limits facing modern computer systems. That leads to another core conclusion of this report. Basic laws of physics and constraints on chip design mean that the growth in the performance of computer systems will become limited by their power and thermal requirements within the next decade. Optimists might hope that new technologies and new research could overcome that limitation and allow hardware to continue to drive future performance scaling akin to what we have seen with single-thread performance, but there are reasons for caution, as described in the next section.

Finding: The growth in the performance of computing systems—even if they are multiple-processor parallel systems—will become limited by power consumption within a decade.

ADVANCED TECHNOLOGY OPTIONS

If CMOS scaling, even in chip-multiprocessor designs, is reaching limits, it is natural to ask whether other technology options might get around the limits and eventually overtake CMOS, as CMOS did to nMOS and bipolar circuits in the 1980s. The answer to the question is mixed.28 It

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28 Mark Bohr, Intel senior fellow, gave a plenary talk at ISSCC 2009 on scaling in an SOC world in which he argues that “our challenge … is to recognize the coming revolutionary



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