Comparing different scheduling schemes for M/G/1 queue

Scheduling has a great influence in improving computer system performance. Many recent system designs use policies which give priority to short jobs as it is often the case that the majority of the jobs are short. SRPT (Shortest Remaining Processing Time First), which servers the jobs that needs the least amount of service to complete, is known to produce optimum mean response time. However SRPT requires having the knowledge of the individual service time exactly in advance, which is not practical. FCFS and PS which shares the system capacity equally among all the jobs can be used if the CPU time requirement of each job is not available beforehand. However, FCFS is detrimental even when there is a moderate variability in job service times. In practice, however, PS must be implemented by RR with time-slicing, which incurs non-negligible job switching overhead for small time-slices. Over time, in addition to PS strategies, for example, LCFSPR (Last Come First Serve with Pre-emptive Resume), LAT (Least-Attained-Time), SRT (Shortest Residual-Time), etc have been presented. A common feature of these strategies is that preemption is employed to favour possibly short jobs. Among these strategies, some are simple to use and incur less overhead, while others incur more overhead. For example, RR and LCFSPR only require insertion of jobs at the front/back of queue (with complexity O(1)), whereas LAT and SRT need to maintain a sorted queue (with complexity O(log(n))), and keep track of the attained times and residual times of individual jobs. LCFSPR also yields M/M/1 result, but it leads to situations where short jobs can get stuck behind long jobs. In this paper, we mainly consider the issue of handling newly arrived jobs in implementing RR strategy. A research issue is then how time-slicing performs if large time-slices have to be used. In this paper, we investigate several RR variants through Discrete Event Simulation. Our results show that, by favouring newly arrived jobs, - - the performance of RR with large time-slices could be improved than that of ideal PS. The simple immediate preemption scheme, which serves the new jobs immediately by preempting the current active job, is shown to further improve the performance of RR.