Dennard scaling

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Dennard scaling, also known as MOSFET scaling, is a scaling law based on a 1974 paper co-authored by Robert H. Dennard, after whom it is named. [1] Originally formulated for MOSFETs, it states, roughly, that as transistors get smaller, their power density stays constant, so that the power use stays in proportion with area; both voltage and current scale (downward) with length. [2]

Robert H. Dennard American engineer and inventor

Robert Dennard is an American electrical engineer and inventor.

MOSFET transistor used for amplifying or switching electronic signals

The metal-oxide-semiconductor field-effect transistor is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, whose voltage determines the conductivity of the device. This ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. A metal-insulator-semiconductor field-effect transistor or MISFET is a term almost synonymous with MOSFET. Another synonym is IGFET for insulated-gate field-effect transistor.

Power density is the amount of power per unit volume.



Dennard observes that transistor dimensions are scaled by 30% (0.7x) every technology generation, thus reducing their area by 50%. This reduces the delay by 30% (0.7x) and therefore increases operating frequency by about 40% (1.4x). Finally, to keep the electric field constant, voltage is reduced by 30%, reducing energy by 65% and power (at 1.4x frequency) by 50%. [note 1] Therefore, in every technology generation the transistor density doubles, the circuit becomes 40% faster, and power consumption (with twice the number of transistors) stays the same. [3]

Relation with Moore's law and computing performance

Moore's law says that the number of transistors doubles about every two years. Combined with Dennard scaling, this means that performance per watt grows at this same rate, doubling about every two years. This trend is referred to as Koomey's law. The rate of doubling was originally suggested by Koomey to be 1.57 years [4] (somewhat faster than the doubling period of Moore's law), but more recent estimates suggest this is slowing. [5]

Moores law heuristic law stating that the number of transistors on a circuit doubles every two years

Moore's law is the observation that the number of transistors in a dense integrated circuit doubles about every two years. The observation is named after Gordon Moore, the co-founder of Fairchild Semiconductor and CEO of Intel, whose 1965 paper described a doubling every year in the number of components per integrated circuit and projected this rate of growth would continue for at least another decade. In 1975, looking forward to the next decade, he revised the forecast to doubling every two years. The period is often quoted as 18 months because of a prediction by Intel executive David House.

In computing, performance per watt is a measure of the energy efficiency of a particular computer architecture or computer hardware. Literally, it measures the rate of computation that can be delivered by a computer for every watt of power consumed. This rate is typically measured by performance on the LINPACK benchmark when trying to compare between computing systems.

Koomeys law

Koomey's law describes a long-term trend in the history of computing hardware. The number of computations per joule of energy dissipated has been doubling approximately every 1.57 years. This trend has been remarkably stable since the 1950s and has been somewhat faster than Moore's law. Jonathan Koomey articulated the trend as follows: "at a fixed computing load, the amount of battery you need will fall by a factor of two every year and a half."

Breakdown of Dennard scaling around 2006

The dynamic (switching) power consumption of CMOS circuits is proportional to frequency. [6] Historically, the transistor power reduction afforded by Dennard scaling allowed manufacturers to drastically raise clock frequencies from one generation to the next without significantly increasing overall circuit power consumption.

Since around 2005–2007 Dennard scaling appears to have broken down. As of 2016, transistor counts in integrated circuits are still growing, but the resulting improvements in performance are more gradual than the speed-ups resulting from significant frequency increases. [2] [7] The primary reason cited for the breakdown is that at small sizes, current leakage poses greater challenges and also causes the chip to heat up, which creates a threat of thermal runaway and therefore further increases energy costs. [2] [7]

Thermal runaway situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result

Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result. It is a kind of uncontrolled positive feedback.

The breakdown of Dennard scaling and resulting inability to increase clock frequencies significantly has caused most CPU manufacturers to focus on multicore processors as an alternative way to improve performance. An increased core count benefits many (though by no means all) workloads, but the increase in active switching elements from having multiple cores still results in increased overall power consumption and thus worsens CPU power dissipation issues. [8] [9] The end result is that only some fraction of an integrated circuit can actually be active at any given point in time without violating power constraints. The remaining (inactive) area is referred to as dark silicon.

Central processing unit power dissipation or CPU power dissipation is the process in which central processing units (CPUs) consume electrical energy, and dissipate this energy in the form of heat due to the resistance in the electronic circuits.

In the electronics industry, dark silicon is the amount of circuitry of an integrated circuit that cannot be powered-on at the nominal operating voltage for a given thermal design power (TDP) constraint. This is a challenge in the era of nanometer semiconductor nodes, where transistor scaling and voltage scaling are no longer in line with each other, resulting in the failure of Dennard scaling. This discontinuation of Dennard scaling has led to sharp increases in power densities that hamper powering-on all the transistors simultaneously at the nominal voltage, while keeping the chip temperature in the safe operating range. According to recent studies, researchers from different groups have projected that, at 8 nm technology nodes, the amount of Dark Silicon may reach up to 50–80% depending upon the processor architecture, cooling technology, and application workloads. Dark Silicon may be unavoidable even in server workloads with abundance of inherent client request-level parallelism.

