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.
Automated office systems are emerging as an interdisciplinary research area with a strong computer science component. In this paper office information systems are defined as entities which perform document storage, retrieval, manipulation, and control within a distributed environment. Some state-of-the-art implementations are described. The research is related to different areas of computer science and several detailed examples are provided.
High-bandwidth applications with time-dependent resource requirements demand certain resource level assurances in order to operate correctly. Quality of Service resource management techniques are being successfully developed that allow network systems to provide such assurances. These solutions generally assume that the operating system at either end of the network is capable of handling the throughput requirements of the applications. However, real operating systems have to manage many concurrent applications with varying resource requirements. Without specialized support, the operating system cannot guarantee the resources needed for any particular application. In support of these kinds of applications we have developed a middleware agent called a dynamic QoS manager (DQM) that mediates application resource usage so as to ensure that applications get the resources they need in order to provide adequate performance. The DQM employs a variety of algorithms to determine application resource allocations. Using application QoS levels, it provides for resource availability based algorithmic variation within applications and varying application periods. It also allows for inaccurate application resource usage estimates through a technique we have developed called dynamic estimate refinement. This paper discusses new developments in the design of the DQM and presents results showing DQM performance with both real and synthetic applications.
The simultaneous evolution of personal computing tools and networks has focused attention on the notion of harnessing computer technology to assist in human collaboration on group work. While personal productivity tool technology and use have reached a high level of sophistication, the most basic ideas for how computer technology should assist in collaboration across the network have not yet converged. The approaches range from ones where coordination of work is uniquely human-controlled, to workflow-based approaches where the computer is involved in scheduling the group's work. This survey paper describes how workflow technology has evolved from a modelling focus to flexible model-based systems to support collaborative work across this range of work styles.
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