On the Expected Relative Performance of List Scheduling

Let XI„ = X1, ', Xn denote an ordered list of service times required by n tasks. The service will be performed by m ≥ 2 processors working in parallel. Each processor serves one task at a time and, having once started a task, finishes it before starting another. A schedule determines how the tasks are to be served. A list schedule keeps the tasks not yet serviced listed in the order prescribed by XI„. Whenever a processor completes a service, it then takes its next task from the head of the list. The makespan of a schedule is the time required for all service to be completed. The makespan LXI„ of a list schedule is usually longer than necessary. Reordering the tasks in an optimal way can reduce the makespan to OPTXI„, the smallest possible makespan, but requires knowing the Xi in advance and solving an NP-complete problem. The ratio RXI„ = LXI„/OPTXI„ measures the penalty paid for serving the tasks in a predetermined order. Here, the service times Xi are treated as independent identically distributed random variables. Two distributions for Xi, uniform and exponential, are considered. Bounds on the mean ERXI„ and on the distribution function P[RXI„ >x] are obtained.