Browsing by Author "Page, Ernest H."

Now showing 1 - 7 of 7
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    Formally Reasoning About and Automatically Generating Sequential and Parallel Simulations
    Abrams, Marc; Page, Ernest H. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1992)
    This paper proposes a methodology to automate the construction of simulation programs within the context of a simulation support environment. The methodology starts with a simulation model specification in the form of a set of coupled state transition systems. The paper provides a mechanical method of mapping the transition systems first into a set of formal assertions, permitting formal verification of the transition systems, and second into an executable program. UNITY, a computational model and proof system suitable for development of parallel and distributed programs through step-wise refinement of specifications, is used as the specification and program notation. The methodology provides a means to independently verify the correctness of the transition systems: one can specify properties formally that the model should obey and prove them as theorems using the formal specification.
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    Linking Simulation Model Specification and Parallel Execution through UNITY
    Abrams, Marc; Page, Ernest H.; Nance, Richard E. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1991)
    Chandy and Misra's UNITY is a computation model and proof system suitable for development of parallel (and distributed) programs through step-wise refinement of specifications. UNITY supports the development of correct programs and the efficient implementation of those programs on parallel computer architectures. This paper assesses the potential of UNITY for simulation model specification and implementation by developing a UNITY specification of the machine interference problem with a patrolling repairman service discipline. The conclusions reached are that the UNITY proof system can assist formal verification of simulation models and the UNITY mappings of programs to various computer architectures offer some potential for assisting the automatic implementation of simulation models on parallel architectures. The paper gives some insights into the relationship of time flow mechanisms, parallel simulation protocols, and target parallel computer architectures.
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    Model Generation Issues in a Simulation Support Environment
    Balci, Osman; Nance, Richard E.; Derrick, Emory Joseph; Page, Ernest H.; Bishop, John L. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1990)
    No longer available as a technical report. Contact authors re reprints of published article.
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    Model generators: Prototyping simulation model definition, specification, and documentation under the conical methodology
    Page, Ernest H. (Virginia Tech, 1990)
    The process of model generation is key to the realization of a Simulation Model Development Environment. Model generation is facilitated in the environment via the Model Generator - a software utility that assists a modeler in the development of a simulation model specification. Since modeling is inherently creative, the correct assistance provided to a modeler can neither be derived algorithmically, nor proved mathematically. Only through experimentation with prototypical assistance forms can we begin to understand the meaning of correctness. This thesis describes the development of a Model Generator prototype for the Simulation Model Development Environment. A review of the literature indicates the need for more extensive questioning of the model generation process to identify the proper foundational support than 1s available in applications designed under the program generation approach. The Conical Methodology provides the Conceptual Framework, and the Condition Specification provides the target specification form for the Model Generator prototype. A set of algorithms to derive a condition specification via a series of interactive dialogues is presented, and the results of early prototype experimentation are discussed. New questions are raised as to the role of relational attributes in the Conical Methodology and the extent and types of model analysis provided by a Model Generator. Finally, an analysis of the Model Generator as a platform for the assessment of the Conical Methodology/Condition Specification is given and directions for future research outlined.
  • Parallel Discrete Event Simulation: A Modeling MethodologicalPerspective
    Page, Ernest H.; Nance, Richard E. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1994)
    The field of parallel discrete event simulation is entering a period of self-assessment. Fifteen years of investigation has seen great strides in techniques for efficiently executing discrete event simulations on parallel and distributed machines. Still, the discrete event simulation community at large has failed to recognize much of these results. One reason for this is perhaps a disagreement in the focus and purpose of the parallel discrete event simulation research community (primarily computer scientists) and the discrete event simulation community (a widely diverse group including operations researchers, statisticians, as well as computer scientists). An examination of the parallel discrete event simulation problem from a modeling methodological perspective illustrates some of these differences and reveals potentials for their resolution.
