Peng Lu

CPU Scheduling 1. Basic Concepts 2. Scheduling Criteria 3. Scheduling Algorithms 4. Multiple-Processor Scheduling 5. Real-Time Scheduling 2

1. Basic Concepts Alternating Sequence of CPU And I/O Bursts CPU Scheduler Preemptive Scheduling Dispatcher 3

Alternating Sequence of CPU And I/O Bursts 4

Histogram of CPU-burst Times 5

CPU Scheduler Short-term scheduler First-in, first-out (FIFO) 6

Preemptive Scheduling CPU scheduling decisions may take place when a process: 1.Switches from running to waiting state 2.Switches from running to ready state 3.Switches from waiting to ready state 4.Terminates Non-preemptive (1 and 4) Preemptive (All other) 7

Dispatcher Dispatcher involves: switching context switching to user mode jumping to the proper location in the user program to restart that program Dispatch latency 8

2. Scheduling Criteria CPU utilization Throughput Turnaround time Waiting time Response time 9

Optimization Criteria Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time 10

3. Scheduling Algorithms First-come, First-served (FCFS) Shortest-Job-First (SJF) Priority Round-Robin (RR) Multilevel Queue Multilevel Feedback Queue 11

First-Come, First-Served (FCFS) Process Burst Time P 1 24 P 2 3 P 3 3 If that the processes arrive in the order: P 1, P 2, P 3,, The Gantt Chart for the schedule is: P1P1 P2P2 P3P

FCFS Scheduling (Cont.) If that the processes arrive in the order P 2, P 3, P 1 The Gantt chart for the schedule is: P1P1 P3P3 P2P

Shortest-Job-First (SJF) Process Burst Time P 1 6 P 2 8 P 3 7 P

Prediction of the Length of the Next CPU Burst 15

Priority Scheduling Process Burst Time Priority P P P P P

Priority Scheduling Indefinite blocking (Starvation) Aging 17

Round-Robin (RR) Time Quantum = 20 Process Burst Time P 1 53 P 2 17 P 3 68 P 4 24 P1P1 P2P2 P3P3 P4P4 P1P1 P3P3 P4P4 P1P1 P3P3 P3P

Time Quantum and Context Switch Time 19

Turnaround Time Varies With The Time Quantum 20

Multilevel Queue 21

Multilevel Feedback Queues 22

4. Multiple-Processor Scheduling More complex when multiple CPUs are available Homogeneous Load sharing Asymmetric multiprocessing 23

5. Real-Time Scheduling Hard real-time Soft real-time 24

Summary Optimization Criteria Max CPU utilization, throughput; Min turnaround, waiting and response time FCFS, SJF, Priority, RR Multilevel Queue and Feedback Queue 25

6. Algorithm Evaluation Deterministic modeling Queueing models Simulations Implementation 26

Operating System Examples Solaris scheduling Windows XP scheduling Linux scheduling 27

Operating System Examples : Solaris 2 Scheduling 28

Solaris Dispatch Table 29

Operating System Examples : Windows XP 30

Operating System Examples : Linux Scheduling Time-sharing Real-time 31

The Relationship Between Priorities and Time- slice length 32