Dynamic fine-grain body biasing of caches with latency and leakage 3T1D-based monitors

In this paper, we propose a dynamically tunable fine-grain body biasing mechanism to reduce active & standby leakage power in caches under process variations.Preprin

Reliability in the face of variability in nanometer embedded memories

In this thesis, we have investigated the impact of parametric variations on the behaviour of one performance-critical processor structure - embedded memories. As variations manifest as a spread in power and performance, as a first step, we propose a novel modeling methodology that helps evaluate the impact of circuit-level optimizations on architecture-level design choices. Choices made at the design-stage ensure conflicting requirements from higher-levels are decoupled. We then complement such design-time optimizations with a runtime mechanism that takes advantage of adaptive body-biasing to lower power whilst improving performance in the presence of variability. Our proposal uses a novel fully-digital variation tracking hardware using embedded DRAM (eDRAM) cells to monitor run-time changes in cache latency and leakage. A special fine-grain body-bias generator uses the measurements to generate an optimal body-bias that is needed to meet the required yield targets. A novel variation-tolerant and soft-error hardened eDRAM cell is also proposed as an alternate candidate for replacing existing SRAM-based designs in latency critical memory structures. In the ultra low-power domain where reliable operation is limited by the minimum voltage of operation (Vddmin), we analyse the impact of failures on cache functional margin and functional yield. Towards this end, we have developed a fully automated tool (INFORMER) capable of estimating memory-wide metrics such as power, performance and yield accurately and rapidly. Using the developed tool, we then evaluate the #effectiveness of a new class of hybrid techniques in improving cache yield through failure prevention and correction. Having a holistic perspective of memory-wide metrics helps us arrive at design-choices optimized simultaneously for multiple metrics needed for maintaining lifetime requirements

Subthreshold SRAM Design for Energy Efficient Applications in Nanometric CMOS Technologies

Embedded SRAM circuits are vital components in a modern system on chip (SOC) that can occupy up to 90% of the total area. Therefore, SRAM circuits heavily affect SOC performance, reliability, and yield. In addition, most of the SRAM bitcells are in standby mode and significantly contribute to the total leakage current and leakage power consumption. The aggressive demand in portable devices and billions of connected sensor networks requires long battery life. Therefore, careful design of SRAM circuits with minimal power consumption is in high demand. Reducing the power consumption is mainly achieved by reducing the power supply voltage in the idle mode. However, simply reducing the supply voltage imposes practical limitations on SRAM circuits such as reduced static noise margin, poor write margin, reduced number of cells per bitline, and reduced bitline sensing margin that might cause read/write failures. In addition, the SRAM bitcell has contradictory requirements for read stability and writability. Improving the read stability can cause difficulties in a write operation or vice versa. In this thesis, various techniques for designing subthreshold energy-efficient SRAM circuits are proposed. The proposed techniques include improvement in read margin and write margin, speed improvement, energy consumption reduction, new bitcell architecture and utilizing programmable wordline boosting. A programmable wordline boosting technique is exploited on a conventional 6T SRAM bitcell to improve the operational speed. In addition, wordline boosting can reduce the supply voltage while maintaining the operational frequency. The reduction of the supply voltage allows the memory macro to operate with reduced power consumption. To verify the design, a 16-kb SRAM was fabricated using the TSMC 65 nm CMOS technology. Measurement results show that the maximum operational frequency increases up to 33.3% when wordline boosting is applied. Besides, the supply voltage can be reduced while maintaining the same frequency. This allows reducing the energy consumption to be reduced by 22.2%. The minimum energy consumption achieved is 0.536 fJ/b at 400 mV. Moreover, to improve the read margin, a 6T bitcell SRAM with a PMOS access transistor is proposed. Utilizing a PMOS access transistor results in lower zero level degradation, and hence higher read stability. In addition, the access transistor connected to the internal node holding V DD acts as a stabilizer and counterbalances the effect of zero level degradation. In order to improve the writability, wordline boosting is exploited. Wordline boosting also helps to compensate for the lower speed of the PMOS access transistor compared to a NMOS transistor. To verify our design, a 2kb SRAM is fabricated in the TSMC 65 nm CMOS technology. Measurement results show that the maximum operating frequency of the test chip is at 3.34 MHz at 290 mV. The minimum energy consumption is measured as 1.1 fJ/b at 400 mV

