The effect of inserted idle time on the performance of dispatching rules in a flowshop

Inserted idle time occurs when a machine is kept waiting deliberately for the arrival of a job that is still being processed on a preceding machine. A common assumption in most of the existing literature on flowshop scheduling is that inserted idle time is not allowed, and whenever there are jobs that are ready for processing on a given machine, that machine is not kept idle for any reason. This study examines the effects of inserted idle times on the performance of a selection of dispatching rules that are commonly used in flowshops, and for the separate objectives of minimizing the mean flowtime, mean tardiness, or number of tardy jobs. In general, idle times are not looked upon favorably in production scheduling because they lengthen production runs and decrease machine utilizations. However, this study finds that there are benefits to insertion of short idle times under some conditions, and that the duration of these idle times depends on the shop congestion levels, the tightness of the job due dates, and the performance objective under consideration. In particular, inserted idle time is appears to be most effective in minimizing the number of tardy jobs in situations where due-dates are tight.