1. Lecture 23: Process Scheduling for Interactive Systems

2. Review: Scheduling Algorithms for Batch Systems
First come first serve
Shortest job first
Shortest remaining time next

3. Review: Scheduling Algorithm Goals
All Systems
Fairness – giving each process a fair share of the CPU
Policy enforcement – seeing that stated policy is carried out
Balance – keeping all parts of the system busy

4. Review: Scheduling Algorithm Goals
Interactive Systems
Response time – respond to requests quickly
Proportionality – meet users’ expectations

5. Scheduling Algorithms for Interactive Systems
Round-Robin scheduling
Priority scheduling
Multiple queues
Lottery scheduling

6. Round Robin Give CPU time to each process by turn
Quantum = CPU time allotted every turn
how to choose the value of the quantum?
Too small might mean high context switch overhead
Too large might mean bad response times

7. Priority Scheduling Different Priorities:
Background process sending mail vs:
Shell process accepting input
Requirement:
Schedule jobs with highest priority first
Issue: high priority jobs should not overwhelm the others
Might decrease priority with time

8. Multiple Queues – static version
System level processes
High
Interactive processes
Background processes
Low
Lower priority queues don’t run if higher priority queues non-empty
Could time slice between queues

9. Multiple Queues – dynamic version
High
Quantum = 1
Quantum = 2
Low
Quantum = 4
Process enters high priority level
If takes more than 1 quantum, moved to second highest priority, …..

10. Multiple Queues – dynamic version
Higher priority = faster service, but short service times
Reduce number of context switches for long processes

11. Lottery Scheduling Example: 3 processes, priorities 2, 3, 5
Distribute 20, 30, 50 tickets respectively
System holds “lottery” 50 times a second
Winner gets 20 ms of CPU time
Gives probabilistic guarantees

12. RR, Q = 10 Arrival time CPU burst P1: 4 36 8 112 1 13 I/O burst 1613 16 20
P1:
I/O burst
P2:
P3:
P4:
Gantt chart

13. RR, Q = 10 13 16 20
P1:
P2:
P3:
P4:
Which process goes next, and when?
Gantt chart
P1 idle P2 P3 P4

14. RR, Q = 10 40 82 47
179 13 16 20
P1:
P2:
P3:
P4:
Gantt chart
P1 idle P2 P3 P4 P2

15. RR, Q = 10
162
End
Run
179 13 16 20
P1:
P2:
P3:
P4:
Gantt chart
P1 idle P2 P3 P4 P2 P1 P3 P2 P3

16. RR, Q = 10
End
205
179 13 16 20
P1:
P2:
P3:
P4:
Gantt chart
P1 idle P2 P3 P4 P2 P1 P3 P2 P3 P1 P3 idle