CPU Scheduling Lecture PDF

Chapter 5: CPU Scheduling Operating System Concepts – 10 th Edition Silberschatz, Galvin and Gagne ©2018  Outline ▪ Basic Concepts ▪ Scheduling Criteria ▪ Scheduling Algorithms Operating System Concepts – 10 th Edition...

Chapter 5: CPU Scheduling Operating System Concepts – 10 th Edition Silberschatz, Galvin and Gagne ©2018  Outline ▪ Basic Concepts ▪ Scheduling Criteria ▪ Scheduling Algorithms Operating System Concepts – 10 th Edition 5.2 Silberschatz, Galvin and Gagne ©2018  Objectives ▪ Describe various CPU scheduling algorithms ▪ Assess CPU scheduling algorithms based on scheduling criteria ▪ Explain the issues related to multiprocessor and multicore scheduling ▪ Describe various real-time scheduling algorithms Operating System Concepts – 10 th Edition 5.3 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 – 10 th Edition 5.4 Silberschatz, Galvin and Gagne ©2018  Histogram of CPU-burst Times Large number of short bursts Small number of longer bursts Operating System Concepts – 10 th Edition 5.5 Silberschatz, Galvin and Gagne ©2018  CPU Scheduler ▪ The CPU scheduler selects from among the processes in ready queue, and allocates a CPU core to one of them 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 ▪ For situations 1 and 4, there is no choice in terms of scheduling. A new process (if one exists in the ready queue) must be selected for execution. ▪ For situations 2 and 3, however, there is a choice. Operating System Concepts – 10 th Edition 5.6 Silberschatz, Galvin and Gagne ©2018  Preemptive and Nonpreemptive Scheduling ▪ When scheduling takes place only under circumstances 1 and 4, the scheduling scheme is nonpreemptive. ▪ Otherwise, it is preemptive. ▪ Under Nonpreemptive scheduling, once the CPU has been allocated to a process, the process keeps the CPU until it releases it either by terminating or by switching to the waiting state. ▪ Virtually all modern operating systems including Windows, MacOS, Linux, and UNIX use preemptive scheduling algorithms. Operating System Concepts – 10 th Edition 5.7 Silberschatz, Galvin and Gagne ©2018  Preemptive Scheduling and Race Conditions ▪ Preemptive scheduling can result in race conditions when data are shared among several processes. ▪ Consider the case of two processes that share data. While one process is updating the data, it is preempted so that the second process can run. The second process then tries to read the data, which are in an inconsistent state. ▪ This issue will be explored in detail in Chapter 6. Operating System Concepts – 10 th Edition 5.8 Silberschatz, Galvin and Gagne ©2018  Dispatcher ▪ Dispatcher module gives control of the CPU to the process selected by the CPU 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 – 10 th Edition 5.9 Silberschatz, Galvin and Gagne ©2018  Scheduling Criteria ▪ CPU utilization – 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 ▪ 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. Operating System Concepts – 10 th Edition 5.10 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 – 10 th 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: P1 P2 P3 0 24 27 30 ▪ Waiting time for P1 = 0; P2 = 24; P3 = 27 ▪ Average waiting time: (0 + 24 + 27)/3 = 17 Operating System Concepts – 10 th Edition 5.12 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: P2 P3 P1 0 3 6 30 ▪ Waiting time for P1 = 6; P2 = 0; P3 = 3 ▪ Average waiting time: (6 + 0 + 3)/3 = 3 ▪ Much better than previous case Operating System Concepts – 10 th Edition 5.13 Silberschatz, Galvin and Gagne ©2018  Shortest-Job-First (SJF) Scheduling ▪ SJF is optimal – gives minimum average waiting time for a given set of processes ▪ Preemptive version called shortest-remaining-time-first Operating System Concepts – 10 th Edition 5.14 Silberschatz, Galvin and Gagne ©2018  Example of SJF Process Burst Time P1 6 P2 8 P3 7 P4 3 ▪ SJF scheduling