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Suggested Citation:"Appendix E: Dennard Scaling and Implications." National Research Council. 2012. The New Global Ecosystem in Advanced Computing: Implications for U.S. Competitiveness and National Security. Washington, DC: The National Academies Press. doi: 10.17226/13472.
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E


Dennard Scaling and Implications

The following description was taken from the 2010 National Research Council Computer Science and Telecommunications Board (CSTB) report The Future of Computing Performance: Game Over or Next Level?1

“In a classic 1974 paper, reprinted in Appendix D, Robert Dennard et al. showed that the MOS transistor has a set of very convenient scaling properties.10 The scaling properties are shown in Table 3.1, taken from that paper. If all the voltages in a MOS device are scaled down with the physical dimensions, the operation of the device scales in a particularly favorable way. The gates clearly become smaller because linear dimensions are scaled. That scaling also causes gates to become faster with lower energy per transition. If all dimensions and voltages are scaled by the scaling factor κ (κ has typically been 1.4), after scaling the gates become (1/κ)2 their previous size, and κ2 more gates can be placed on a chip of roughly the same size and cost as before. The delay of the gate also decreases by 1/κ, and, most importantly, the energy dissipated each time the gate switches decreases by (1/κ)3. To understand why the energy drops so rapidly, note that the energy that the gate dissipates is proportional to the energy that is stored at the output of the gate. That energy is proportional to a quantity called capacitance11 and the square of the supply voltage. The load capacitance of the wiring decreases by 1/κ because the smaller gates make all the wires shorter and capacitance is proportional to length. Therefore, the power requirements per unit of space on the chip (mm2), or energy per second per mm2, remain constant:

Power = (number of gates)(CLoad/gate)(Clock Rate)(Vsupply2)
Power density = NgCloadFclkVdd2
              Ng = CMOS gates per unit area
              Cload = capacitive load per CMOS gate
              Fclk = clock frequency
              Vdd = supply voltage
Power density = (κ2)(1/κ)(κ)(1/κ)2 = 1

That the power density (power requirements per unit space on the chip, even when each unit space contains many, many more gates) can remain constant across generations of CMOS scaling has been a critical property underlying progress in microprocessors and in ICs in general. In every technology generation, ICs can double in complexity and increase in clock frequency while consuming the same power and not increasing in cost. Given that description of classic CMOS scaling, one would expect the power of processors to have remained constant since the CMOS transition, but this has not been the case. During the late 1980s and early 1990s, supply voltages were stuck at 5 V for system reasons. So power density would have been expected to increase as technology scaled from 2 mm to 0.5 mm. However, until recently supply voltage has scaled with technology, but power densities continued to increase.”

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1NRC, 2011, The Future of Computing Performance: Game Over or Next Level?, Washington, D.C.: The National Academies Press (available online at http://www.nap.edu/catalog.php?record_id=12980).

Suggested Citation:"Appendix E: Dennard Scaling and Implications." National Research Council. 2012. The New Global Ecosystem in Advanced Computing: Implications for U.S. Competitiveness and National Security. Washington, DC: The National Academies Press. doi: 10.17226/13472.
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Suggested Citation:"Appendix E: Dennard Scaling and Implications." National Research Council. 2012. The New Global Ecosystem in Advanced Computing: Implications for U.S. Competitiveness and National Security. Washington, DC: The National Academies Press. doi: 10.17226/13472.
×
Page 69
Suggested Citation:"Appendix E: Dennard Scaling and Implications." National Research Council. 2012. The New Global Ecosystem in Advanced Computing: Implications for U.S. Competitiveness and National Security. Washington, DC: The National Academies Press. doi: 10.17226/13472.
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Computing and information and communications technology (ICT) has dramatically changed how we work and live, has had profound effects on nearly every sector of society, has transformed whole industries, and is a key component of U.S. global leadership. A fundamental driver of advances in computing and ICT has been the fact that the single-processor performance has, until recently, been steadily and dramatically increasing year over years, based on a combination of architectural techniques, semiconductor advances, and software improvements. Users, developers, and innovators were able to depend on those increases, translating that performance into numerous technological innovations and creating successive generations of ever more rich and diverse products, software services, and applications that had profound effects across all sectors of society. However, we can no longer depend on those extraordinary advances in single-processor performance continuing. This slowdown in the growth of single-processor computing performance has its roots in fundamental physics and engineering constraints--multiple technological barriers have converged to pose deep research challenges, and the consequences of this shift are deep and profound for computing and for the sectors of the economy that depend on and assume, implicitly or explicitly, ever-increasing performance. From a technology standpoint, these challenges have led to heterogeneous multicore chips and a shift to alternate innovation axes that include, but are not limited to, improving chip performance, mobile devices, and cloud services. As these technical shifts reshape the computing industry, with global consequences, the United States must be prepared to exploit new opportunities and to deal with technical challenges. The New Global Ecosystem in Advanced Computing: Implications for U.S. Competitiveness and National Security outlines the technical challenges, describe the global research landscape, and explore implications for competition and national security.

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