Multicast protocols that provide message ordering and delivery guarantees are becoming increasingly important in distributed system design. However, despite the large number of such protocols, little analytical work h...
The problem of single-vehicle scheduling with ready time and deadline constraints can be apphcable to many practical transportation problems. But the general case is a generalization of classical Traveling Salesman Pr...
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This paper describes version three of UltraSAN, a software environment for the performance, dependability, and performability evaluation of computer networks and systems. The package makes use of both analytic-and sim...
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Group-oriented multicast protocols that provide message ordering and delivery guarantees are becoming increasingly important in distributed system design. However, despite the large number of such protocols, little an...
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Group-oriented multicast protocols that provide message ordering and delivery guarantees are becoming increasingly important in distributed system design. However, despite the large number of such protocols, little analytical work has been done concerning their performance, especially in the presence of message loss. This paper illustrates a method for determining the performability of group-oriented multicast protocols using stochastic activity networks, a stochastic extension to Petri nets, and reduced base model construction. In particular, we study the performability of one such protocol, called Psync, under a wide variety of workload and message loss probabilities. The specific focus is on measuring two quantities, the stabilization time-that is, the time required for messages to arrive at all hosts-and channel utilization. The analysis shows that Psync works well when message transmissions are frequent, but exhibits extremely long message stabilization times when transmissions are infrequent and message losses occur. The results provide useful insight on the behavior of Psync, as well as serve as a guide for evaluating the performability of other group-oriented multicast protocols.
UltraSAN is a software package for model-based evaluation of systems represented as stochastic activity networks. The software has been implemented in a modular fashion, with clearly delineated interfaces between mode...
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UltraSAN is a software package for model-based evaluation of systems represented as stochastic activity networks. The software has been implemented in a modular fashion, with clearly delineated interfaces between model specification, construction, and solution. UltraSAN offers an X Windows-based user interface and both analytical and simulation solution modules for transient and steady-state performance, dependability, and performability measures. Furthermore, the tool facilitates graphical representation of the obtained results by its report generator. This paper gives a very brief overview and pointers to more detailed descriptions of the software.
Large-scale data storage systems rely on magnetic tape cartridges to store millions of data objects. As these tapes age, the resident data objects become invalid; consequently, less and less of the tape potential capa...
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Large-scale data storage systems rely on magnetic tape cartridges to store millions of data objects. As these tapes age, the resident data objects become invalid; consequently, less and less of the tape potential capacity is effectively utilized. To address this problem, data storage systems have a facility, called "recycle" in this paper, that transfers valid data objects from sparsely populated tapes onto new tapes, thus creating empty tapes for reuse. A highperformance recycle process is needed to keep the number of tape cartridges to a minimum, and to maintain a continuous supply of empty tapes for storing newly created data objects. The performance of such processes is not easy to determine, and depends strongly on the data stored on the tapes, the speed and characteristics of the computer on which recycle is executed, and the nature of the algorithms themselves. This paper documents an extensive effort to evaluate a proposed recycle algorithm, using held workload data, laboratory measurements, and modeling. The results of the study were used to improve the recycle process, and were later verified through field trials. In addition yielding the results themselves, the effort illustrated that modeling and measurement, in an industrial setting, can indeed be used successfully in the design process.< >
Model-based evaluation of reliable distributed and parallel systems is difficult due to the complexity of these systems and the nature of the dependability measures of interest. The complexity creates problems for ana...
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Model-based evaluation of reliable distributed and parallel systems is difficult due to the complexity of these systems and the nature of the dependability measures of interest. The complexity creates problems for analytical model solution techniques, and the fact that reliability and availability measures are based on rare events makes traditional simulation methods inefficient. Importance sampling is a well-known technique for improving the efficiency of rare event simulations. However, finding an importance sampling strategy that works well in general is a difficult problem. The best strategy for importance sampling depends on the characteristics of the system and the dependability measure of interest. This fact motivated the development of an environment for importance sampling that would support the wide variety of model characteristics and interesting measures. The environment is based on stochastic activity networks, and importance sampling strategies are specified using the new concept of the importance sampling governor. The governor supports dynamic importance sampling strategies by allowing the stochastic elements of the model to be redefined based on the evolution of the simulation. The utility of the new environment is demonstrated by evaluating the unreliability of a highly dependable fault-tolerant unit used in the well-known MARS architecture. The model is non-Markovian, with Weibull distributed failure times and uniformly distributed repair times.< >
In numerical simulations of fluid-dynamics problems, solution-adaptive methods have proven to be very powerful. The implementation of the modified Shepard’s interpolation to the structured grids used in CFD is sugges...
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