chart P4 P1 P3 P2 0 3 9 16 24 ▪ Average waiting time = (3 + 16 + 9 + 0) / 4 = 7 Operating System Concepts – 10 th Edition 5.15 Silberschatz, Galvin and Gagne ©2018  Shortest Remaining Time First Scheduling ▪ Preemptive version of SJN ▪ Whenever a new process arrives in the ready queue, the decision on which process to schedule next is redone using the SJN algorithm. ▪ Is SRT more “optimal” than SJN in terms of the minimum average waiting time for a given set of processes? Operating System Concepts – 10 th Edition 5.16 Silberschatz, Galvin and Gagne ©2018  Example of Shortest-remaining-time-first ▪ Now we add the concepts of varying arrival times and preemption to the analysis Process i Arrival TimeT Burst Time P1 0 8 P2 1 4 P3 2 9 P4 3 5 ▪ Preemptive SJF Gantt Chart P1 P2 P4 P1 P3 0 1 5 10 17 26 ▪ Average waiting time = [(10-1)+(1-1)+(17-2)+(5-3)]/4 = 26/4 = 6.5 Operating System Concepts – 10 th Edition 5.17 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 (FCFS) q small  RR ▪ Note that q must be large with respect to context switch, otherwise overhead is too high Operating System Concepts – 10 th Edition 5.18 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: P1 P2 P3 P1 P1 P1 P1 P1 0 4 7 10 14 18 22 26 30 ▪ Typically, higher average turnaround than SJF, but better response ▪ q should be large compared to context switch time q usually 10 milliseconds to 100 milliseconds, Context switch < 10 microseconds Operating System Concepts – 10 th Edition 5.19 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 – 10 th Edition 5.20 Silberschatz, Galvin and Gagne ©2018  Example of Priority Scheduling Process Burst Time 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 Operating System Concepts – 10 th Edition 5.21 Silberschatz, Galvin and Gagne ©2018  Priority Scheduling w/ Round-Robin ▪ Run the process with the highest priority. Processes with the same priority run round-robin ▪ Example: Process a Burst Time Priority P1 4 3 P2 5 2 P3 8 2 P4 7 1 P5 3 3 ▪ Gantt Chart with time quantum = 2 Operating System Concepts – 10 th Edition 5.22 Silberschatz, Galvin and Gagne ©2018  Multilevel Queue ▪ The ready queue consists of multiple queues ▪ Multilevel queue scheduler defined by the following parameters: Number of queues Scheduling algorithms for each queue Method used to determine which queue a process will enter when that process needs service Scheduling among the queues Operating System Concepts – 10 th Edition 5.23 Silberschatz, Galvin and Gagne ©2018  Multilevel Queue ▪ With priority scheduling, have separate queues for each priority. ▪ Schedule the process in the highest-priority queue! Operating System Concepts – 10 th Edition 5.24 Silberschatz, Galvin and Gagne ©2018  Multilevel Queue ▪ Prioritization based upon process type Operating System Concepts – 10 th Edition 5.25 Silberschatz, Galvin and Gagne ©2018  Multilevel Feedback Queue ▪ A process can move between the various queues. ▪ Multilevel-feedback-queue scheduler defined by the following parameters: Number of queues Scheduling algorithms 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 ▪ Aging can be implemented using multilevel feedback queue Operating System Concepts – 10 th Edition 5.26 Silberschatz, Galvin and Gagne ©2018  Example of Multilevel Feedback Queue ▪ Three queues: Q0 – RR with time quantum 8 milliseconds Q1 – RR time quantum 16 milliseconds Q2 – FCFS ▪ Scheduling A new process enters queue Q0 which is served in RR  When it gains CPU, the process receives 8 milliseconds  If it does not finish in 8 milliseconds, the process is moved to queue Q1 At Q1 job is again served in RR and receives 16 additional milliseconds  If it still does not complete, it is preempted and moved to queue Q2 Operating System Concepts – 10 th Edition 5.27 Silberschatz, Galvin and Gagne ©2018  End of Chapter 5 Operating System Concepts – 10 th Edition Silberschatz, Galvin and Gagne ©2018