Art Scott and Michael Frank on energy-efficient computing

Abstract

Editor's IntroductionClock speeds of computing chips have leveled off dramatically since 2005, and putting more cores in systems on a chip (SoC) has produced more heat, adding a new ceiling to further advances. Leading-edge researchers, like Mike Frank, and dedicated technologists with a wealth of experience, like Art Scott, represent a new vanguard of the leap-forward beyond Dennard scaling and Landauer's limit. Art looks for ways to reduce energy consumption and Mike looks for ways to "architect" future chips according to principles of reversibility. Is the future in reversible, adiabatic computing and simpler architectures using posit arithmetic? My guests think so.Ted Lewis: You have been in the computing industry for a long time, going all the way back to the beginning of Moore's law. Generally speaking, you have participated in making computers run faster and hotter, but since around 2005 Moore's law has hit a wall because of heat-the end of Dennard scaling has forced chip-makers to face the problem of heat dissipation. As a consequence, clock speeds have not improved for a decade, see Figure 1. Some say the answer is reversible computing. What is reversible computing and how did you become interested in it?Art Scott: Energy efficient is the answer to Landauer's limit, the act of erasing a bit of information gives off an amount of heat related to the temperature and Boltzmann's constant-a total of 3×10 -21 joules at room temperature. Reversible computing is energy efficient; therefore, reversible computing is the answer to Landauer's limit. Your readers can find out more online where Wikipedia describes the reason reversible computing is now center stage, "Probably the largest motivation for the study of technologies aimed at actually implementing reversible computing is that they offer what is predicted to be the only potential way to improve the computational energy efficiency of computers beyond the fundamental von Neumann-Landauer limit of kT ln 2 energy dissipated per irreversible bit operation." 1 There, k is the Boltzmann constant (approximately 1.38×10 −23 J/K), T is the absolute temperature of the environment, and ln 2 is the natural logarithm of 2 (approximately 0.69315). Reversible means the computation can be reversed, in other words, the inputs can be obtained from the outputs, by running circuits backwards. However, the purpose of reversible design is not to run backwards, but to avoid heat generation associated with increased thermodynamic entropy,

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“…which states that E is the heat dissipated by a logically irreversible gate to its environment, k B is the Boltzmann constant, T is the temperature of the environment in kelvins, and ln(2) is the natural logarithm of 2. At room temperature (293.15 K), erasing one bit of information generates about 2.805 • 10 −21 joules of heat [69].…”

“…The charging and discharging of circuit elements must be adiabatic. The rules [69] to achieve this are 1) Do not turn on a switch if there is a significant voltage difference between the channel terminals. 2) Do not turn off a switch if there is a significant electrical current flowing through the channel of the switch.…”