Lecture 16 Syed Mansoor Sarwar Operating Systems Lecture 16 Syed Mansoor Sarwar

© Copyright Virtual University of Pakistan Agenda for Today Review of previous lecture SJF is optimal Round-Robin scheduling Multi-level queues scheduling Multi-level feedback queues scheduling Recap of lecture 20 November 2018 © Copyright Virtual University of Pakistan

© Copyright Virtual University of Pakistan Review of Lecture 15 Shortest-Job-First (SJF) Shortest-Remaining-Time-First (SRTF) Exponential averaging Priority scheduling 20 November 2018 © Copyright Virtual University of Pakistan

n+1 = tn + (1- ) n Review of Lecture 15 Exponential Averaging for  = ½ n+1 = tn + (1- ) n n+1 = tn/2 + tn-1/22 + tn-2/23 + tn-3/24 + … 20 November 2018 © Copyright Virtual University of Pakistan

© Copyright Virtual University of Pakistan SJF is Optimal Logical Argument: Decrease in the wait times for short processes is much more than increase in the wait times for long processes P1 P2 P3 5 3 2 P3 P2 P1 2 3 5 20 November 2018 © Copyright Virtual University of Pakistan

© Copyright Virtual University of Pakistan Round Robin (RR) Each process gets a small unit of CPU time, called time slice or quantum, which is usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. 20 November 2018 © Copyright Virtual University of Pakistan

© Copyright Virtual University of Pakistan Round Robin (RR) If there are n processes in the ready queue, the time quantum is q, and context switch time is tcs, then no process waits more than (n-1)(q+tcs) time units Used in time-sharing systems where response time is an important performance criteria 20 November 2018 © Copyright Virtual University of Pakistan

© Copyright Virtual University of Pakistan Round Robin (RR) Performance q large  FCFS q small  q must be large with respect to context switch, otherwise overhead is too high. 20 November 2018 © Copyright Virtual University of Pakistan

© Copyright Virtual University of Pakistan Round Robin Example Process Burst Time P1 53 — 33 — 13 P2 17 P3 68 — 48 — 28 — 8 P4 24 — 4 The Gantt chart with quantum 20 is: P1 P2 P3 P4 20 37 57 77 97 117 121 134 154 162 20 November 2018 © Copyright Virtual University of Pakistan

© Copyright Virtual University of Pakistan Round Robin Example Process Turnaround Time Waiting Time P1 134 134 – 53 = 81 P2 37 37 – 17 = 20 P3 162 162 – 68 = 94 P4 121 121 – 24 = 97 Average waiting time = 73 Average waiting time for SJF = 38 Typically, higher average turnaround than SJF, but better response. 20 November 2018 © Copyright Virtual University of Pakistan

Quantum vs Context Switch Process Time = 10 Quantum Context Switches 12 0 6 1 1 9 20 November 2018 © Copyright Virtual University of Pakistan

Turnaround Time vs Quantum 20 November 2018 © Copyright Virtual University of Pakistan

© Copyright Virtual University of Pakistan Multilevel Queues Ready queue is partitioned into separate queues: - foreground (interactive) - background (batch) Each queue has its own priority and scheduling algorithm: - foreground – RR - background – FCFS 20 November 2018 © Copyright Virtual University of Pakistan

© Copyright Virtual University of Pakistan Multilevel Queues Scheduling must be done across queues. Fixed priority scheduling; i.e., serve all from foreground then from background. Time slice – each queue gets a certain percentage of CPU time, e.g., 80% to foreground in RR and 20% to background in FCFS 20 November 2018 © Copyright Virtual University of Pakistan

© Copyright Virtual University of Pakistan Multilevel Queues 20 November 2018 © Copyright Virtual University of Pakistan

© Copyright Virtual University of Pakistan Recap of Lecture SJF is optimal Round-Robin scheduling Multi-level queues scheduling 20 November 2018 © Copyright Virtual University of Pakistan