Solve-and-robustify: Synthesizing partial order schedules by chaining

Nicola Policella, Amedeo Cesta, Angelo Oddi and Stephen Smith
Conference Paper, Journal of Scheduling, 12(3), 2009., May, 2009

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Goal separation is often a fruitful approach when solving complex problems. It provides a way to focus on relevant aspects in a stepwise fashion and hence bound the problem solving scope along a specific direction at any point. This work applies goal separation to the problem of synthesizing robust schedules. The problem is addressed by separating the phase of problem solution, which may pursue a standard optimization criterion (e.g., minimal makespan), from a subsequent phase of solution robustification in which a more flexible set of solutions is obtained and compactly represented through a temporal graph, called a Partial Or-der Schedule (POS ). The key advantage of a POS is that it provides the capability to promptly respond to temporal changes (e.g., activity duration changes or activity start-time delays) and to hedge against further changes (e.g., new ac-tivities to perform or unexpected variations in resource ca-pacity). On the one hand, the paper focuses on specific heuris-tic algorithms for synthesis of POS s, starting from a pre-existing schedule (hence the name Solve-and-Robustify). Different extensions of a technique called chaining, which progressively introduces temporal flexibility into the rep-resentation of the solution, are introduced and evaluated. These extensions follow from the fact that in multi-capaci-tated resource settings more than one POS can be derived from a specific fixed-times solution via chaining, and carry out a search for the most robust alternative. On the other hand, an additional analysis is performed to investigate the performance gain possible by further broadening the search process to consider multiple initial seed solutions. A detailed experimental analysis using state-of-the-art RCPSP/max benchmarks is carried out to demonstrate the performance advantage of these more sophisticated solve and robustify procedures, corroborating prior results ob-tained on smaller problems and also indicating how this leverage increases as problem size is increased.

author = {Nicola Policella and Amedeo Cesta and Angelo Oddi and Stephen Smith},
title = {Solve-and-robustify: Synthesizing partial order schedules by chaining},
booktitle = {Journal of Scheduling, 12(3), 2009.},
year = {2009},
month = {May},
keywords = {Iterative improvement techniques, Scheduling under uncertainty, Constraint-based scheduling},
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