1. OPERATING SYSTEM

4. Schedulers A process migrates between various scheduling queues throughout its lifetime. The process of selecting processes from these queues is carried out by a scheduler . 2 types of scheduler Long-term scheduler (job scheduler) Short-term scheduler (CPU scheduler)

9. Addition of Medium Term Scheduling The Medium Term Scheduler facilitates efficient swapping It removes processes (less priority?) from MM (from active contention of CPU) and thus reduce the DoM At some later time, the process can be reintroduced into MM and its execution can be completed where it left off – this is scheme is called swapping

16. Preemptive vs Non-Preemptive Scheduling Scheduling is non-preemptive if once the CPU has been allocated to a process, the process can keep the CPU until it releases it, either by terminating or switching to the waiting state. Scheduling is preemptive if the CPU can be taken away from a process during execution.

26. Nonpreemptive SJF example Example of non-preemptive SJF: Process CPU burst time P1 6 P2 8 P3 7 P4 3 P1 P2 P3 0 3 9 16 24 P4 Average waiting time = (0 + 3 + 9 + 16)/4 = 7 ms

27. SJF Scheduling, cont. Example of Preemptive SJF (Shortest-Remaining-Time-First) Process Arrival Time Burst Time P1 0 8 P2 1 4 P3 2 9 P4 3 5 P2 P1 P4 P1 P3 0 1 5 10 17 26 When process P2 arrives, the remaining time for P1 (7 ms) is larger than the time required for P2 (4 ms), so process P1 is preempted and P2 is scheduled. Average waiting time is : ((10-1) + (1-1) + (17-2) + (5-3)) / 4 = 6.5 ms.

32. Priority scheduling example Process Burst Time Priority P 1 10 3 P 2 1 1 P 3 2 4 P 4 1 5 P 5 5 2 assuming processes all arrived at time 0, Gantt Chart for the schedule is: average waiting time: (6 + 0 + 16 + 18 + 1)/5 = 8.2 average turnaround time: (16 + 1 + 18 + 19 + 6)/5 = 12 P 5 P 3 P 2 6 1 16 0 P 4 18 19 P 1