Fault- and Yield-Aware On-Chip Memory Design and Management

Ever decreasing device size causes more frequent hard faults, which becomes a serious burden to processor design and yield management. This problem is particularly pronounced in the on-chip memory which consumes up to 70% of a processor' s total chip area. Traditional circuit-level techniques, such as redundancy and error correction code, become less effective in error-prevalent environments because of their large area overhead. In this work, we suggest an architectural solution to building reliable on-chip memory in the future processor environment. Our approaches have two parts, a design framework and architectural techniques for on-chip memory structures. Our design framework provides important architectural evaluation metrics such as yield, area, and performance based on low level defects and process variations parameters. Processor architects can quickly evaluate their designs' characteristics in terms of yield, area, and performance. With the framework, we develop architectural yield enhancement solutions for on-chip memory structures including L1 cache, L2 cache and directory memory. Our proposed solutions greatly improve yield with negligible area and performance overhead. Furthermore, we develop a decoupled yield model of compute cores and L2 caches in CMPs, which show that there will be many more L2 caches than compute cores in a chip. We propose efficient utilization techniques for excess caches. Evaluation results show that excess caches significantly improve overall performance of CMPs

Exploiting Adaptive Techniques to Improve Processor Energy Efficiency

Rapid device-miniaturization keeps on inducing challenges in building energy efficient microprocessors. As the size of the transistors continuously decreasing, more uncertainties emerge in their operations. On the other hand, integrating more and more transistors on a single chip accentuates the need to lower its supply-voltage. This dissertation investigates one of the primary device uncertainties - timing error, in microprocessor performance bottleneck in NTC era. Then it proposes various innovative techniques to exploit these opportunities to maintain processor energy efficiency, in the context of emerging challenges. Evaluated with the cross-layer methodology, the proposed approaches achieve substantial improvements in processor energy efficiency, compared to other start-of-art techniques

Multi-criteria optimization for energy-efficient multi-core systems-on-chip

The steady down-scaling of transistor dimensions has made possible the evolutionary progress leading to today’s high-performance multi-GHz microprocessors and core based System-on-Chip (SoC) that offer superior performance, dramatically reduced cost per function, and much-reduced physical size compared to their predecessors. On the negative side, this rapid scaling however also translates to high power densities, higher operating temperatures and reduced reliability making it imperative to address design issues that have cropped up in its wake. In particular, the aggressive physical miniaturization have increased CMOS fault sensitivity to the extent that many reliability constraints pose threat to the device normal operation and accelerate the onset of wearout-based failures. Among various wearout-based failure mechanisms, Negative biased temperature instability (NBTI) has been recognized as the most critical source of device aging. The urge of reliable, low-power circuits is driving the EDA community to develop new design techniques, circuit solutions, algorithms, and software, that can address these critical issues. Unfortunately, this challenge is complicated by the fact that power and reliability are known to be intrinsically conflicting metrics: traditional solutions to improve reliability such as redundancy, increase of voltage levels, and up-sizing of critical devices do contrast with traditional low-power solutions, which rely on compact architectures, scaled supply voltages, and small devices. This dissertation focuses on methodologies to bridge this gap and establishes an important link between low-power solutions and aging effects. More specifically, we proposed new architectural solutions based on power management strategies to enable the design of low-power, aging aware cache memories. Cache memories are one of the most critical components for warranting reliable and timely operation. However, they are also more susceptible to aging effects. Due to symmetric structure of a memory cell, aging occurs regardless of the fact that a cell (or word) is accessed or not. Moreover, aging is a worst-case matric and line with worst-case access pattern determines the aging of the entire cache. In order to stop the aging of a memory cell, it must be put into a proper idle state when a cell (or word) is not accessed which require proper management of the idleness of each atomic unit of power management. We have proposed several reliability management techniques based on the idea of cache partitioning to alleviate NBTI-induced aging and obtain joint energy and lifetime benefits. We introduce graceful degradation mechanism which allows different cache blocks into which a cache is partitioned to age at different rates. This implies that various sub-blocks become unreliable at different times, and the cache keeps functioning with reduced efficiency. We extended the capabilities of this architecture by integrating the concept of reconfigurable caches to maintain the performance of the cache throughout its lifetime. By this strategy, whenever a block becomes unreliable, the remaining cache is reconfigured to work as a smaller size cache with only a marginal degradation of performance. Many mission-critical applications require guaranteed lifetime of their operations and therefore the hardware implementing their functionality. Such constraints are usually enforced by means of various reliability enhancing solutions mostly based on redundancy which are not energy-friendly. In our work, we have proposed a novel cache architecture in which a smart use of cache partitions for redundancy allows us to obtain cache that meet a desired lifetime target with minimal energy consumption

