Department of Computer Engineering PSU Wannarat Suntiamorntut 240

Department of Computer Engineering, PSU Wannarat Suntiamorntut 240 -323, Part II CPU Scheduling 240 -323 OS, 2000 1

Department of Computer Engineering, PSU Wannarat Suntiamorntut Basic Concepts The basic problem is as follows: How can OS schedule the allocation of CPU cycles to processes in system, to achieve “good performance”? Scheduling is a fundamental OS function. 240 -323 OS, 2000 2

Department of Computer Engineering, PSU Wannarat Suntiamorntut CPU- I/O Burst Cycle cpu burst I/O burst CPU burst 240 -323 OS, 2000 I/O burst 3

Department of Computer Engineering, PSU Wannarat Suntiamorntut Preemptive Scheduling • process switches from running -> waiting )I/O request(. . , • process switches from running -> ready )interrupt occur( • process switches from waiting -> ready )completion of I/O( • process terminate 240 -323 OS, 2000 4

Department of Computer Engineering, PSU Wannarat Suntiamorntut Dispatcher Module to give control of CPU to process selected by short-term scheduler. This function involves: -Switching context -Switching to user mode -Jumping to the proper location in user program to restart that program 240 -323 OS, 2000 5

Department of Computer Engineering, PSU Wannarat Suntiamorntut Scheduling Criteria • CPU utilization : in a real system, it range from 90 - 40 percents. • Throughput : number of processes that are completed. • Turnaround time : interval from time of submission to completion. • Waiting time : time that a process spends waiting in ready queue. • Response time : time from the submission of a request until the first response. 240 -323 OS, 2000 6

Department of Computer Engineering, PSU Wannarat Suntiamorntut Scheduling Algorithms • First-come, First-served Scheduling (FCFS( mange with FIFO Process P 1 P 2 P 3 Burst Time 24 3 3 P 1 Average wait time = 17 P 2 P 3 3027240 240 -323 OS, 2000 7

Department of Computer Engineering, PSU Wannarat Suntiamorntut FCFS Process P 1 P 2 P 3 Burst Time 24 3 3 P 2 306 3 240 -323 OS, 2000 Average wait time = 3 P 1 0 8

Department of Computer Engineering, PSU Wannarat Suntiamorntut Scheduling Algorithm • Shortest-Job-First Scheduling mange with SJF , minimum average wait time Process P 1 P 2 P 3 P 4 Burst Time 6 8 7 3 P 4 2416 Average wait time = 7 P 1 9 240 -323 OS, 2000 P 3 3 0 P 2 9

Department of Computer Engineering, PSU Wannarat Suntiamorntut Scheduling Algorithm • Shortest-remaining-time-First Scheduling mange with SJF , minimum average wait time Process P 1 P 2 P 3 P 4 P 1 2617 Arrival time 0 1 2 3 P 2 240 -323 OS, 2000 10 P 4 Burst Time 8 4 9 5 P 1 5 1 0 Average wait time = 6. 5 P 3 10

Department of Computer Engineering, PSU Wannarat Suntiamorntut Scheduling Algorithm • Priority Scheduling scheduling in term of high priority and low. Process P 1 P 2 P 3 P 4 P 5 Burst Time 10 1 2 1 5 P 2 P 5 19 18 16 240 -323 OS, 2000 Priority 3 1 3 4 2 P 1 6 1 0 Average wait time = 8. 2 P 3 P 4 11

Department of Computer Engineering, PSU Wannarat Suntiamorntut Scheduling Algorithm • Round-Robin Scheduling round-robin (RR), especially for time-sharing. -time quantum , time slice = small unit of time Process P 1 P 2 P 3 302622 18 240 -323 OS, 2000 Burst Time 24 3 3 P 1 14 P 1 10 If time quantum = 4 P 1 7 P 1 4 Average wait time = 5. 66 P 1 0 12

Department of Computer Engineering, PSU Wannarat Suntiamorntut Scheduling Algorithm • Multilevel Queue Scheduling Highest priority System process Interactive editing process Bath process student process lowest priority 240 -323 OS, 2000 13

Department of Computer Engineering, PSU Wannarat Suntiamorntut Scheduling Algorithm • Multilevel Feedback Queue Scheduling Quantum = 8 Quantum = 16 FCFS 240 -323 OS, 2000 14

Department of Computer Engineering, PSU Wannarat Suntiamorntut Scheduling Algorithm Do the example Process P 1 P 2 P 3 P 4 P 5 240 -323 OS, 2000 Burst Time Priority 10 3 5 1 4 3 2 Arrival time 0 2 3 5 7 15

Department of Computer Engineering, PSU Wannarat Suntiamorntut Next Lecture in Process Synchronization 240 -323 OS, 2000 16