See also


  1. Active power = CV2f

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An integrated circuit or monolithic integrated circuit is a set of electronic circuits on one small flat piece of semiconductor material that is normally silicon. The integration of large numbers of tiny transistors into a small chip results in circuits that are orders of magnitude smaller, cheaper, and faster than those constructed of discrete electronic components. The IC's mass production capability, reliability and building-block approach to circuit design has ensured the rapid adoption of standardized ICs in place of designs using discrete transistors. ICs are now used in virtually all electronic equipment and have revolutionized the world of electronics. Computers, mobile phones, and other digital home appliances are now inextricable parts of the structure of modern societies, made possible by the small size and low cost of ICs.

Very Large Scale Integration process of creating an integrated circuit by combining thousands of transistors into a single chip. VLSI began in the 1970s when complex semiconductor and communication technologies were being developed

Very-large-scale integration (VLSI) is the process of creating an integrated circuit (IC) by combining hundreds of thousands of transistors or devices into a single chip. VLSI began in the 1970s when complex semiconductor and communication technologies were being developed. The microprocessor is a VLSI device. Before the introduction of VLSI technology most ICs had a limited set of functions they could perform. An electronic circuit might consist of a CPU, ROM, RAM and other glue logic. VLSI lets IC designers add all of these into one chip.

CMOS technology for constructing integrated circuits

Complementary metal–oxide–semiconductor (CMOS) is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. CMOS technology is also used for several analog circuits such as image sensors, data converters, and highly integrated transceivers for many types of communication. Frank Wanlass patented CMOS in 1963 while working for Fairchild Semiconductor.

Static random-access memory Semiconductor memory

Static random-access memory is a type of semiconductor memory that uses bistable latching circuitry (flip-flop) to store each bit. SRAM exhibits data remanence, but it is still volatile in the conventional sense that data is eventually lost when the memory is not powered.

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Subthreshold conduction

Subthreshold conduction or subthreshold leakage or subthreshold drain current is the current between the source and drain of a MOSFET when the transistor is in subthreshold region, or weak-inversion region, that is, for gate-to-source voltages below the threshold voltage. The terminology for various degrees of inversion is described in Tsividis.

The CPU core voltage (VCORE) is the power supply voltage supplied to the CPU, GPU, or other device containing a processing core. The amount of power a CPU uses, and thus the amount of heat it dissipates, is the product of this voltage and the current it draws. In modern CPUs, which are CMOS circuits, the current is almost proportional to the clock speed, the CPU drawing almost no current between clock cycles.

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Multigate device

A multigate device or multiple-gate field-effect transistor (MuGFET) refers to a MOSFET that incorporates more than one gate into a single device. The multiple gates may be controlled by a single gate electrode, wherein the multiple gate surfaces act electrically as a single gate, or by independent gate electrodes. A multigate device employing independent gate electrodes is sometimes called a multiple-independent-gate field-effect transistor (MIGFET).

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Low-power electronics are electronics, such as notebook processors, that have been designed to use less electric power.

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  1. Dennard, Robert H.; Gaensslen, Fritz; Yu, Hwa-Nien; Rideout, Leo; Bassous, Ernest; LeBlanc, Andre (October 1974). "Design of ion-implanted MOSFET's with very small physical dimensions" (PDF). IEEE Journal of Solid-State Circuits. SC-9 (5).
  2. 1 2 3 McMenamin, Adrian (April 15, 2013). "The end of Dennard scaling" . Retrieved January 23, 2014.
  3. Borkar, Shekhar; Chien, Andrew A. (May 2011). "The Future of Microprocessors". Communications of the ACM. 54 (5): 67. doi:10.1145/1941487.1941507 . Retrieved 2011-11-27.
  4. Greene, Katie (September 12, 2011). "A New and Improved Moore's Law: Under "Koomey's law," it's efficiency, not power, that doubles every year and a half". Technology Review . Retrieved January 23, 2014.
  6. "CMOS Power Consumption and CPD Calculation" (PDF). Texas Instruments. June 1997. Retrieved March 9, 2016.
  7. 1 2 Bohr, Mark (January 2007). "A 30 Year Retrospective on Dennard's MOSFET Scaling Paper" (PDF). Solid-State Circuits Society. Retrieved January 23, 2014.
  8. Esmaeilzedah, Hadi; Blem, Emily; St. Amant, Renee; Sankaralingam, Kartikeyan; Burger, Doug (2012). "Dark Silicon and the end of multicore scaling" (PDF).
  9. Hruska, Joel (February 1, 2012). "The death of CPU scaling: From one core to many — and why we're still stuck". ExtremeTech . Retrieved January 23, 2014.