Chap 5 CPU Scheduling.ppt

Chapter 5: CPU Scheduling Operating System Concepts – 10th Edition Silberschatz, Galvin and Gagne ©2018 Objectives  To introduce CPU scheduling, which is the basis for multiprogrammed operating systems  To describe various CPU-scheduling algorithms  To discuss evaluation criteria for sel...

Chapter 5: CPU Scheduling Operating System Concepts – 10th Edition Silberschatz, Galvin and Gagne ©2018 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  To examine the scheduling algorithms of several operating systems Operating System Concepts – 10th Edition 5.2 Silberschatz, Galvin and Gagne ©2018 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 followed by I/O burst  CPU burst distribution is of main concern Operating System Concepts – 10th Edition 5.3 Silberschatz, Galvin and Gagne ©2018 Schedulers  Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocated CPU    Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue  Long-term scheduler is invoked infrequently (seconds, minutes)  (may be slow)  The long-term scheduler controls the degree of multiprogramming Processes can be described as either:  I/O-bound process – spends more time doing I/O than computations, many short CPU bursts: Examples: Index a file system   Short-term scheduler is invoked frequently (milliseconds)  (must be fast) CPU-bound process – spends more time doing computations; few very long CPU bursts. Examples: mp3 encoding, Scientific applications Long-term scheduler strives for good process mix Operating System Concepts – 10th Edition 5.4 Silberschatz, Galvin and Gagne ©2018 Addition of Medium-Term Scheduling  Medium-term scheduler can be added if degree of multiple programming needs to decrease  Remove process from memory, store on disk, bring back in from disk to continue execution: swapping Operating System Concepts – 10th Edition 5.5 Silberschatz, Galvin and Gagne ©2018 Context Switch  When CPU switches to another process, the system must save the state of the old process and load the saved state for the new process via a context switch  Context of a process represented in the PCB  Context-switch time is overhead; the system does no useful work while switching  The more complex the OS and the PCB  the longer the context switch Operating System Concepts – 10th Edition 5.6 Silberschatz, Galvin and Gagne ©2018 CPU Scheduler  Short-term scheduler selects from among the processes in ready queue, and allocates the CPU to one of them   Ready Queue may be ordered in various ways 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  All other scheduling is preemptive  Example: interrupts occurring during crucial OS activities Operating System Concepts – 10th Edition 5.7 Silberschatz, Galvin and Gagne ©2018 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 Operating System Concepts – 10th Edition 5.8 Silberschatz, Galvin and Gagne ©2018 Scheduling Criteria  CPU utilization – Goal is to keep the CPU as busy as possible  Throughput – # of processes that complete their execution per time unit  Turnaround time – amount of time to execute a particular process –from creation to termination  Waiting time – amount of time a process has been waiting in the 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 or interactive programs) Operating System Concepts – 10th Edition 5.9 Silberschatz, Galvin and Gagne ©2018 Scheduling Algorithm Optimization Criteria  Max CPU utilization  Max throughput  Min turnaround time  Min waiting time  Min response time Operating System Concepts – 10th Edition 5.10 Silberschatz, Galvin and Gagne ©2018 Waiting Time Non-Preemptive Scheduling Algorithm Waiting Time = Turn Around Time – Burst Time Preemptive Scheduling Algorithm Turn Around Time = Completion Time – Arrival Time Waiting Time = Turn Around Time – Burst Time Operating System Concepts – 10th Edition 5.11 Silberschatz, Galvin and Gagne ©2018 First- Come, First-Served (FCFS) Scheduling  Process Burst Time P1 24 P2 3 P3 3 Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is: P P 1 0 24  Waiting time for P1 = 0; P2 = 24; P3 = 27  Average waiting time: (0 + 24 + 27)/3 = 17 Operating System Concepts – 10th Edition 5.12 P3 2 27 30 Silberschatz, Galvin and Gagne ©2018 FCFS Scheduling (Cont.) Suppose that the processes arrive in the order: P2 , P3 , P1  The Gantt chart for the schedule is: P 0 P 2 3 P 3 1 6 30  Waiting time for P1 = 6; P2 = 0; P3 = 3  Average waiting time: (6 + 0 + 3)/3 = 3  Much better than previous case  Convoy effect - short process behind long process  Consider one CPU-bound and many I/O-bound processes Operating System Concepts – 10th Edition 5.13 Silberschatz, Galvin and Gagne ©2018 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 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 Operating System Concepts – 10th Edition 5.14 Silberschatz, Galvin and Gagne ©2018 Example of SJF ProcessArriva l Time  0.0 6 P2 2.0 8 P3 4.0 7 P4 5.0 3 SJF scheduling chart P 0  P1 Burst Time P 4 3 P3 1 9 P2 16 24 Average waiting time = (3 + 16 + 9 + 0) / 4 = 7 Operating System Concepts – 10th Edition 5.15 Silberschatz, Galvin and Gagne ©2018 Example of Shortest-remaining-time-first (Preemptive)  Now we add the concepts of varying arrival times and preemption to the analysis ProcessAarri Arrival TimeT  P1 0 8 P2 1 4 P3 2 9 P4 3 5 Preemptive SJF Gantt Chart P 0  Burst Time P 1 1 P4 2 5 P1 10 P3 17 26 Average waiting time = [(10-1)+(1-1)+(17-2)+5-3)]/4 = 26/4 = 6.5 msec Operating System Concepts – 10th Edition 5.16 Silberschatz, Galvin and Gagne ©2018 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 priority scheduling where priority is the inverse of predicted next CPU burst time  Problem  Starvation – low priority processes may never execute  Solution  Aging – as time progresses increase the priority of the process Operating System Concepts – 10th Edition 5.17 Silberschatz, Galvin and Gagne ©2018 Example of Priority Scheduling ProcessA arri Burst TimeT Priority P1 10 3 P2 1 1 P3 2 4 P4 1 5 P5 5 2  Priority scheduling Gantt Chart  Average waiting time = 8.2 msec Operating System Concepts – 10th Edition 5.18 Silberschatz, Galvin and Gagne ©2018 Round Robin (RR)  Each process gets a small unit of CPU time (time quantum q), usually 10-100 milliseconds. After this time has elapsed, the process is preempted 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.  Timer interrupts every quantum to schedule next process  Performance  q large  FIFO  q small  q must be large with respect to context switch, otherwise overhead is too high Operating System Concepts – 10th Edition 5.19 Silberschatz, Galvin and Gagne ©2018 Example of RR with Time Quantum = 4  Process Burst Time P1 24 P2 3 P3 3 The Gantt chart is: P 0 P 1 P 2 4 7 P 3 10 P 1 14 P 1 18 P 1 22 P1 1 26 30  Typically, higher average turnaround than SJF, but better response  q should be large compared to context switch time  q usually 10ms to 100ms, context switch < 10 usec Operating System Concepts – 10th Edition 5.20 Silberschatz, Galvin and Gagne ©2018 Summary of Scheduling Algorithms  FCFS: Not fair, and average waiting time is poor.  Round Robin: Fair, but average waiting time is poor.  SJF: Not fair, but average waiting time is minimized assuming we can accurately predict the length of the next CPU burst. Starvation is possible.  Multilevel Queuing: An implementation (approximation) of SJF. Taken from another book Operating System Concepts – 10th Edition 5.21 Silberschatz, Galvin and Gagne ©2018 End of Chapter 5 Operating System Concepts – 10th Edition Silberschatz, Galvin and Gagne ©2018