MANAGING AND LEVERAGING VARIATIONS AND NOISE IN NANOMETER CMOS

Advanced CMOS technologies have enabled high density designs at the cost of complex fabrication process. Variation in oxide thickness and Random Dopant Fluctuation (RDF) lead to variation in transistor threshold voltage Vth. Current photo-lithography process used for printing decreasing critical dimensions result in variation in transistor channel length and width. A related challenge in nanometer CMOS is that of on-chip random noise. With decreasing threshold voltage and operating voltage; and increasing operating temperature, CMOS devices are more sensitive to random on-chip noise in advanced technologies. In this thesis, we explore novel circuit techniques to manage the impact of process variation in nanometer CMOS technologies. We also analyze the impact of on-chip noise on CMOS circuits and propose techniques to leverage or manage impact of noise based on the application. True Random Number Generator (TRNG) is an interesting cryptographic primitive that leverages on-chip noise to generate random bits; however, it is highly sensitive to process variation. We explore novel metastability circuits to alleviate the impact of variations and at the same time leverage on-chip noise sources like Random Thermal Noise and Random Telegraph Noise (RTN) to generate high quality random bits. We develop stochastic models for metastability based TRNG circuits to analyze the impact of variation and noise. The stochastic models are used to analyze and compare low power, energy efficient and lightweight post-processing techniques targeted to low power applications like System on Chip (SoC) and RFID. We also propose variation aware circuit calibration techniques to increase reliability. We extended this technique to a more generic application of designing Post-Si Tunable (PST) clock buffers to increase parametric yield in the presence of process variation. Apart from one time variation due to fabrication process, transistors undergo constant change in threshold voltage due to aging/wear-out effects and RTN. Process variation affects conventional sensors and introduces inaccuracies during measurement. We present a lightweight wear-out sensor that is tolerant to process variation and provides a fine grained wear-out sensing. A similar circuit is designed to sense fluctuation in transistor threshold voltage due to RTN. Although thermal noise and RTN are leveraged in applications like TRNG, they affect the stability of sensitive circuits like Static Random Access Memory (SRAM). We analyze the impact of on-chip noise on Bit Error Rate (BER) and post-Si test coverage of SRAM cells

Circuits and Systems Advances in Near Threshold Computing

Modern society is witnessing a sea change in ubiquitous computing, in which people have embraced computing systems as an indispensable part of day-to-day existence. Computation, storage, and communication abilities of smartphones, for example, have undergone monumental changes over the past decade. However, global emphasis on creating and sustaining green environments is leading to a rapid and ongoing proliferation of edge computing systems and applications. As a broad spectrum of healthcare, home, and transport applications shift to the edge of the network, near-threshold computing (NTC) is emerging as one of the promising low-power computing platforms. An NTC device sets its supply voltage close to its threshold voltage, dramatically reducing the energy consumption. Despite showing substantial promise in terms of energy efficiency, NTC is yet to see widescale commercial adoption. This is because circuits and systems operating with NTC suffer from several problems, including increased sensitivity to process variation, reliability problems, performance degradation, and security vulnerabilities, to name a few. To realize its potential, we need designs, techniques, and solutions to overcome these challenges associated with NTC circuits and systems. The readers of this book will be able to familiarize themselves with recent advances in electronics systems, focusing on near-threshold computing

Active timing margin management to improve microprocessor power efficiency

Improving power/performance efficiency is critical for today’s micro- processors. From edge devices to datacenters, lower power or higher performance always produces better systems, measured by lower cost of ownership or longer battery time. This thesis studies improving microprocessor power/performance efficiency by optimizing the pipeline timing margin. In particular, this thesis focuses on improving the efficacy of Active Timing Margin, a young technology that dynamically adjusts the margin. Active timing margin trims down the pipeline timing margin with a control loop that adjusts voltage and frequency based on real-time chip environment monitoring. The key insight of this thesis is that in order to maximize active timing margin’s efficiency enhancement benefits, synergistic management from processor architecture design and system software scheduling are needed. To that end, this thesis covers the major consumers of pipeline timing margin, including temperature, voltage, and process variation. For temperature variation, the thesis proposes a table-lookup based active timing margin mechanism, and an associated temperature management scheme to minimize power consumption. For voltage variation, the thesis characterizes the limiting factors of adaptive clocking’s power saving and proposes application scheduling to maximize total system power reduction. For process variation, the thesis proposes core-level adaptive clocking reconfiguration to automatically expose inter-core variation and discusses workload scheduling and throttling management to control critical application performance. The author believes the optimization presented in this thesis can potentially benefit a variety of processor architectures as the conclusions are based on the solid measurement on state-of-the-art processors, and the research objective, active timing margin, already has wide applicability in the latest microprocessors by the time this thesis is written.Electrical and Computer Engineerin

