Modified from Silberschatz, Galvin and Gagne ©2009 Lecture 8 Chapter 5: CPU Scheduling

2 CS 446 Principles of Computer Operating Systems Chapter 5: CPU Scheduling Basic Concepts Scheduling Criteria Scheduling Algorithms Thread Scheduling Multiple-Processor Scheduling Operating Systems Examples Algorithm Evaluation

3 CS 446 Principles of Computer Operating Systems Objectives To introduce CPU scheduling, which is the basis for multiprogrammed operating systems To describe various CPU-scheduling algorithms To discuss evaluation criteria for selecting a CPU-scheduling algorithm for a particular system

4 CS 446 Principles of Computer Operating Systems Basic Concepts Maximum CPU utilization obtained with multiprogramming CPU–I/O Burst Cycle: Process execution consists of a cycle of CPU execution and I/O wait Alternating Sequence of CPU And I/O Bursts

5 CS 446 Principles of Computer Operating Systems Histogram of CPU-burst Times

6 CS 446 Principles of Computer Operating Systems CPU Scheduler Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them 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 4. Terminates Scheduling under 1 and 4 is nonpreemptive Processes keep CPU until it releases either by terminating or I/O wait. All other scheduling is preemptive Interrupts

7 CS 446 Principles of Computer Operating Systems Dispatcher Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves: switching context switching to user mode jumping to the proper location in the user program to restart that program Dispatch latency – time it takes for the dispatcher to stop one process and start another running

8 CS 446 Principles of Computer Operating Systems Scheduling Criteria CPU utilization – keep the CPU as busy as possible Typically between 40% to 90% Throughput – # of processes that complete their execution per time unit Depends on the length of process Turnaround time – amount of time to execute a particular process Sum of wait for memory, ready queue, execution, and I/O. Waiting time – amount of time a process has been waiting in the ready queue Sum of wait in ready queue Response time – amount of time it takes from when a request was submitted until the first response is produced, not output for time-sharing environment

9 CS 446 Principles of Computer Operating Systems Scheduling Algorithm Optimization Criteria Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time In most cases, systems optimize average measure It is important to minimize variance Users prefer predictable response time to faster system with high variances.

10 CS 446 Principles of Computer Operating Systems First-Come, First-Served (FCFS) Scheduling ProcessBurst Time P 1 24 P2 3 P2 3 P3 3 P3 3 Suppose that the processes arrive in the order: P 1, P 2, P 3 The Gantt Chart for the schedule is: Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: ( )/3 = 17 P1P1 P2P2 P3P

11 CS 446 Principles of Computer Operating Systems FCFS Scheduling (Cont) Suppose that the processes arrive in the order P 2, P 3, P 1 The Gantt chart for the schedule is: Waiting time for P 1 = 6; P 2 = 0 ; P 3 = 3 Average waiting time: ( )/3 = 3 Much better than previous case Nonpreemtive Convoy effect short process behind long process P1P1 P3P3 P2P

12 CS 446 Principles of Computer Operating Systems Shortest-Job-First (SJF) Scheduling Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time  shortest-next-CPU-burst SJF is optimal Gives minimum average waiting time for a given set of processes The difficulty is knowing the length of the next CPU request SFJ scheduling is preferred for long-term scheduling

13 CS 446 Principles of Computer Operating Systems Example of SJF ProcessArrival TimeBurst Time P P P P SJF scheduling chart Average waiting time = ( ) / 4 = 7 P4P4 P3P3 P1P P2P2 24

14 CS 446 Principles of Computer Operating Systems Determining Length of Next CPU Burst Can only estimate the length Can be done by using the length of previous CPU bursts using exponential averaging

15 CS 446 Principles of Computer Operating Systems Examples of Exponential Averaging  =0  n+1 =  n Recent history does not count  =1  n+1 =  t n Only the actual last CPU burst counts If we expand the formula, we get:  n+1 =  t n +(1 -  )  t n -1 + … +(1 -  ) j  t n -j + … +(1 -  ) n +1  0 Since both  and (1 -  ) are less than or equal to 1, each successive term has less weight than its predecessor

16 CS 446 Principles of Computer Operating Systems Prediction of the Length of the Next CPU Burst

17 CS 446 Principles of Computer Operating Systems Priority Scheduling A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority (smallest integer  highest priority) Preemptive Nonpreemptive SJF is a priority scheduling where priority is the predicted next CPU burst time Problem  Starvation low priority processes may never execute Solution  Aging as time progresses increase the priority of the process