Emery Berger

1
Operating SystemsCMPSCI 377Lecture 6 Scheduling

• Emery Berger
• University of Massachusetts, Amherst

2
Last Time Threads Scheduling

• Thread execution stream within process
• User-level, kernel-level, hybrid
• Theres no perfect scheduling algorithm!
• Competing goals
• Minimize response time
• Maximize throughput
• Fairness
• Policy decision

3
This Time Scheduling Algorithms

• FCFS
• First-Come, First-Served
• Round-robin
• SJF
• Multilevel Feedback Queues
• Lottery Scheduling

4
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

5
Example Round-Robin
6
Example Round-Robin
7
Example Round-Robin
8
Example Round-Robin
9
Example Round-Robin
10
Example Round-Robin
11
Example Round-Robin
12
Example Round-Robin
13
Example Round-Robin
14
Example Round-Robin
15
Example Round-Robin

• Fair
• Long average wait times

16
Round-Robin vs. FCFS

• Example 1
• 5 jobs, 100 seconds each, quantum 1s
• ignore context switch time

17
Round-Robin vs. FCFS

• Example 1
• 5 jobs, 100 seconds each, quantum 1s
• ignore context switch time

18
Round-Robin vs. FCFS

• Example 2
• 5 jobs 50, 40, 30, 20, 10 seconds each

19
Round-Robin vs. FCFS

• Example 2
• 5 jobs 50, 40, 30, 20, 10 seconds each

20
This Time Scheduling Algorithms

• FCFS
• First-Come, First-Served
• Round-robin
• SJF
• Multilevel Feedback Queues
• Lottery Scheduling

21
Example SJF
10
20
30
40
50

• Schedule job with least work until I/O or done

22
Example SJF
10
20
30
40
50

• Schedule job with least work until I/O or done

23
Example SJF
10
20
30
40
50

• Schedule job with least work until I/O or done

24
Example SJF
10
20
30
40
50

• Schedule job with least work until I/O or done

25
Example SJF
10
20
30
40
50

• Schedule job with least work until I/O or done

26
Example SJF
10
20
30
40
50

• Schedule job with least work until I/O or done

27
Example SJF

• 5 jobs, length 50, 40, 30, 20, 10 seconds

28
Example SJF

• 5 jobs, length 50, 40, 30, 20, 10 seconds

29
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

30
This Time Scheduling Algorithms

• FCFS
• First-Come, First-Served
• Round-robin
• SJF
• Multilevel Feedback Queues
• Lottery Scheduling

31
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

32
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

33
MLFQ Example
34
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

35
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

36
This Time Scheduling Algorithms

• FCFS
• First-Come, First-Served
• Round-robin
• SJF
• Multilevel Feedback Queues
• Lottery Scheduling

37
Lottery Scheduling

• Every job gets lottery tickets
• Each quantum randomly pick winner
• On averageCPU 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

38
Example Lottery Scheduling
2
1
1
1
1
1
1
2

• Paying customers 40, guests 60
• 21 ticket ratio

39
Example Lottery Scheduling
2
1
1
1
1
1
1
2

• Paying customers 40, guests 60
• 21 ticket ratio

40
Example Lottery Scheduling
2
1
1
1
1
1
1
2

• Paying customers 40, guests 60
• 21 ticket ratio

41
Example Lottery Scheduling
2
1
1
1
1
1
1
2

• Paying customers 40, guests 60
• 21 ticket ratio

42
Example Lottery Scheduling
2
1
1
1
1
1
1
2

• Paying customers 40, guests 60
• 21 ticket ratio

43
Example Lottery Scheduling
2
1
1
1
1
1
1
2

• Paying customers 40, guests 60
• 21 ticket ratio

44
Example Lottery Scheduling
2
1
1
1
1
1
1
2

• Paying customers 40, guests 60
• 21 ticket ratio

45
Example Lottery Scheduling
2
1
1
1
1
1
1
2
2/540
46
Example Lottery Scheduling
2
1
1
1
1
1
1
2
3/560
2/540

• Probabilistically achieves desired proportions

47
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

48
Next Time

• Synchronization