6780360
An exit-flow model used in dynamic traffic assignment
We consider the behaviour of a link exit-flow model that has been used to model link flows in dynamic traffic assignment (DTA) on networks. In particular, we investigate how the model behaves when time and space (the link length) discretised and the discretisation is varied. We present numerical examples based on various inflow patterns and exit-flow functions and draw conclusions for applications of the model in discrete space and in discrete or continuous time. If inflows are always less than capacity and the link is homogeneous, with no obstructions at the exit, then if the discretisation is refined to the continuous limit the model goes to the solution of the well-known LWR model. However, we observe, somewhat counter intuitively, that the usual continuous-time model does not give as good an approximation to the LWR solution as does the discrete-time model: for a best approximation, the discretisation of space and time should be synchronised. We also investigate the 'dip' in outflows, or 'jamming' of outflows, that the model display, if inflows are permitted to exceed a capacity limit (as they sometimes do in published applications of the model) and the exit-flow function has a downward sloping part (which has usually been assumed away in DTA applications). In that case, if the number of spatial segments is increased, or the number of time intervals is reduced, then any dip in outflows occurs sooner and is more pronounced, and leads to earlier jamming. In the continuous limit, the jam occurs at the link entrance, preventing inflows in excess of capacity.
28682270
Extending and solving a multiperiod congested network flow model
Abstract We consider an existing model for optimizing time-varying flows on a congested network, and extend it so as to improve its accuracy while reducing computation. The model has been developed in a series of articles and has a variety of applications. Solving the model is a significant problem, since it is a non-linear program and can be very large for even small or medium size applications. Hitherto the model has been solved by taking a piecewise linear approximation and solving this as a linear program, perhaps taking advantage of the staircase structure of the constraints. However, this staircase is not available in the extended model. Further, the existing solution methods do not take advantage of the network structure of the problem. Here we show that a piecewise linear version of the model can be stated as a pure processing network (PPN); this allows algorithms for PPNs to be used to solve the model. We also propose a penalty (or barrier) function solution method. This reduces the problem to successively reoptimizing a program equivalent to the well-known static traffic assignment model. The latter is a convex cost, linearly constrained, un capacitated network flow problem. The above solution methods apply both to the basic model and the extended model.
5780278
A Status Report on the oo7 OODBMS Benchmarking Effort
The OO7 Benchmark was first published in 1993, and has since found a home in the marketing literature of various object-oriented database management system (OODBMS) vendors. The OO7 Benchmark (as published) was the initial result of an ongoing OODBMS performance evaluation effort at the University of Wisconsin. This paper provides an update on the status of the effort on two fronts: single-user and multi-user. On the single-user front, we review and critique the design of the initial OO7 Benchmark. We discuss some of its faults, the reasons for those faults, and things that might be done to correct them. On the multi-user front, we describe our current work on the development of a multi-user benchmark for OODBMSs. This effort includes changes and extensions to the OO7 database and the design of a family of interesting multi-user workloads.
5949420
Indexing Alternatives for Multiversion Locking
In this paper, we present several options for extending single-version indexing schemes for use with multiversion two-phase locking. The proposed options are orthogonal to the underlying indexing structure (e.g., hashing or B+ trees). The options differ in where they place references to individual versions; this version selection information is placed either with the data or in the leaf entries of one or more indices. We also briefly summarize the results of a performance study showing that storing version selection information with the data is usually the best option because it keeps indices smaller, thus enabling a larger fraction of the indices to remain cached in the buffer pool.