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Ch5 cpu-scheduling
1.
Silberschatz, Galvin and
Gagne ©2009 Operating System Concepts – 8th Edition, Chapter 5: CPU Scheduling
2.
5.2 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition Chapter 5: CPU Scheduling Basic Concepts Scheduling Criteria Scheduling Algorithms Thread Scheduling Multiple-Processor Scheduling Operating Systems Examples Algorithm Evaluation
3.
5.3 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition 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.
5.4 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition 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.
5.5 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition Histogram of CPU-burst Times
6.
5.6 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition Alternating Sequence of CPU And I/O Bursts
7.
5.7 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition 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 All other scheduling is preemptive
8.
5.8 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition 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, not output (for time- sharing environment)
9.
5.9 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition Scheduling Algorithm Optimization Criteria Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time
10.
5.10 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition 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: Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17 P1 P2 P3 24 27 30 0
11.
5.11 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition FCFS Scheduling (Cont) Suppose that the processes arrive in the order P2 , P3 , P1 The Gantt chart for the schedule is: 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 P1 P3 P2 6 3 30 0
12.
5.12 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition 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
13.
5.13 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition Example of SJF Process Arrival Time Burst Time P1 0.0 6 P2 2.0 8 P3 4.0 7 P4 5.0 3 SJF scheduling chart Average waiting time = (3 + 16 + 9 + 0) / 4 = 7 P4 P3 P1 3 16 0 9 P2 24
14.
5.14 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition 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 : Define 4. 1 0 , 3. burst CPU next the for value predicted 2. burst CPU of length actual 1. 1 n th n n t . 1 1 n n n t
15.
5.15 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition Prediction of the Length of the Next CPU Burst
16.
5.16 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition Examples of Exponential Averaging =0 n+1 = n Recent history does not count =1 n+1 = tn Only the actual last CPU burst counts If we expand the formula, we get: n+1 = tn+(1 - ) tn -1 + … +(1 - )j tn -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
17.
5.17 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition 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
18.
5.18 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition Round Robin (RR) Each process gets a small unit of CPU time (time quantum), 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. Performance q large FIFO q small q must be large with respect to context switch, otherwise overhead is too high
19.
5.19 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition Example of RR with Time Quantum = 4 Process Burst Time P1 24 P2 3 P3 3 The Gantt chart is: Typically, higher average turnaround than SJF, but better response P1 P2 P3 P1 P1 P1 P1 P1 0 4 7 10 14 18 22 26 30
20.
5.20 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition Time Quantum and Context Switch Time
21.
5.21 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition Turnaround Time Varies With The Time Quantum
22.
5.22 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition Multilevel Queue Ready queue is partitioned into separate queues: foreground (interactive) background (batch) Each queue has its own scheduling algorithm foreground – RR background – FCFS Scheduling must be done between the queues Fixed priority scheduling; (i.e., serve all 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 processes; i.e., 80% to foreground in RR 20% to background in FCFS
23.
5.23 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition Multilevel Queue Scheduling
24.
5.24 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition Multilevel Feedback Queue Problem with above approach 1)inflexible(but low scheduling overhead) A process can move between the various queues; aging can be implemented this way 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
25.
5.25 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition 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 job enters queue Q0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1. At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.
26.
5.26 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition Multilevel Feedback Queues
27.
5.27 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition Multiple-Processor Scheduling CPU scheduling more complex when multiple CPUs are available Homogeneous processors within a multiprocessor Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing Symmetric multiprocessing (SMP) – each processor is self-scheduling, all processes in common ready queue, or each has its own private queue of ready processes
28.
5.28 Silberschatz, Galvin
and Gagne ©2009 Operating System Concepts – 8th Edition Real Time Scheduling Hard Real time OS Required resource reservation Soft Real time OS (less restrictive)
29.
Silberschatz, Galvin and
Gagne ©2009 Operating System Concepts – 8th Edition, End of Chapter 5
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