# Lecture 6- CPU Scheduling Continued

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1 Lecture 6- CPU Scheduling Continued Instructor : Bibhas Ghoshal Autumn Semester, 2015 Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

2 Shortest Remaining Job First (SRJF) Preemptive version of SJF While a job A is running, if a new job B comes whose length is shorter than the remaining time of job A, then B preempts A and B is started to run. Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

3 SRJF-Example Process Arrival Time Burst Time P P P SRJF scheduling chart P 2 P 4 P Average waiting time = ( ) / 4 = 6.5 ms Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

4 Numerical Example Assume we have the following processes. Find out the finish time, waiting time and turnaround time of each process for the following scheduling algorithms: FCFS, SJF, SRJF. Process Arrival Time CPU Burst A 0 30 B 5 20 C D Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

5 Numerical Example - FCFS FCFS : Processes will run in the order they arrive. The following is the completion, turnaround, waiting time of each process. AT : Arrival Time ; BT : Burst Time CT : Completion Time; TAT : Turnaround Time WT : Waiting Time AT BT CT TAT WT A B C D Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

6 Numerical Example - SJF SJF: running order will be: A(30) D(10) C(12) B(20) AT : Arrival Time ; BT : Burst Time CT : Completion Time; TAT : Turnaround Time WT : Waiting Time AT BT CT TAT WT A B C D Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

7 Numerical Example - SRJF SRJF: running order will be: A(5) B(5) C(12) D(10) B(15) A(25) AT : Arrival Time ; BT : Burst Time CT : Completion Time; TAT : Turnaround Time WT : Waiting Time AT BT CT TAT WT A B C D Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

8 Priority Scheduling A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority (smallest integer is equivalent to highest priority) Preemptive - higher priority process preempts the running one 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 Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

9 Priority Scheduling - Example AT CPU Burst Priority A B C D E Nonpreemptive priority scheduling: AAAACCCCDDDEEEEBBB assuming each letter is 5 time units Completion times: A: 20, B: 90, C: 40, D: 55, E: 75 Preemptive priority scheduling: ABCCCCDDDEEEEBBAAA Completion times: A: 90, B: 75, C:30, D: 45, E: 65 Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

10 Round Robin (RR) Each process gets a small unit of CPU time (time quantum), usually 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 Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

11 Example of RR with Time Quantum = 4 Process Burst Time 24 P 2 3 P 3 3 The Gantt chart is: P 2 P Typically, higher average turnaround than SJF, but better response Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

12 Example of RR with Time Quantum = 20 Process Burst Time 53 P 2 17 P 3 68 P 4 24 The Gantt chart is: P 2 P 3 P 4 P 3 P 4 P 3 P Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

13 Time Quantum and Context Switch Time Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

14 Turnaround Time Varies With The Time Quantum Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

15 Multilevel Queue Ready queue is partitioned into separate queues:foreground (interactive) and 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 Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

16 Multilevel Queue Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

17 Multilevel Feedback Queue 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 Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

18 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. Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

19 Multilevel Feedback Queue Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

20 Multiple-Processor Scheduling CPU scheduling more complex when multiple CPUs are available Homogeneous processors within a multiprocessor Load sharing : Preserve locality of data and state Asymmetric multiprocessing only one processor accesses the operating system data structures, alleviating the need for kernel data sharing among processors Some cooperative processes like to run with n processors or none at all : Gang scheduling to assign group of processors Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

21 Real Time Scheduling Hard real-time systems required to complete a critical task within a guaranteed amount of time Soft real-time computing requires that critical processes receive priority over less fortunate ones In both cases, RT behaviour is achieved by dividing the program into number of process, each of whose behaviour is predictable. When an external event is detected, it is the job of the scheduler to schedule the processes in such a way that all deadlines are met. The events a RT system has to handle are periodic(occuring at regular intervals) or aperiodic(occuring unpredictably) If there are m periodic events and event i occurs with period P i and requires C i seconds of CPU to handle each event, then the load can be handled if m i=1 1 - schedulable RT system C i P i Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

22 Thread Scheduling Local Scheduling How the threads library decides which thread to put onto an available light weight process (LWP) (kernel thread) Global Scheduling How the kernel decides which kernel thread to run next Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

23 Windows Xp Priorities Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

24 Linux Scheduling Two algorithms: time-sharing and real-time Time-sharing Prioritized credit-based process with most credits is scheduled next Credit subtracted when timer interrupt occurs When credit = 0, another process chosen When all processes have credit = 0, recrediting occurs Real Time Softy real time Posix.1b compliant two classes FCFS and RR Highest Priority Process always run first Bibhas Ghoshal IOSY 332C & IOPS 332C: OS Autumn Semester, / 24

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