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For real-time management of waiting-lines in manufacturing system, priority dispatching rules appear to be the most frequently used method. Over the years, numerous dispatching rules have been proposed by the researchers. This article provides a survey of priority dispatching rules that have been extensively used by researchers and practitioners. Although approximation methods to solve job shop scheduling problem do not guarantee achieving optimal solutions, they are able to attain acceptable solutions within moderate computing times and are therefore more suitable for larger problems. Koulamas (1994) states that there is abundant of optimization procedures available for a variety of standard scheduling environments such as single-machine, flow shop and job shop settings. However, these procedures are generally limited to static problems of relatively smaller in size (Panwalker and Iskander, 1977). Hence, more scheduling research introducing efficient heuristics has been called for especially for large-size dynamic problems. According to (Jain and Meeran, 1998), approximation procedures applied to job shop scheduling problem were first developed on the basis of priority dispatching rules (pdrs). Many different terms such as priority rule, dispatch heuristic, and scheduling rule are used to refer to the principles that determine the relative importance of a job among all waiting jobs when selecting the next one for processing without inserting idle time. However, it is possible in practice that some idle time is inserted in the schedules while waiting for a soon-to-arrive urgent job instead of prioritizing back-logged jobs. The general approach for a dispatching rule, according to (Montana, 2005), is to define a score associated with assigning a given task to a given resource and select the eligible task that minimizes (or maximizes) that score for the chosen resource. (Haupt, 1989) defines it as a policy with a specific sequencing decision, applied each time a resource gets idle. According to him, priority rule-based scheduling approach considers the sequencing decision as a set of independent decentralized one-machine problems. A priority dispatching rule (pdr) determines a value of priority index, κj , for any job Jj in a given scheduling situation, and of a ranking procedure using the priority indices for selecting one of the schedulable jobs waiting on an idle machine to be started next (Baker, 1974; Panwalker and Iskander, 1977). Dynamic scheduling uses dispatching rules to prioritize jobs waiting for processing at a resource (Vieira et al., 2003). Due to their ease of implementation and their substantially reduced computational requirement they remained a very popular technique despite of their poor performance in the long run (Baker, 1974; French, 1982; Morton and Pentico, 1993). This approach is best when there is on-line scheduling (i.e. tasks being revealed during execution), the fundamental time scale of the problem is short, and the future is very uncertain (Montana, 2005). ==References== * * * * 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Priority Dispatching Rules」の詳細全文を読む スポンサード リンク
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