A New Approach to Object Oriented Simulation of Concurrent Systems

High speed simulation of concurrent systems requires distributed processing if meaningful results are to be obtained for large systems in a reasonable timeframe. One of the most common methods used for such simulation is Parallel Discrete Event Simulation (PDES). A range of PDES simulation kernels have been developed and much research has been devoted to optimistic execution strategies such as TimeWarp. Unfortunately in all this e ort some fundamental aspects of object oriented modelling for simulation have received scant attention. In particular the ability of simulation kernels to act on truly generic simulation objects. In this context we de ne a truly generic object to be one which totally de nes its responeses to external stimuli, but which has no concept of its place in the interconnected web of objects that comprise the total simulation environment. To address this problem, we propose a new modelling approach based on interacting objects, and an associated simulation kernel architecture. This paper describes the architecture and features of our simulation kernel in detail, and demonstrates, using a small example, the bene ts of adopting such a modelling approach. The major speci c bene ts include true object genericity, enhanced scope for object re-use, and enhanced opportunities to use polymorphism. Keywords: object-oriented simulator, generic distributed kernel, parallel and distributed systems This paper describes the design and implementation of OBJECTSIM. OBJECTSIM represents a new exible and generic approach to performance modelling for parallel and distributed systems. The design is based on a powerful combination of a generic object library (with its associated hierarchies) and a distributed simulation kernel. The introduction motivates the development of OBJECTSIM. Design choices are presented in the context of recent research and the problems with related tools are highlighted. The main body of the paper outlines the conceptual design of OBJECTSIM, and demonstrates how the design addresses the key issues identi ed. The superiority of the OBJECTSIM tool is demonstrated using an example. The paper concludes that the OBJECTSIM approach has produced an e cient and powerful generic tool for simulating parallel and distributed systems. A nal paragraph provides a brief discussion of future directions. Simulation is one of the most important basic tools available to designers and implementers of large scale computers. Development of CPU and system architectures can no longer be realistically accomplished using prototyping approaches. There are two reasons for this. First, the cost of prototyping is very high in terms of both hardware fabrication expenses and time. Second, modern systems are becoming increasingly complex. Thus performance and system behaviour are much harder to predict and evaluate than was previously the case. Architecture simulation provides a cost e ective means to: analyse the performance of a system (in order to better understand its operation); predict system computational power (in terms of the processing capability available through its instruction set); and measure expected temporal performance. Simulation may also be used to evaluate system reliability and fault tolerance and provide support for real-time decisions and interactive training. Programmers also use simulation to evaluate novel programming models for future systems. Simulations are currently used to evaluate processor designs, computer networks, and novel multiprocessor architectures.
Other author information: (Send correspondence to Arnold N. Pears) Arnold N. Pears: Email: pears@.au; Telephone: 61-3-9479 1144; Fax: 61-3-9479 306
Once simulated and precisely evaluated, a design (whether it be for a chip, computer , or network) can be realised with the most up-to-date technology. This allows designers to avoid situations where a nal system has been realised incrementally over many years of prototyping, resulting in a product which is obsolete before it reaches the marketplace. Moreover, new developments made in hardware, software or formal system design can easily be integrated into the system at successive stages of the design. OBJECTSIM draws upon research in a range of elds to produce an e cient object oriented approach to modelling and to support the simulation of the resulting models. To address the issues involved in modelling concurent systems OBJECTSIM uses research on object oriented design, and simulation methodology. An investigation of simulationtechniques has provided the necessary background to develop an e cient simulation kernel architecture. Recent research on the principles and structure of object oriented systems and simulation environments has provided valuable insights for developers of object oriented models. In particular Page and Nance, who emphasise the importance of system correctness and testability, software quality, and the ability to provide a good match between the structure of the system being modelled and the forms of expression provided by the simulation environment. Another important result is the attempt by Tanir and Sevinc, to establish a reference model which characterises the constructs of simulation environments. They emphasise the choice of a general-purpose programming language and suggest that an object oriented languages with high level data structures is to be preferred. Interestingly, they opine that the choice of strongly typed languages when implementing simulation environments may impose restrictions that do not improve the modelling capabilities of the system. In this context C++ seemed the obvious choice for implementingthe OBJECTSIM prototype. The object oriented nature of C++ also assisted in realising the correctness and software quality requirements identi ed by Page and Nance.