상변화 메모리 시스템의 간섭 오류 완화 및 RMW 성능 향상 기법

학위논문(박사) -- 서울대학교대학원 : 공과대학 전기·정보공학부, 2021.8. 이혁재.Phase-change memory (PCM) announces the beginning of the new era of memory systems, owing to attractive characteristics. Many memory product manufacturers (e.g., Intel, SK Hynix, and Samsung) are developing related products. PCM can be applied to various circumstances; it is not simply limited to an extra-scale database. For example, PCM has a low standby power due to its non-volatility; hence, computation-intensive applications or mobile applications (i.e., long memory idle time) are suitable to run on PCM-based computing systems. Despite these fascinating features of PCM, PCM is still far from the general commercial market due to low reliability and long latency problems. In particular, low reliability is a painful problem for PCM in past decades. As the semiconductor process technology rapidly scales down over the years, DRAM reaches 10 nm class process technology. In addition, it is reported that the write disturbance error (WDE) would be a serious issue for PCM if it scales down below 54 nm class process technology. Therefore, addressing the problem of WDEs becomes essential to make PCM competitive to DRAM. To overcome this problem, this dissertation proposes a novel approach that can restore meta-stable cells on demand by levering two-level SRAM-based tables, thereby significantly reducing the number WDEs. Furthermore, a novel randomized approach is proposed to implement a replacement policy that originally requires hundreds of read ports on SRAM. The second problem of PCM is a long-latency compared to that of DRAM. In particular, PCM tries to enhance its throughput by adopting a larger transaction unit; however, the different unit size from the general-purpose processor cache line further degrades the system performance due to the introduction of a read-modify-write (RMW) module. Since there has never been any research related to RMW in a PCM-based memory system, this dissertation proposes a novel architecture to enhance the overall system performance and reliability of a PCM-based memory system having an RMW module. The proposed architecture enhances data re-usability without introducing extra storage resources. Furthermore, a novel operation that merges commands regardless of command types is proposed to enhance performance notably. Another problem is the absence of a full simulation platform for PCM. While the announced features of the PCM-related product (i.e., Intel Optane) are scarce due to confidential issues, all priceless information can be integrated to develop an architecture simulator that resembles the available product. To this end, this dissertation tries to scrape up all available features of modules in a PCM controller and implement a dedicated simulator for future research purposes.상변화 메모리는(PCM) 매력적인 특성을 통해 메모리 시스템의 새로운 시대의 시작을 알렸다. 많은 메모리 관련 제품 제조업체(예 : 인텔, SK 하이닉스, 삼성)가 관련 제품 개발에 박차를 가하고 있다. PCM은 단순히 대규모 데이터베이스에만 국한되지 않고 다양한 상황에 적용될 수 있다. 예를 들어, PCM은 비휘발성으로 인해 대기 전력이 낮다. 따라서 계산 집약적인 애플리케이션 또는 모바일 애플리케이션은(즉, 긴 메모리 유휴 시간) PCM 기반 컴퓨팅 시스템에서 실행하기에 적합하다. PCM의 이러한 매력적인 특성에도 불구하고 PCM은 낮은 신뢰성과 긴 대기 시간으로 인해 여전히 일반 산업 시장에서는 DRAM과 다소 격차가 있다. 특히 낮은 신뢰성은 지난 수십 년 동안 PCM 기술의 발전을 저해하는 문제다. 반도체 공정 기술이 수년에 걸쳐 빠르게 축소됨에 따라 DRAM은 10nm 급 공정 기술에 도달하였다. 