Priority Scheduling Example Fixed (strict) priority scheduling Process Burst Time Priority P1 10 3 P2 1 1 P3 2 4 P4 1 5 P5 5 2 Priority scheduling Gantt Chart Average waiting time = 8.2 msec P2 P3 P5 1 18 16 P4 19 6 P1

Priority Scheduling A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority (usually smallest integer  highest priority) Preemptive Nonpreemptive SJF is priority scheduling where priority is the inverse of predicted next CPU burst time Equal-priority processes are scheduled in FCFS order Problem  Starvation (indefinite blocking) – low priority processes may never execute Solution  Aging – as time progresses increase the priority of the process

Internal / External Priorities Priorities can be defined either internally to OS or externally Internally defined priorities use some measurable quantity or quantities to compute the priority of a process. For example, time limits memory requirements the number of open files the ratio of average I/O burst to average CPU burst have been used in computing priorities External priorities are set by criteria that are external to the operating system such as the Importance of the process Urgency …

Round Robin (RR) The round-robin (RR) scheduling algorithm is designed especially for timesharing systems. It is similar to FCFS scheduling, but preemption is added to switch between processes. A small unit of time, called a time quantum (or time slice), is defined. Adv:The RR provides acceptable response time for the processes. DAdv:The average waiting time under the RR policy is often quite long. Performance depends on time quantum length.

Example of RR with Time Quantum = 4 ms Process Burst Time P1 24 P2 3 P3 3 The Gantt chart is: Typically, higher average turnaround than SJF, but better response q should be large compared to context switch time q usually 10ms to 100ms, context switch < 10 usec P1 P2 P3 4 7 10 14 18 22 26 30

Time Quantum and Context Switch Time If the time quantum is 1 time unit, then 9 context switches will occur, slowing the execution of the process accordingly.

Turnaround Time Varies With The Time Quantum Empiric Conclusion 80% of CPU bursts should be shorter than q The average turnaround time of a set of processes does not necessarily improve as the time-quantum size increases. In general, the average turnaround time can be improved if most processes finish their next CPU burst in a single time quantum.