Immunology as a Metaphor for Computational ... - Napier University

Chapter 3. Immune Systems for Scheduling 73permutations¢ When machine X breaks down, often task Z can be performed while waiting¢ Job C is often late arriving, so process Job D insteadThese processes are captured to some extent in the historical schedules themselves;For example, if we examine the order in which jobs are processed on a specific machine,or the order in which jobs are selected from the shop-floor for processing, thenit is common to observe patterns occurring across subsets of these processes. Thus,if a set of common patterns or parts of schedules could be built up using the knowledgeencapsulated in past schedules, then these patterns can be used as building blockswhen constructing a new schedule. The simple idea is that building blocks formulatedas a result of past experiences encapsulate past learning and knowledge, and thereforeshould be an efficient and rapid way of forming a new schedule. This type of approachto scheduling also seems more realistic than the typical GA approach which starts froma random starting point and searches for a new schedule, using no knowledge of pastbehaviour.Thus, in contrast to SCHED1 IS which adopts a bottom-up approach to producingschedules, we are now proposing a top-down approach. Figure 3.8 shows themajor differences in the approach taken in each case. In SCHED1 IS, the systemstarts with a set of potential scheduling conditions, and generates an evolved immunesystem consisting of libraries of partial instructions for creating schedules. In contrast,SCHED2 IS starts with historical information describing actual past schedules; a setof building blocks is derived from these schedules, which can then be recombined toproduce new schedules. Thus, although both models ultimately result in a step whichbuilds new schedules from smaller building blocks, the preliminary steps taken to generatethese building blocks are very different.3.10 SCHED2 3 IS - Description of ModelThis section describes the new model, SCHED2 IS, which adopts the top-down approachoutlined in figure 3.8. The model must satisfy two aims; firstly, derive a sensible