이어서, 쓰기 방해 오류 (WDE)가 54nm 등급 프로세스 기술 아래로 축소되면 PCM에 심각한 문제가 될 것으로 보고되었다. 따라서, WDE 문제를 해결하는 것은 PCM이 DRAM과 동등한 경쟁력을 갖추도록 하는 데 있어 필수적이다. 이 문제를 극복하기 위해 이 논문에서는 2-레벨 SRAM 기반 테이블을 활용하여 WDE 수를 크게 줄여 필요에 따라 준 안정 셀을 복원할 수 있는 새로운 접근 방식을 제안한다. 또한, 원래 SRAM에서 수백 개의 읽기 포트가 필요한 대체 정책을 구현하기 위해 새로운 랜덤 기반의 기법을 제안한다. PCM의 두 번째 문제는 DRAM에 비해 지연 시간이 길다는 것이다. 특히 PCM은 더 큰 트랜잭션 단위를 채택하여 단위시간 당 데이터 처리량 향상을 도모한다. 그러나 범용 프로세서 캐시 라인과 다른 유닛 크기는 읽기-수정-쓰기 (RMW) 모듈의 도입으로 인해 시스템 성능을 저하하게 된다. PCM 기반 메모리 시스템에서 RMW 관련 연구가 없었기 때문에 본 논문은 RMW 모듈을 탑재 한 PCM 기반 메모리 시스템의 전반적인 시스템 성능과 신뢰성을 향상하게 시킬 수 있는 새로운 아키텍처를 제안한다. 제안된 아키텍처는 추가 스토리지 리소스를 도입하지 않고도 데이터 재사용성을 향상시킨다. 또한, 성능 향상을 위해 명령 유형과 관계없이 명령을 병합하는 새로운 작업을 제안한다. 또 다른 문제는 PCM을 위한 완전한 시뮬레이션 플랫폼이 부재하다는 것이다. PCM 관련 제품(예 : Intel Optane)에 대해 발표된 정보는 대외비 문제로 인해 부족하다. 하지만 알려져 있는 정보를 적절히 취합하면 시중 제품과 유사한 아키텍처 시뮬레이터를 개발할 수 있다. 이를 위해 본 논문은 PCM 메모리 컨트롤러에 필요한 모든 모듈 정보를 활용하여 향후 이와 관련된 연구에서 충분히 사용 가능한 전용 시뮬레이터를 구현하였다.1 INTRODUCTION 1 1.1 Limitation of Traditional Main Memory Systems 1 1.2 Phase-Change Memory as Main Memory 3 1.2.1 Opportunities of PCM-based System 3 1.2.2 Challenges of PCM-based System 4 1.3 Dissertation Overview 7 2 BACKGROUND AND PREVIOUS WORK 8 2.1 Phase-Change Memory 8 2.2 Mitigation Schemes for Write Disturbance Errors 10 2.2.1 Write Disturbance Errors 10 2.2.2 Verification and Correction 12 2.2.3 Lazy Correction 13 2.2.4 Data Encoding-based Schemes 14 2.2.5 Sparse-Insertion Write Cache 16 2.3 Performance Enhancement for Read-Modify-Write 17 2.3.1 Traditional Read-Modify-Write 17 2.3.2 Write Coalescing for RMW 19 2.4 Architecture Simulators for PCM 21 2.4.1 NVMain 21 2.4.2 Ramulator 22 2.4.3 DRAMsim3 22 3 IN-MODULE DISTURBANCE BARRIER 24 3.1 Motivation 25 3.2 IMDB: In Module-Disturbance Barrier 29 3.2.1 Architectural Overview 29 3.2.2 Implementation of Data Structures 30 3.2.3 Modification of Media Controller 36 3.3 Replacement Policy 38 3.3.1 Replacement Policy for IMDB 38 3.3.2 Approximate Lowest Number Estimator 40 3.4 Putting All Together: Case Studies 43 3.5 Evaluation 45 3.5.1 Configuration 45 3.5.2 Architectural Exploration 47 3.5.3 Effectiveness of the Replacement Policy 48 3.5.4 Sensitivity to Main Table Configuration 49 3.5.5 Sensitivity to Barrier Buffer Size 51 3.5.6 Sensitivity to AppLE Group Size 52 3.5.7 Comparison with Other Studies 54 3.6 Discussion 59 3.7 Summary 63 4 INTEGRATION OF AN RMW MODULE IN A PCM-BASED SYSTEM 64 4.1 Motivation 65 4.2 Utilization of DRAM Cache for RMW 67 4.2.1 Architectural Design 67 4.2.2 Algorithm 70 4.3 Typeless Command Merging 73 4.3.1 Architectural Design 73 4.3.2 Algorithm 74 4.4 An Alternative Implementation: SRC-RMW 78 4.4.1 Implementation of SRC-RMW 78 4.4.2 Design Constraint 80 4.5 Case Study 82 4.6 Evaluation 85 4.6.1 Configuration 85 4.6.2 Speedup 88 4.6.3 Read Reliability 91 4.6.4 Energy Consumption: Selecting a Proper Page Size 93 4.6.5 Comparison with Other Studies 95 4.7 Discussion 97 4.8 Summary 99 5 AN ALL-INCLUSIVE SIMULATOR FOR A PCM CONTROLLER 100 5.1 Motivation 101 5.2 PCMCsim: PCM Controller Simulator 103 5.2.1 Architectural Overview 103 5.2.2 Underlying Classes of PCMCsim 104 5.2.3 Implementation of Contention Behavior 108 5.2.4 Modules of PCMCsim 109 5.3 Evaluation 116 5.3.1 Correctness of the Simulator 116 5.3.2 Comparison with Other Simulators 117 5.4 Summary 119 6 Conclusion 120 Abstract (In Korean) 141 Acknowledgment 143박