A MODIFIED SI RULE IN JOB SHOP SCHEDULING

Abstract

A job shop is defined as a production centre consisting of several machines, each of which may be required for processing jobs of different makeups. The scheduling problem is concerned with determining the sequence in which these jobs should be processed on each machine, and this has led to several studies of alternative loading or priority rules. One rule, which has been found to be effective in reducing the average throughput time of jobs, is the SI rule, which can be described as follows: of jobs waiting in a queue to be processed on any given machine, select first the one with the shortest imminent processing time. The study described in this paper involved a simulation of a hypothetical shop with several machines. A number of alternative rules and variations on the SI rule were investigated. One modification of the SI rule called for the creation of two queues, one having a priority over the other and each queue involving an SI discipline. The decision as to which queue a job should belong to is determined by its float, which is defined as the difference between the time available until the prescribed due-date of the job and the time required for operations still outstanding. The effectiveness of this rule is investigated.