온 칩 네트워크 설계: 매핑, 관리, 라우팅

학위논문 (박사)-- 서울대학교 대학원 : 전기·정보공학부, 2016. 2. 최기영.지난 수십 년간 이어진 반도체 기술의 향상은 매니 코어의 시대를 가져다 주었다. 우리가 일상 생활에 쓰는 데스크톱 컴퓨터조차도 이미 수 개의 코어를 가지고 있으며, 수백 개의 코어를 가진 칩도 상용화되어 있다. 이러한 많은 코어들 간의 통신 기반으로서, 네트워크-온-칩(NoC)이 새로이 대두되었으며, 이는 현재 많은 연구 및 상용 제품에서 널리 사용되고 있다. 그러나 네트워크-온-칩을 매니 코어 시스템에 사용하는 데에는 여러 가지 문제가 따르며, 본 논문에서는 그 중 몇 가지를 풀어내고자 하였다. 본 논문의 두 번째 챕터에서는 NoC 기반 매니코어 구조에 작업을 할당하고 스케쥴하는 방법을 다루었다. 매니코어에의 작업 할당을 다룬 논문은 이미 많이 출판되었지만, 본 연구는 메시지 패싱과 공유 메모리, 두 가지의 통신 방식을 고려함으로써 성능과 에너지 효율을 개선하였다. 또한, 본 연구는 역방향 의존성을 가진 작업 그래프를 스케쥴하는 방법 또한 제시하였다. 3차원 적층 기술은 높아진 전력 밀도 때문에 열 문제가 심각해지는 등, 여러 가지 도전 과제를 내포하고 있다. 세 번째 챕터에서는 DVFS 기술을 이용하여 열 문제를 완화하고자 하는 기술을 소개한다. 각 코어와 라우터가 전압, 작동 속도를 조절할 수 있는 구조에서, 가장 높은 성능을 이끌어 내면서도 최대 온도를 넘어서지 않도록 한다. 세 번째와 네 번째 챕터는 조금 다른 측면을 다룬다. 3D 적층 기술을 사용할 때, 층간 통신은 주로 TSV를 이용하여 이루어진다. 그러나 TSV는 일반 wire보다 훨씬 큰 면적을 차지하기 때문에, 전체 네트워크에서의 TSV 개수는 제한되어야 할 경우가 많다. 이 경우에는 두 가지 선택지가 있는데, 첫째는 각 층간 통신 채널의 대역폭을 줄이는 것이고, 둘째는 각 채널의 대역폭은 유지하되 일부 노드만 층간 통신이 가능한 채널을 제공하는 것이다. 우리는 각각의 경우에 대하여 라우팅 알고리즘을 하나씩 제시한다. 첫 번째 경우에 있어서는 deflection 라우팅 기법을 사용하여 층간 통신의 긴 지연 시간을 극복하고자 하였다. 층간 통신을 균등하게 분배함으로써, 제시된 알고리즘은 개선된 지연 시간을 보이며 라우터 버퍼의 제거를 통한 면적 및 에너지 효율성 또한 얻을 수 있다. 두 번째 경우에서는 층간 통신 채널을 선택하기 위한 몇 가지 규칙을 제시한다. 약간의 라우팅 자유도를 희생함으로써, 제시된 알고리즘은 기존 알고리즘의 가상 채널 요구 조건을 제거하고, 결과적으로는 성능 또는 에너지 효율의 증가를 가져 온다.For decades, advance in semiconductor technology has led us to the era of many-core systems. Today's desktop computers already have multi-core processors, and chips with more than a hundred cores are commercially available. As a communication medium for such a large number of cores, network-on-chip (NoC) has emerged out, and now is being used by many researchers and companies. Adopting NoC for a many-core system incurs many problems, and this thesis tries to solve some of them. The second chapter of this thesis is on mapping and scheduling of tasks on NoC-based CMP architectures. Although mapping on NoC has a number of papers published, our work reveals that selecting communication types between shared memory and message passing can help improve the performance and energy efficiency. Additionally, our framework supports scheduling applications containing backward dependencies with the help of modified modulo scheduling. Evolving the SoCs through 3D stacking makes us face a number of new problems, and the thermal problem coming from increased power density is one of them. In the third chapter of this thesis, we try to mitigate the hotspot problem using DVFS techniques. Assuming that all the routers as well as cores have capabilities to control voltage and frequency individually, we find voltage-frequency pairs for all cores and routers which yields the best performance within the given thermal constraint. The fourth and the fifth chapters of this thesis are from a different aspect. In 3D stacking, inter-layer interconnections are implemented using through-silicon vias (TSV). TSVs usually take much more area than normal wires. Furthermore, they also consume silicon area as well as metal area. For this reason, designers would want to limit the number of TSVs used in their network. To limit the TSV count, there are two options: the first is to reduce the width of each vertical links, and the other is to use fewer vertical links, which results in a partially connected network. We present two routing methodologies for each case. For the network with reduced bandwidth vertical links, we propose using deflection routing to mitigate the long latency of vertical links. By balancing the vertical traffics properly, the algorithm provides improved latency. Also, a large amount of area and energy reduction can be obtained by the removal of router buffers. For partially connected networks, we introduce a set of routing rules for selecting the vertical links. At the expense of sacrificing some amount of routing freedom, the proposed algorithm removes the virtual channel requirement for avoiding deadlock. As a result, the performance, or energy consumption can be reduced at the designer's choice.Chapter 1 Introduction 1 1.1 Task Mapping and Scheduling 2 1.2 Thermal Management 3 1.3 Routing for 3D Networks 5 Chapter 2 Mapping and Scheduling 9 2.1 Introduction 9 2.2 Motivation 10 2.3 Background 12 2.4 Related Work 16 2.5 Platform Description 17 2.5.1 Architcture Description 17 2.5.2 Energy Model 21 2.5.3 Communication Delay Model 22 2.6 Problem Formulation 23 2.7 Proposed Solution 25 2.7.1 Task and Communication Mapping 27 2.7.2 Communication Type Optimization 31 2.7.3 Design Space Pruning via Pre-evaluation 34 2.7.4 Scheduling 35 2.8 Experimental Results 42 2.8.1 Experiments with Coarse-grained Iterative Modulo Scheduling 42 2.8.2 Comparison with Different Mapping Algorithms 43 2.8.3 Experiments with Overall Algorithms 45 2.8.4 Experiments with Various Local Memory Sizes 47 2.8.5 Experiments with Various Placements of Shared Memory 48 Chapter 3 Thermal Management 50 3.1 Introduction 50 3.2 Background 51 3.2.1 Thermal Modeling 51 3.2.2 Heterogeneity in Thermal Propagation 52 3.3 Motivation and Problem Definition 53 3.4 Related Work 56 3.5 Orchestrated Voltage-Frequency Assignment 56 3.5.1 Individual PI Control Method 56 3.5.2 PI Controlled Weighted-Power Budgeting 57 3.5.3 Performance/Power Estimation 59 3.5.4 Frequency Assignment 62 3.5.5 Algorithm Overview 64 3.5.6 Stability Conditions for PI Controller 65 3.6 Experimental Result 66 3.6.1 Experimental Setup 66 3.6.2 Overall Algorithm Performance 68 3.6.3 Accuracy of the Estimation Model 70 3.6.4 Performance of the Frequency Assignment Algorithm 70 Chapter 4 Routing for Limited Bandwidth 3D NoC 72 4.1 Introduction 72 4.2 Motivation 73 4.3 Background 74 4.4 Related Work 75 4.5 3D Deflection Routing 76 4.5.1 Serialized TSV Model 76 4.5.2 TSV Link Injection/ejection Scheme 78 4.5.3 Deadlock Avoidance 80 4.5.4 Livelock Avoidance 84 4.5.5 Router Architecture: Putting It All Together 86 4.5.6 System Level Consideration 87 4.6 Experimental Results 89 4.6.1 Experimental Setup 89 4.6.2 Results on Synthetic Traffic Patterns 91 4.6.3 Results on Realistic Traffic Patterns 94 4.6.4 Results on Real Application Benchmarks 98 4.6.5 Fairness Issue 103 4.6.6 Area Cost Comparison 104 Chapter 5 Routing for Partially Connected 3D NoC 106 5.1 Introduction 106 5.2 Background 107 5.3 Related Work 109 5.4 Proposed Algorithm 111 5.4.1 Preliminary 112 5.4.2 Routing Algorithm for 3-D Stacked Meshes with Regular Partial Vertical Connections 115 5.4.3 Routing Algorithm for 3-D Stacked Meshes with Irregular Partial Vertical Connections 118 5.4.4 Extension to Heterogeneous Mesh Layers 122 5.5 Experimental Results 126 5.5.1 Experimental Setup 126 5.5.2 Experiments on Synthetic Traffics 128 5.5.3 Experiments on Application Benchmarks 133 5.5.4 Comparison with Reduced Bandwidth Mesh 139 Chapter 6 Conclusion 141 Bibliography 144 초록 163Docto

