Energy challenges for ICT

The energy consumption from the expanding use of information and communications technology (ICT) is unsustainable with present drivers, and it will impact heavily on the future climate change. However, ICT devices have the potential to contribute signi - cantly to the reduction of CO2 emission and enhance resource e ciency in other sectors, e.g., transportation (through intelligent transportation and advanced driver assistance systems and self-driving vehicles), heating (through smart building control), and manu- facturing (through digital automation based on smart autonomous sensors). To address the energy sustainability of ICT and capture the full potential of ICT in resource e - ciency, a multidisciplinary ICT-energy community needs to be brought together cover- ing devices, microarchitectures, ultra large-scale integration (ULSI), high-performance computing (HPC), energy harvesting, energy storage, system design, embedded sys- tems, e cient electronics, static analysis, and computation. In this chapter, we introduce challenges and opportunities in this emerging eld and a common framework to strive towards energy-sustainable ICT

Limits on Fundamental Limits to Computation

An indispensable part of our lives, computing has also become essential to industries and governments. Steady improvements in computer hardware have been supported by periodic doubling of transistor densities in integrated circuits over the last fifty years. Such Moore scaling now requires increasingly heroic efforts, stimulating research in alternative hardware and stirring controversy. To help evaluate emerging technologies and enrich our understanding of integrated-circuit scaling, we review fundamental limits to computation: in manufacturing, energy, physical space, design and verification effort, and algorithms. To outline what is achievable in principle and in practice, we recall how some limits were circumvented, compare loose and tight limits. We also point out that engineering difficulties encountered by emerging technologies may indicate yet-unknown limits.Comment: 15 pages, 4 figures, 1 tabl

LLM: Realizing Low-Latency Memory by Exploiting Embedded Silicon Photonics for Irregular Workloads

As emerging workloads exhibit irregular memory access patterns with poor data reuse and locality, they would benefit from a DRAM that achieves low latency without sacrificing bandwidth and energy efficiency. We propose LLM (Low Latency Memory), a codesign of the DRAM microarchitecture, the memory controller and the LLC/DRAM interconnect by leveraging embedded silicon photonics in 2.5D/3D integrated system on chip. LLM relies on Wavelength Division Multiplexing (WDM)-based photonic interconnects to reduce the contention throughout the memory subsystem. LLM also increases the bank-level parallelism, eliminates bus conflicts by using dedicated optical data paths, and reduces the access energy per bit with shorter global bitlines and smaller row buffers. We evaluate the design space of LLM for a variety of synthetic benchmarks and representative graph workloads on a full-system simulator (gem5). LLM exhibits low memory access latency for traffics with both regular and irregular access patterns. For irregular traffic, LLM achieves high bandwidth utilization (over 80% peak throughput compared to 20% of HBM2.0). For real workloads, LLM achieves 3 × and 1.8 × lower execution time compared to HBM2.0 and a state-of-the-art memory system with high memory level parallelism, respectively. This study also demonstrates that by reducing queuing on the data path, LLM can achieve on average 3.4 × lower memory latency variation compared to HBM2.0

Design and demonstration of integrated micro-electro-mechanical relay circuits for VLSI applications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 115-121).Complementary-Metal-Oxide-Semiconductor (CMOS) feature size scaling has resulted in significant improvements in the performance and energy efficiency of integrated circuits in the past 4 decades. However, in the last decade and for technology nodes below 90 nm, the scaling of threshold and supply voltages has slowed, as a result of subthreshold leakage, and power density has increased with each new technology node. This has forced a move toward multi-core architectures, but the energy efficiency benefits of parallelism are limited by the sub-thresahold leakage and the minimum energy point for a given function. Avoiding this roadblock requires an alternative device with more ideal switching characteristics. One promising class of such devices is the electro-statically actuated micro-electro-mechanical (MEM) relay which offers zero leakage current and abrupt turn-on behavior. Although a MEM relay is inherently slower than a CMOS transistor due to the mechanical movement, we have developed circuit design methodologies to mitigate this problem at the system level. This thesis explores such design optimization techniques and investigates the viability of MEM relays as an alternative switching technology for very-large scale integration (VLSI) applications. In the first part of this thesis, the feasibility of MEM relays for power management applications is discussed. Due to their negligibly low leakage, in certain applications, chips utilizing power gates built with MEM relays can achieve lower total energy than those built with CMOS transistors. A simple comparative analysis is presented and provides design guidelines and energy savings estimates as a function of technology parameters, and quantifies the further benefits of scaled relay designs. We also demonstrate a relay chip successfully power-gating a CMOS chip, and show a relay-based pulse generator suitable for self-timed operation. Going beyond power-gating applications, this work also describes circuit techniques and trade-offs for logic design with MEM-relays, focusing on multipliers which are commonly known as the most complex arithmetic units in a digital system. These techniques leverage the large disparity between mechanical and electrical time-constants of a relay, partitioning the logic into large, complex gates to minimize the effect of mechanical delay and improve circuit performance. At the component design level, innovations in compressor unit design minimize the required number of relays for each block and facilitate component cascading with no delay penalty. We analyze the area/energy/delay trade-offs vs. CMOS designs, for typical bit-widths, and show that scaled relays offer 10-20x lower energy per operation for moderate throughputs (<10-100MOPS). In addition to this analysis, we demonstrate the functionality of some of the most complex MEM relay circuits reported to date. Finally, considering the importance of signal generation and transmission in VLSI systems, this thesis presents MEM relay-based I/O units, focusing on design and demonstration of digital to analog converters (DAC). It also explores the concept of faster-than-mechanical-delay signal transmission.by Hossein Fariborzi.Ph.D

Cross-Layer Optimization for Power-Efficient and Robust Digital Circuits and Systems

With the increasing digital services demand, performance and power-efficiency become vital requirements for digital circuits and systems. However, the enabling CMOS technology scaling has been facing significant challenges of device uncertainties, such as process, voltage, and temperature variations. To ensure system reliability, worst-case corner assumptions are usually made in each design level. However, the over-pessimistic worst-case margin leads to unnecessary power waste and performance loss as high as 2.2x. Since optimizations are traditionally confined to each specific level, those safe margins can hardly be properly exploited. To tackle the challenge, it is therefore advised in this Ph.D. thesis to perform a cross-layer optimization for digital signal processing circuits and systems, to achieve a global balance of power consumption and output quality. To conclude, the traditional over-pessimistic worst-case approach leads to huge power waste. In contrast, the adaptive voltage scaling approach saves power (25% for the CORDIC application) by providing a just-needed supply voltage. The power saving is maximized (46% for CORDIC) when a more aggressive voltage over-scaling scheme is applied. These sparsely occurred circuit errors produced by aggressive voltage over-scaling are mitigated by higher level error resilient designs. For functions like FFT and CORDIC, smart error mitigation schemes were proposed to enhance reliability (soft-errors and timing-errors, respectively). Applications like Massive MIMO systems are robust against lower level errors, thanks to the intrinsically redundant antennas. This property makes it applicable to embrace digital hardware that trades quality for power savings.Comment: 190 page