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Operating Systems CMPSCI 377 Lecture 6: Scheduling. Emery Berger University of Massachusetts, Amherst. Last Time: Threads & Scheduling. Thread = execution stream within process User-level, kernel-level, hybrid There’s no perfect scheduling algorithm! Competing goals: Minimize response time
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Operating SystemsCMPSCI 377Lecture 6: Scheduling Emery Berger University of Massachusetts, Amherst
Last Time: Threads & Scheduling • Thread = execution stream within process • User-level, kernel-level, hybrid • There’s no perfect scheduling algorithm! • Competing goals: • Minimize response time • Maximize throughput • Fairness • Policy decision
This Time: Scheduling Algorithms • FCFS • First-Come, First-Served • Round-robin • SJF • Multilevel Feedback Queues • Lottery Scheduling
Round-Robin Scheduling • Quantum expires: move to back of queue • Variants used in most real systems • Quantum length – • Large: response time increases • quantum )1 = FCFS • Small: throughput decreases • quantum ) 0 = overhead dominates • context switches, cache misses
waiting running Example: Round-Robin
waiting running Example: Round-Robin
waiting running Example: Round-Robin
waiting running Example: Round-Robin
waiting running Example: Round-Robin
waiting running Example: Round-Robin
waiting running Example: Round-Robin
waiting running Example: Round-Robin
waiting running Example: Round-Robin
waiting running Example: Round-Robin
waiting running Example: Round-Robin • Fair • Long average wait times
Round-Robin vs. FCFS • Example 1: • 5 jobs, 100 seconds each, quantum = 1s • ignore context switch time
Round-Robin vs. FCFS • Example 1: • 5 jobs, 100 seconds each, quantum = 1s • ignore context switch time
Round-Robin vs. FCFS • Example 2: • 5 jobs: 50, 40, 30, 20, 10 seconds each
Round-Robin vs. FCFS • Example 2: • 5 jobs: 50, 40, 30, 20, 10 seconds each
This Time: Scheduling Algorithms • FCFS • First-Come, First-Served • Round-robin • SJF • Multilevel Feedback Queues • Lottery Scheduling
0 10 30 60 100 150 Example: SJF • Schedule job with least work until I/O or done 10 20 30 40 50
0 10 30 60 100 150 Example: SJF • Schedule job with least work until I/O or done 10 20 30 40 50
0 10 30 60 100 150 Example: SJF • Schedule job with least work until I/O or done 10 20 30 40 50
0 10 30 60 100 150 Example: SJF • Schedule job with least work until I/O or done 10 20 30 40 50
0 10 30 60 100 150 Example: SJF • Schedule job with least work until I/O or done 10 20 30 40 50
0 10 30 60 100 150 Example: SJF • Schedule job with least work until I/O or done 10 20 30 40 50
Example: SJF • 5 jobs, length 50, 40, 30, 20, 10 seconds
Example: SJF • 5 jobs, length 50, 40, 30, 20, 10 seconds
SJF/SRTF: Shortest-Job First • Advantages: • Optimal – minimizes average waiting time • Works for preemptive & non-preemptive schedulers • Preemptive SJF = SRTF • Shortest remaining time first • I/O-bound jobs get priority over CPU-bound jobs • Disadvantages: • Impossible to predict CPU time job has left • Long-running CPU-bound jobs can starve
This Time: Scheduling Algorithms • FCFS • First-Come, First-Served • Round-robin • SJF • Multilevel Feedback Queues • Lottery Scheduling
Multilevel Feedback Queues (MLFQ) • Use past behavior to predict future! • Overcome prediction problem in SJF • Assumption: • I/O-bound in past, I/O-bound in future • Scheduler favors jobs that use least CPU time • Adaptive: • Change in behavior ) change in scheduling decisions
MLFQ: Approximating SJF • Multiple queues, different priorities • Round-robin scheduling at each priority level • Run all at highest priority first, then next, etc. • Can lead to starvation • Increase quantum exponentially at lower priorities
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
Improving Fairness • SJF: optimal, but unfair • Increase fairness = give long jobs CPU time • degrades average waiting time • Solutions: • Each queue – fraction of CPU time • Fair iff even distribution of jobs among queues • Adjust priority of jobs w/o service • Originally done by UNIX • Avoids starvation • Under load, waiting time suffers
This Time: Scheduling Algorithms • FCFS • First-Come, First-Served • Round-robin • SJF • Multilevel Feedback Queues • Lottery Scheduling
Lottery Scheduling • Every job gets lottery tickets • Each quantum: randomly pick winner • On average:CPU time proportional to # of tickets • Give most tickets to short-running jobs (approximates SJF) • Give every job at least one ticket • Degrades gracefully as load changes
Example: Lottery Scheduling • Paying customers: 40%, guests: 60% • 2:1 ticket ratio 2 1 1 1 1 1 1 2
Example: Lottery Scheduling 2 1 1 1 1 1 1 2 • Paying customers: 40%, guests: 60% • 2:1 ticket ratio
Example: Lottery Scheduling 2 1 1 1 1 1 1 2 • Paying customers: 40%, guests: 60% • 2:1 ticket ratio
Example: Lottery Scheduling 2 1 1 1 1 1 1 2 • Paying customers: 40%, guests: 60% • 2:1 ticket ratio
Example: Lottery Scheduling 2 1 1 1 1 1 1 2 • Paying customers: 40%, guests: 60% • 2:1 ticket ratio
Example: Lottery Scheduling 2 1 1 1 1 1 1 2 • Paying customers: 40%, guests: 60% • 2:1 ticket ratio
Example: Lottery Scheduling 2 1 1 1 1 1 1 2 • Paying customers: 40%, guests: 60% • 2:1 ticket ratio
Example: Lottery Scheduling 2 1 1 1 1 1 1 2 2/5=40%
Example: Lottery Scheduling 2 1 1 1 1 1 1 2 • Probabilistically achieves desired proportions 3/5=60% 2/5=40%
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 Queueing: • approximates SJF • Lottery scheduling: • fair, low average waiting time • poor fit to priority
Next Time • Synchronization