Chapter 5: CPU Scheduling. Operating System Concepts 8 th Edition,

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1 Chapter 5: CPU Scheduling, Silberschatz, Galvin and Gagne 2009

2 Objectives To introduce CPU scheduling, which is the basis for multiprogrammed operating systems To describe various scheduling algorithms To discuss evaluation criteria for selecting a scheduling algorithm for a particular system 5.2 Silberschatz, Galvin and Gagne 2009

3 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 CPU burst distribution 5.3 Silberschatz, Galvin and Gagne 2009

4 Histogram of CPU-burst Times 5.4 Silberschatz, Galvin and Gagne 2009

5 Alternation of CPU and I/O Bursts 5.5 Silberschatz, Galvin and Gagne 2009

6 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 non-preemptive All other scheduling is preemptive 5.6 Silberschatz, Galvin and Gagne 2009

7 Dispatcher The dispatcher module gives control of the CPU to the process selected by the scheduler, which involves: switching context switching to user mode jumping to the proper location in the user program to restart that program Dispatch latency the time it takes for the dispatcher to stop one process and start another running 5.7 Silberschatz, Galvin and Gagne 2009

8 Scheduling Criteria CPU utilization keep the CPU as busy as possible (i.e. maximize) Throughput the number of processes that complete their execution per time unit (maximize) Turnaround time amount of time to execute a particular process (minimize) Waiting time amount of time a process has been waiting in the ready queue (minimize) Response time the amount of time it takes from when a request is submitted until the first response (not output) is produced (minimize) 5.8 Silberschatz, Galvin and Gagne 2009

9 Scheduling Algorithms First-Come First-Served (FCFS) Shortest Job First (SJF) Shortest Remaining Time (i.e. pre-emptive SJF) Round Robin (RR) Multilevel Queues (MQ) Multilevel Feedback Queues (MFQ) 5.9 Silberschatz, Galvin and Gagne 2009

10 First-Come First-Served Process Burst Time P 1 24 P 2 3 P 3 3 Suppose that the processes arrive in the order: P 1, P 2, P 3 The Gantt Chart for the schedule is: P 1 P 2 P Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: ( )/3 = Silberschatz, Galvin and Gagne 2009

11 FCFS Scheduling Suppose that the processes arrive in the order P 2, P 3, P 1 The Gantt chart for the schedule is: P 2 P 3 P Waiting time for P 1 = 6; P 2 = 0 ; P 3 = 3 Average waiting time: ( )/3 = 3 Much better than previous case Convoy effect - short process behind long process 5.11 Silberschatz, Galvin and Gagne 2009

12 Shortest Job First Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest 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 burst 5.12 Silberschatz, Galvin and Gagne 2009

13 Example of SJF Process Burst Time P 1 6 P 2 8 P 3 7 P 4 3 SJF scheduling chart P 4 P 1 P 3 P Average waiting time = ( ) / 4 = Silberschatz, Galvin and Gagne 2009

14 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 t n 0 n n 1 1 actuallength of t n 1 n n th CPU burst predicted value for thenext CPU burst Silberschatz, Galvin and Gagne 2009

15 Predicting the Length of the Next CPU Burst 5.15 Silberschatz, Galvin and Gagne 2009

16 Examples of Exponential Averaging =0 n+1 = n i.e. recent history does not count =1 n+1 = t n i.e. only the actual last CPU burst counts If we expand the formula, we get: n+1 = t n +(1 - ) t n (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 5.16 Silberschatz, Galvin and Gagne 2009

17 Priority Scheduling A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority (e.g. smallest integer) Can be preemptive or non-preemptive (SJF is an example of priority scheduling where the 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 5.17 Silberschatz, Galvin and Gagne 2009

18 Round Robin Each process gets a small slice of CPU time (a time quantum), usually ms. After this time has elapsed, the process is pre-empted and added to the end of the ready queue. If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units. Performance: q large FIFO q must be significantly larger than the context switch time, otherwise the overhead is too high 5.18 Silberschatz, Galvin and Gagne 2009

19 Example of RR (tq = 4) Process Burst Time P 1 24 P 2 3 P 3 3 The Gantt chart is: P 1 P 2 P 3 P 1 P 1 P 1 P 1 P Typically, higher average turnaround time than SJF, but better response time 5.19 Silberschatz, Galvin and Gagne 2009

20 Time Quantum and Context Switch Time 5.20 Silberschatz, Galvin and Gagne 2009

21 Turnaround Time Varies with the Time Quantum 5.21 Silberschatz, Galvin and Gagne 2009

22 Multilevel Queues The ready queue is partitioned into separate queues, e.g. foreground (interactive) background (batch) Each queue has its own scheduling algorithm: foreground RR and background FCFS Scheduling must be done between the queues Fixed priority scheduling; (i.e. serve all jobs from foreground then from background). Possibility of starvation. Time slice each queue gets a certain amount of CPU time which it can schedule amongst its jobs (e.g. 80% to foreground in RR and 20% to background in FCFS) Silberschatz, Galvin and Gagne 2009

23 Multilevel Queues 5.23 Silberschatz, Galvin and Gagne 2009

24 Multilevel Feedback Queues A process can move between the various queues (aging can be implemented this way) Multilevel-feedback-queues scheduling is defined by the following parameters: number of queues scheduling algorithm for each queue method used to determine when to upgrade a process method used to determine when to demote a process method used to determine which queue a process will enter when that process needs service 5.24 Silberschatz, Galvin and Gagne 2009

25 Example of Multilevel Feedback Queues Three queues: Q 0 RR with time quantum 8 ms Q 1 RR time quantum 16 ms Q 2 FCFS Scheduling A new job enters queue Q 0 (served FCFS). When it gains the CPU, the job receives 8 ms. If it does not finish in 8 ms, the job is moved to queue Q 1. At Q 1 job is again served FCFS and receives 16 additional ms. If it still does not complete, it is preempted and moved to queue Q Silberschatz, Galvin and Gagne 2009

26 Multilevel Feedback Queues 5.26 Silberschatz, Galvin and Gagne 2009

27 How to Evaluate Scheduling Algorithms Deterministic modeling predefined workload and a specific criterion Queuing models predefined arrival and service processes and a specific criterion Simulation (may need to make assumptions) Implementation 5.27 Silberschatz, Galvin and Gagne 2009

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