计算机系统体系结构的层次设计

Hierarchical Architecture Design of Computer System

计算机系统由许多连通的层和子系统组成,这些层和子系统的交互模式复杂,整体体系结构设计尤为重要.在计算机系统体系结构演变的过程中形成了一些基本设计原理,其中层次设计是设计大规模系统体系结构的有效途径.从Dijkstra的工作开始,计算机系统体系结构的层次设计原理已经被提出很长时间,是计算机系统体系结构设计的重要组成部分.它广泛存在于计算机体系结构设计、网络体系结构设计、云计算、网络虚拟化、软件工程以及计算机科学的很多其他分支.计算机科学技术的演变和革新异常频繁,适用范围广的层次设计模型框架和层次设计方法尤为重要.虽然有不少工作对体系结构层次设计进行研究,但很少有工作对层次设计原则和方法的内涵进行探索,同时缺少统一的层次设计模型框架和评价指标.现有工作的不足主要表现为:(1)对计算机系统层次设计的描述通常是非形式化阐述;(2)现有的层次设计分析以具体系统和应用分析为主,缺少对层次设计机制内涵的理解分析;(3)现有层次设计模型主要局限于所研究的对象系统,缺少统一的层次设计模型框架和评价指标.针对计算机系统层次结构设计的上述不足,该文首先给出了层次设计相关的基本概念及其形式化定义,然后对层次设计研究现状从层次模型设计、层次构件设计、层次跨层设计和层次覆盖设计四个方面进行归类综述.层次模型设计主要包含层次描述模型和层次量化模型,对层次结构针对对象系统特征进行数学描述和推导;层次构件设计将层次结构的某一子结构以单一或较少模块抽象来实现;层次跨层设计是指打破既定层次结构,根据特定需求生成新的层间交互关系;层次覆盖主要是指以虚拟节点和逻辑连接构成的灵活的虚拟平面设计.在此基础上,对层次设计的内涵、设计原则、主要机制和设计路径进行探究和归纳.该文认为简化和效率是计算机系统体系结构层次设计的两个设计原则,抽象和虚拟是支撑设计原则的两个设计机制.相应地,该文给出计算机系统体系结构层次设计复杂性和性能的模型评价框架,得到一些基本定理.该文还对超级计算机系统、软件定义网络和云计算三个层次设计经典系统例子进行讨论,并在文章的结尾对计算机系统体系结构层次设计的进一步研究进行展望.

Computer system is composed by several layers and sub-systems which link with each other with complex interactions between them. The overall architecture design of computer system is very important. Many design principles were proposed during the evolution and development of computer system architecture, among which hierarchical architecture design is an efficient way for large system design. Since the literature elaborated by Dijkstra, hierarchical architecture design principle of computer system architecture has been proposed for many years, and it is an important part of computer system architecture design. Hierarchy design exists widely in computer science and system, such as computer architecture design, network architecture design, cloud computing,network virtualization, soitware engmeelms technology is evolving and renovating with a high frequency, and it is extremely important to propose a general model framework and design mechanism for hierarchy design. Though some literatures have been explored on hierarchy design of system architecture, few works explored the essence of the principles and the methods of hierarchy design, let alone general model framework and evaluation metric. The main insufficiencies appear as follows： （1） the descriptions of hierarchy design lack of formal descriptions; （2） the analyses of hierarchy design mainly focus on specific system and application lacking of understanding on the essence of the principles and the methods; （3） the models of hierarchy design are also limited to the target system lack of general model framework and evaluation metrics. This paper shed lights on these mentioned issues of hierarchical architecture design of computer systems in the following procedures. Firstly, the basic concepts and preliminary formal definitions related to hierarchy design are given. After that, this paper surveys the state of the arts of hierarchy design through four categories named as hierarchy model design, hierarchy component design, hierarchy cross-layer design, and hierarchy overlay design respectively. Hierarchy model design includes description model and quantification model giving mathematical description and deduction to specific characteristics of the target system; Hierarchy component design abstractsa given substructure of the target system into a simple one with its core function; Hierarchy cross-layer design generates new virtual links and interactions beside the original hierarchical structure for specific demands; Hierarchy overlay design uses virtual nodes and logical links to form a flexible virtual plane. On that basis, the essence of basic concepts, design principles, main mechanisms, and implementation method of hierarchy design are explored. We summarize simpli- fication and efficiency as two essential principles. They also imply the objectives of hierarchy design at the same time. These two principles are achieved by abstraction mechanism and virtualization mechanism which are formally defined by plane mappings. Accordingly, we establish an evaluation framework for complexity and performance of hierarchical structure, give their formal expressions, and obtain some theorems. This paper also makes essential introduction and discussion on the hierarchical structure of three typical systems, i.e. super computer system, software defined network, and cloud computing system with complexity and performance evaluation methodology. Conclusion and prospective future research challenges are summarized at the end of this paper.