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CH 5. CPU Scheduling 2015-04-14

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5.1 Basic Concepts F CPU Scheduling context switching u CPU switching for another process u saving old PCB and loading new PCB dispatcher u give control of the CPU to the process selected by the CPU scheduler F CPU-I/O Burst Cycle Process execution begins with a CPU burst

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5.1 Basic Concepts Alternating sequence of CPU and I/O bursts Load store add store read from file Wait for I/O Store increment index write to file Wait for I/O Load store add store read from file Wait for I/O CPU burst I/O burst CPU burst I/O burst frequency 160 140 120 100 80 60 40 20 0816243240 burst duration (milliseconds) Histogram of CPU-burst times

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5.2 Scheduling Criteria F Performance comparison criteria of CPU scheduling algorithm CPU utilization : 40% ~ 90% throughput : finished job # per time unit turnaround time : waiting time in the Ready Queue response time : interactive system F preemptive & nonpreemptive Scheduling context switching : pure overhead (PCB save, load) F Interval timer & interrupting clocking prevent a monopoly of system F Deadline Scheduling pay remnant cost for processing in explicit time

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5.3 Scheduling Algorithms 1.FIFO(FCFS) CBACPU Finish Nonpreemptive Ex) Job Burst Time 1 24 2 3 3 3 If arriving order 1, 2, 3 If arriving order is 2, 3, 1 A.T.T = (3 + 6 + 30) / 3 = 13 A.W.T = (0 + 3 + 6) / 3 = 3 Job1 Job2 Job3 24 27 30 Average Turnaround Time = (24 + 27 + 30)/3 = 27 Average Waiting Time = (0 + 24 + 27)/3 = 17 ms

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5.3 Scheduling Algorithms 2.Round Robin Scheduling for Timesharing System Time Quantum of Time Slice (10 ~ 100 msec) New job Preemptive Finish Ex) In the case of jobs in previous example, if time quantum is 4, Average turnaround time is -- J1 J2 J3 J1 J1 · · · · · J1 47 / 3 16 Average Waiting Time = (4 + 7 + (10 - 4)) / 3 = 17 / 3 = 5.66 ms 4 7 10 14 30 R.R Scheduling = Preemptive Scheduling Time Quantum : if large FCFS short like processor sharing context switching overhead

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5.3 Scheduling Algorithms Process time = 10 Quantum Context switches 12 0 6 1 1 9 0 10 0 1 2 3 4 5 6 7 8 9 10 6 Showing how a smaller time quantum increases context switching

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5.3 Scheduling Algorithms Average turnaround time 12.5 12.0 11.5 11.0 10.5 10.0 9.5 9.0 1 2 3 4 5 6 7 Process time P 1 6 P 2 3 P 3 1 P 4 7 Time quantum Showing how turnaround time varies with the time quantum

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5.3 Scheduling Algorithms 3.Shortest - Job - First (SJF) Non preemptive Scheduling Long - Term Scheduling in Batch system : good. Short-Term Scheduling : impossible u A.W.T = (3 + 16 + 9 + 0)/4 = 7 ms. –FCFS A.W.T = (0 + 6 + 14 + 21)/4 = 41/4 = 10.25 ms. –AVG Turnaround Time = (3 + 9 + 16 + 24)/4 = 52/4 = 13 ms. Job Burst Time 1 6 2 8 3 7 4 3 0 3 9 16 24 J 4 J 1 J 3 J 2

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5.3 Scheduling Algorithms 4.Shortest-Remaining-Time-First(SRT) Transformation of Shortest-Job-First Preemptive SJF u A.W.T = ((10-1) + (1-1) + (17 -2) + (5-3))/4 = 26/4 = 6.5 ms. –A.T.T = 17 + (5-1) + (26-2) + (10-3) =52/4 = 13 ms. –SJF A.T.T = 14.25 ms. –SJF A.W.T = 5.75 ms. Ex) Job Arrive Time Burst Time 1 0 8 2 1 4 3 2 9 4 3 5 * Assign the Thrasholded Value J 1 J 2 J 4 J 1 J 3 0 1 5 10 17 26

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5.3 Scheduling Algorithms 5.Priority Scheduling SJF : Special case of the general priority scheduling. FCFS : Equal priority. Major problem : indefinite blocking or starvation. u Solution HRN Ex) Job Burst Time Priority 110 3 2 1 1 3 2 3 4 1 4 5 5 2 J 2 J 5 J 1 J 3 J 4 0 1 6 16 18 19 A.W.T = 8.2 ms A.T.T = 12 ms

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5.3 Scheduling Algorithms 6.HRN(Highest-Response-ratio-Next) : Brinch Hansen priority = (Waiting Time + Service Time) / Service Time Nonpreemptive method that apply process service time and total time to wait service

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5.3 Scheduling Algorithms 7.Multilevel Queue processes each job classified into queues ex) classify into Foreground and Background (80% : 20%) u F.G is scheduled by Round Robin algorithm u B.G is scheduled by an FCFS algorithm system processes Interactive processes Interactive editing processes batch processes student processes highest priority lowest priority Multilevel Queue scheduling

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5.3 Scheduling Algorithms 8.Multilevel Feedback Queue allow a process to move between queues quantum = 8 quantum = 16 FCFS Fig 5.7 Multilevel feedback queues

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5.5 Real-Time Scheduling F Hard real-time within a guaranteed time F soft real-time less restrictive

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5.6 Algorithm Evaluation F Deterministic modeling ProcessBurst Time P 1 10 P 2 1 P 3 2 P 4 1 P 5 5 maximize CPU utilization maximize throughput) F Queueing Models result mathematical formula that estimate the distribution of CPU and I/O bursts. F Simulations

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5.6 Algorithm Evaluation FCFS RR (Quantum=1) S.J.F F Evaluation method Analytical evaluation u use mathematical analysis by predetermined workload, … u ex) Deterministic modeling Queuing Model u mathematical formula, arrival rate, service rate èutilization, queue length, wait time Simulation

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