Originating from basic research conducted in the 1970's and 1980's, the parallel and distributed simulation field has matured over the last few decades. Today, operational systems have been fielded for applications such as military training, analysis of communication networks, and air traffic control systems, to mention a few. The article gives an overview of technologies to distribute the execution of simulation programs over multiple computer systems. Particular emphasis is placed on synchronization (also called time management) algorithms as well as data distribution techniques.

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Parallel and Distributed Simulation Systems

The traditional assumption about memory is that a read returns the value written by the most recent write. However, in a shared memory multiprocessor several processes independently and simultaneously submit reads and writes resulting in a partial order of memory operations. In this partial order, the definition of most recent write may be ambiguous. Memory consistency models have been developed to specify what values may be returned by a read given that memory operations may only be partially ordered. Before this work, consistency models were defined independently. Each model followed a set of rules which was separate from the rules of every other model. In our work, we have defined a set of four consistency properties. Any subset of the four properties yields a set of rules which constitute a consistency model. Every consistency model previously described in the literature can be defined based on our four properties. Therefore, we present these properties as a unfied theory of shared memory consistency.Our unified theory provides several benefits. First, we claim that these four properties capture the underlying structure of memory consistency. That is, the goal of memory consistency is to ensure certain declarative properties which can be intuitively understood by a programmer, and hence allow him or her to write a correct program. Our unified theory provides a uniform, formal definition of all previously described consistency models, and in addition some combinations of properties produce new models that have not yet been described. We believe these new models will prove to be useful because they are based on declarative properties which programmers desire to be enforced. Finally, we introduce the idea of selecting a consistency model as an on-line activity. Before our work, a shared memory program would run start to finish under a single consistency model. Our unified theory allows the consistency model to change as the program runs while maintaining a consistent definition of what values may be returned by each read.

A unified theory of shared memory consistency

When an extreme event hits an urban area, the efficiency and effectiveness of the first response have a profound effect on disaster relief efforts. The redefinition of the civil engineers' role and responsibilities as first response team members, along with an enhanced collaboration between disaster relief organizations, will greatly improve first response efforts and the securing of affected infrastructures. To improve collaboration efforts, the currently used radio systems-based interaction medium needs to be modified due to the impossibility of storing, retrieving, and transferring digital information, and limited support to implement information dissemination policies. This paper presents a reliable, transparent, and portable mobile ad hoc space for collaboration (MASC) based on a short range wireless communication platform to address these limitations in order to provide more consistent and efficient collaboration among first responders. The system was designed around a robust data redundancy core, and tested through software simulations and by conducting a search and rescue exercise involving civil engineers and firefighters. The simulation results highlight that the number of machines, the replication level, the size of the replication unit, and the wireless communication range are key design elements of the system in providing high availability. The search and rescue exercise allowed this research to confirm the high availability simulation results and to demonstrate that MASC is able to adequately manage and disseminate information in disaster scenarios. These encouraging results allow this research effort to conclude that MASC is able to address these new challenges.

Robust Mobile Ad Hoc Space for Collaboration to Support Disaster Relief Efforts Involving Critical Physical Infrastructure

In this paper, we consider the problem of designing optimal asynchronous wake-up schedules to facilitate distributed power management and neighbor discovery in multihop wireless networks. We first formulate it as a block design problem and derive the fundamental trade-offs between wake-up latency and the average duty cycle of a node. After the theoretical foundation is laid, we then devise a neighbor discovery and schedule bookkeeping protocol that can operate on the optimal wake-up schedule derived. To demonstrate the usefulness of asynchronous wake-up, we investigate the efficiency of neighbor discovery and the application of on-demand power management, which overlays a desirable communication schedule over the wake-up schedule mandated by the asynchronous wake-up mechanism. Simulation studies demonstrate that the proposed asynchronous wake-up protocol has short discovery time which scales with the density of the network; it can accommodate various traffic characteristics and loads to achieve an energy savings that can be as high as 70 percent, while the packet delivery ratio is comparable to that without power management

Optimal Block Design for Asynchronous Wake-Up Schedules and Its Applications in Multihop Wireless Networks

Fault injection is typically used to characterize failures and to validate and compare fault-tolerant mechanisms. However fault injection is rarely used for all these purposes to guide the design and implementation of a fault tolerant system. We present a systematic and quantitative approach for using software-implemented fault injection to guide the design and implementation of a fault-tolerant system. Our system design goal is to build a write-back file cache on Intel PCs that is as reliable as a write-through file cache. We follow an iterative approach to improve robustness in the presence of operating system errors. In each iteration, we measure the reliability of the system, analyze the fault symptoms that lead to data con option, and apply fault-tolerant mechanisms that address the fault symptoms. Our initial system is 13 times less reliable than a write-through file cache. The result of several iterations is a design that is both more reliable (1.9% vs. 3.1% corruption rate) and 5-9 times as fast as a write-through file cache.

The systematic improvement of fault tolerance in the Rio file cache

Distributed Operating Systems

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Distributed Operating Systems