  • Simulation Model Specifications: On the Role of Representation in the Model-Centered Sciences
    Page, Ernest H.; Nance, Richard E. (Department of Computer Science, Virginia Polytechnic Institute & State University, 1994)
    The old adage that "a problem correctly formulated is half solved" is rarely challenged. Model-centered problem solving relies critically on a correct model. Complete, clearly stated assumptions, a precise statement of objectives, and an adequate representation of the model of the system under study are readily accepted as responsibilities of the modeler. But just how important are both the process and product of model representation? The modeler's responsibilities in producing a correct model can be either significantly aided or drastically inhibited by the representational mechanism - the language - being used. The comprehensibility of the model, subsequent extension, adaptation, or maintenance of the model, are highly dependent on the representational language. Language for model representation is therefore an indispensible tool. The pivotal role of this tool is described through its relationship to modeling methodology, model development environments, and automated model diagnosis. The need for an emphasis on the capabilities for conceptual expressiveness, in contrast with execution efficiency is discussed. While software engineering research clearly advocates this redefinition of emphasis, we observe that much of the research in the rising disciplines within parallel computation appears to ignore the lessons of this history.
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    Simulation modeling methodology: principles and etiology of decision support
    Page, Ernest H. (Virginia Tech, 1994)
    Investigation in discrete event simulation modeling methodology has persisted for over thirty years. Fundamental is the recognition that the overriding objectives for simulation must involve decision support. Rapidly advancing technology is today exerting major influences on the course of simulation in many areas, e.g. distributed interactive simulation and parallel discrete event simulation, and evidence suggests that the role of decision support is being subjugated to accommodate new technologies and system-level constraints. Two questions are addressed by this research: (1) can the existing theories of modeling methodology contribute to these new types of simulation, and (2) how, if at all, should directions of modeling methodological research be redefined to support the needs of advancing technology. Requirements for a next-generation modeling framework (NGMF) are proposed, and a model development abstraction is defined to support the framework. The abstraction identifies three levels of model representation: (1) modeler-generated specifications, (2) transformed specifications, and (3) implementations. This hierarchy may be envisaged as consisting of either a set of narrow-spectrum languages, or a single wide-spectrum language. Existing formal approaches to discrete event simulation modeling are surveyed and evaluated with respect to the NGMF requirements. All are found deficient in one or more areas. The Conical Methodology (CM), in conjunction with the Condition Specification (CS), is identified as a possible NGMF candidate. Initial assessment of the CS relative to the model development abstraction indicates that the CS is most suited for the middle level of the hierarchy of representations — specifically functioning as a form for analysis. The CS is extended to provide wide-spectrum support throughout the entire hierarchy via revisions of its supportive facilities for both model representation and model execution. Evaluation of the pertinent model representation concepts is accomplished through a complete development of four models. The collection of primitives for the CS is extended to support CM facilities for set definition. A higher-level form for the report specification is defined, and the concept of an augmented specification is outlined whereby the object specification and transition specification may be automatically transformed to include the objects, attributes and actions necessary to provide statistics gathering. An experiment specification is also proposed to capture details, e.g. the condition for the start of steady state, necessary to produce an experimental model. In order to provide support for model implementation, the semantic rules for the CS are refined. Based on a model of computation provided by the action cluster incidence graph (ACIG), an implementation structure referred to as a direct execution of action clusters (DEAC) simulation is defined. A DEAC simulation is simply an execution of an augmented CS transition specification. Two algorithms for DEAC simulations are presented. Support for parallelizing model execution is also investigated. Parallel discrete event simulation (PDES) is presented as a case study. PDES research is evaluated from the modeling methodological perspective espoused by this effort, and differences are noted in two areas: (1) the enunciation of the relationship between simulation and decision support, and the guidance provided by the life cycle in this context, and (2) the focus of the development effort. Recommendations are made for PDES research to be reconciled with the “mainstream” of DES. The capability of incorporating parallel execution within the CM/CS approach is investigated. A new characterization of inherent parallelism is given, based on the time and state relationships identified in prior research. Two types of inherent parallelism are described: (1) inherent event parallelism, which relates to the independence of attribute value changes that occur during a given instant, and (2) inherent activity parallelism, which relates to the independence of attribute value changes that occur over all instants of a given model execution. An analogy between an ACIG and a Petri net is described, and a synchronous model of parallel execution is developed based on this analogy. Revised definitions for the concepts time ambiguity and state ambiguity in a CS are developed, and a necessary condition for state ambiguity is formulated. A critical path algorithm for parallel direct execution of action clusters (PDEAC) simulations is constructed. The algorithm is an augmentation of the standard DEAC algorithm and computes the synchronous critical path for a given model representation. Finally, a PDEAC algorithm is described.