Software-based and regionally-oriented traffic management in Networks-on-Chip

Since the introduction of chip-multiprocessor systems, the number of integrated cores has been steady growing and workload applications have been adapted to exploit the increasing parallelism. This changed the importance of efficient on-chip communication significantly and the infrastructure has to keep step with these new requirements. The work at hand makes significant contributions to the state-of-the-art of the latest generation of such solutions, called Networks-on-Chip, to improve the performance, reliability, and flexible management of these on-chip infrastructures

Energy-aware synthesis for networks on chip architectures

The Network on Chip (NoC) paradigm was introduced as a scalable communication infrastructure for future System-on-Chip applications. Designing application specific customized communication architectures is critical for obtaining low power, high performance solutions. Two significant design automation problems are the creation of an optimized configuration, given application requirement the implementation of this on-chip network. Automating the design of on-chip networks requires models for estimating area and energy, algorithms to effectively explore the design space and network component libraries and tools to generate the hardware description. Chip architects are faced with managing a wide range of customization options for individual components, routers and topology. As energy is of paramount importance, the effectiveness of any custom NoC generation approach lies in the availability of good energy models to effectively explore the design space. This thesis describes a complete NoC synthesis flow, called NoCGEN, for creating energy-efficient custom NoC architectures. Three major automation problems are addressed: custom topology generation, energy modeling and generation. An iterative algorithm is proposed to generate application specific point-to-point and packet-switched networks. The algorithm explores the design space for efficient topologies using characterized models and a system-level floorplanner for evaluating placement and wire-energy. Prior to our contribution, building an energy model required careful analysis of transistor or gate implementations. To alleviate the burden, an automated linear regression-based methodology is proposed to rapidly extract energy models for many router designs. The resulting models are cycle accurate with low-complexity and found to be within 10% of gate-level energy simulations, and execute several orders of magnitude faster than gate-level simulations. A hardware description of the custom topology is generated using a parameterizable library and custom HDL generator. Fully reusable and scalable network components (switches, crossbars, arbiters, routing algorithms) are described using a template approach and are used to compose arbitrary topologies. A methodology for building and composing routers and topologies using a template engine is described. The entire flow is implemented as several demonstrable extensible tools with powerful visualization functionality. Several experiments are performed to demonstrate the design space exploration capabilities and compare it against a competing min-cut topology generation algorithm