Explain any three policies for process scheduling that uses resource consumption information. What is response ratio?
Ans: Three policies for process scheduling are described below in brief: 1. First-come First-served (FCFS) (FIFO) – Jobs are scheduled in order of arrival – Non-preemptive • Problem: – Average waiting time can be large if small jobs wait behind long ones – May lead to poor overlap of I/O and CPU and convoy effects 2. Shortest Job First (SJF) – Choose the job with the shortest next CPU burst – Provably optimal for minimizing average waiting time • Problem: – Impossible to know the length of the next CPU burst 3. Round Robin(RR) – Often used for timesharing – Ready queue is treated as a circular queue (FIFO) – Each process is given a time slice called a quantum – It is run for the quantum or until it blocks – RR allocates the CPU uniformly (fairly) across all participants. If average queue length is n, each participant gets 1/n – As the time quantum grows, RR becomes FCFS – Smaller quanta are generally desirable, because they improve response time • Problem: – Context switch overhead of frequent context switch Highest Response Ratio Next (HRRN) scheduling is a non-preemptive discipline, similar to Shortest Job First (SJF) in which the priority of each job is dependent on its estimated run time, and also the amount of time it has spent waiting. Jobs gain higher priority the longer they wait which prevents indefinite postponement . In fact, the jobs that have spent a long time waiting compete against those which are estimated to have short run times.