A note on “Two-machine flow-shop scheduling with rejection” and its link with flow-shop scheduling and common due date assignment

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Abstract

In a recent paper by Shabtay and Gasper “Two-machine flow-shop scheduling with rejection, Computers and Operations Research”, forthcoming, doi:10.1016/j.cor.2011.05.023, several complexity and approximation results are proposed for a two-criteria two-machine flow-shop scheduling problem with rejection. The two criteria to be minimized are the makespan the total rejection cost. This note positions the contribution of such results with respect to the contributions of the literature on common due date assignment and flow-shop scheduling not considered in the work of Shabtay and Gasper.

Introduction

In [5] Shabtay and Gasper have recently tackled the two-machine flow-shop problem with rejection where two objectives are considered, namely the makespan and the total rejection cost, and four combinations of such objectives are analyzed. Such combinations are the problem of minimizing the weighted sum of such objectives (denoted by P1), the ϵ constraint problem with respect to the total rejection cost (denoted by P2), the ϵ constraint problem with respect to the makespan (denoted by P3) and the more general problem devoted to the search for the Pareto optimal solutions with respect to both objectives (denoted by P4). Several complexity and approximation results are provided on problems P1, …, P4 but some related literature [3], [8] on common due date assignment and flow-shop scheduling is missing. Purpose of this note is to position the contribution of [5] with respect to such literature.

Consider the more general problem P4. We have n jobs available for processing at time zero to be scheduled on m=2 machines in a flow-shop scheduling system. Each job j has a processing time aj on the first machine, a processing time bj on the second machine and a rejection cost wj. The jobs can be either accepted (belonging then to the set A of accepted jobs) or rejected (belonging then to the set A¯ of rejected jobs). The objectives are the makespan of the accepted jobs and the total rejection cost jA¯wj and the aim is to search for the Pareto optimal solutions with respect to both objectives.

However, as all the jobs in A will be completed within the makespan Cmax, such makespan can be seen as a common due date d to be respected by the jobs in A (hence all jobs A will be early with respect to d), while the jobs in A¯ can be assumed w.l.o.g. to be completed after d (hence all jobs A¯ will be tardy with respect to d).

But then, using the extended three-field classification of [7] (which is more common for multi-objective scheduling), problem P4 can be denoted by F2|dj=d, unknown d|d,jwjUj. Correspondingly, P1 can be denoted by F2|dj=d, unknown d|d+jwjUj, P2 can be denoted by F2|dj=d, unknown d|ϵ(d/jwjUj) and P3 can be denoted by F2|dj=d, unknown d|ϵ(jwjUj/d).

It turns out that P4 is actually the weighted generalization of problem F2|dj=d, unknown d|d,jUj considered in [8] (denoted hereafter by P5) and is also strictly related to problem F2|dj=D|jwjUj (minimization of the weighted sum of tardy jobs in a two-machine flow-shop with common due date D—denoted hereafter P6) considered in [3]. This has several implications with respect to the overall contribution of [5].

Section snippets

NP-hardness of problem P1—Section 2.1. in [5]

It can be directly derived from the NP-hardness of problem P6 proved in [3] by showing that P6 reduces to P1. Here is a sketch of the proof. Let us denote by D the makespan obtained by Johnson's algorithm [2] when computing the optimal schedule for the F2Cmax problem. Given an instance of P6 (where we assume w.l.o.g. wi1 integer, i=1, …, n), consider solving several instances of P1 that keep the same processing times but multiplies the weights by a coefficient γ with γ[0,D]. Notice that

Final remarks

In this note we positioned the contribution of [5] with respect to some relevant literature not considered in that paper. We notice, however, that, apparently, in the scheduling literature, there has been little attention in ascertaining the (dis)-similarities between scheduling with rejection and bi-objective scheduling with common due date assignment and this is particularly unfortunate in [5] as one of the authors is the author of several other publications on due date assignment such as,

References (9)

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