1 scheduling algorithms fcfs first-come, first-served round-robin sjf multilevel feedback queues
Post on 22-Dec-2015
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Scheduling Algorithms
FCFS First-Come, First-Served
Round-robin SJF Multilevel Feedback Queues
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Round-Robin Scheduling Quantum expires: move to back of
ready queue Variants used in most real systems
Tradeoffs: length of quantum Large: response time increases Small: throughput decreases
quantum →0 = overhead dominates context switches, cache misses
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Example: Round-Robin
waiting
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Example: Round-Robin
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Example: Round-Robin
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Example: Round-Robin
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Example: Round-Robin
waiting
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Example: Round-Robin
waiting
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Example: Round-Robin
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Example: Round-Robin
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Example: Round-Robin
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Example: Round-Robin
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Example: Round-Robin
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+ Fair- Long
average wait times
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Round-Robin vs. FCFS
Example 1: 5 jobs, 100 seconds each, quantum
= 1s ignore context switch time
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Round-Robin vs. FCFS
Example 1: 5 jobs, 100 seconds each, quantum
= 1s ignore context switch time
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Round-Robin vs. FCFS
Example 2: 5 jobs: 50, 40, 30, 20, 10 seconds
each, quantum = 1s
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Round-Robin vs. FCFS
Example 2: 5 jobs: 50, 40, 30, 20, 10 seconds
each, quantum = 1s
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This Time: Scheduling Algorithms
FCFS First-Come, First-Served
Round-robin SJF Multilevel Feedback Queues
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Example: SJF
50
40
30
20
10
Schedule job with least work until I/O or done
0 10 30 60 100 150
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Example: SJF
50
40
30
20
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Schedule job with least work until I/O or done
0 10 30 60 100 150
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Example: SJF
50
40
30
20
10
Schedule job with least work until I/O or done
0 10 30 60 100 150
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Example: SJF
50
40
30
20
10
Schedule job with least work until I/O or done
0 10 30 60 100 150
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Example: SJF
50
40
30
20
10
Schedule job with least work until I/O or done
0 10 30 60 100 150
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Example: SJF
50
40
30
20
10
0 10 30 60 100 150
Schedule job with least work until I/O or done
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Example: SJF
5 jobs, length 50, 40, 30, 20, 10 seconds, quantum = 1s
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Example: SJF
5 jobs, length 50, 40, 30, 20, 10 seconds
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SJF/SRTF: Shortest-Job First
Advantages: Provably optimal – minimizes average
waiting time Works for preemptive & non-preemptive
schedulers Preemptive SJF = SRTF
Shortest remaining time first
Disadvantages: Impossible to predict CPU time job has left Long-running CPU-bound jobs can starve
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This Time: Scheduling Algorithms
FCFS First-Come, First-Served
Round-robin SJF Multilevel Feedback Queues
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Multilevel Feedback Queues (MLFQ)
Use past behavior to predict future, assign job priorities Overcome prediction problem in SJF
Assumption: I/O-bound in past, I/O-bound in future Scheduler favors jobs that used least
CPU time Adaptive:
Change in behavior → change in scheduling decisions
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MLFQ: Approximating SJF
Multiple queues, different priorities Round-robin scheduling at each
priority level Run all jobs at highest priority first (till
this queue is empty), then next, etc. Can lead to starvation
Increase quantum exponentially at lower priorities
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MLFQ Example
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MLFQ: Assigning Priorities
Job starts in highest priority queue Quantum expires → CPU-bound
Drop priority one level Quantum does not expire → I/O-bound
Increase priority one level
CPU-bound jobs move down,I/O-bound jobs move up
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Summary of Scheduling Algorithms
FCFS: unfair, average waiting time poor
Round robin: fair, average waiting time poor
SJF: unfair, minimizes average waiting time requires accurate prediction
Multilevel Feedback Queuing: approximates SJF