CPU Scheduling

1
CPU Scheduling

• Scheduling Algorithms

2
Introduction

• Scheduling is the basic OS function
• Nearly all computer resources are scheduled
  before use
• CPU scheduling is the basic of multi -programmed
  OS
• The main objective of multi-programming is to
  have some processes running at all times, to
  maximize CPU utilization

3
Scheduling Criteria

• CPU utilization
• Range from 0 to 100
• Throughput
• Number of processes completed per unit time
• Turnaround time
• Interval from submission to completion
• Waiting time
• Time spent waiting in the ready queue
• Response time
• Time to start responding

4
FCFS or First-Come First-Served

• FIFO queue
• Simple to implement
• The average waiting time is long
• Process Burst Time ms
• P1 24
• P2 3
• P3 3
• Gantt chart
• AWT is 17ms
• Compare to P2,P3,P1

P1 P2 P3
5
SJF or Shortest-Job-First

• Shortest next CPU burst
• Process Burst Time ms
• P1 6
• P2 8
• P3 7
• P4 3 Gantt chart
• AWT is 7ms
• AWT for FCFS would be
• Used in long term scheduling

P4 P1 P3 P2
6
Shortest Remaining Time First

• SJF may be preemptive or non-preemptive
• Process Arrival Time Burst Time ms
• P1 0 8
• P2 1 4
• P3 2 9
• P4 3 5
• AWT
• ((10-1)(1-1)(17-2)(5-3))/46.5ms

P1 P2 P4 P1 P3
7
Priority Scheduling

• FCFS, SJF are priority algorithms,
• More general priority can be defined as a number
  p in the range 0-7, 0-4095
• Process Burst Time Priority (0 high)
• P1 10 3
• P2 1 1
• P3 2 3
• P4 1 4
• P5 5 2
• Gannt chart, AWT
• Aging is solution to indefinite blocking or
  starvation

8
RR or Round-Robin Scheduling

• Like FCFS with preemption
• Time quantum is defined (10-100ms)
• Process Burst Time Time Quantum 4ms
• P1 24 ms
• P2 3 ms
• P3 3 ms
• AWT 5.66, Gantt chart
• Context switching

9
Multilevel Queue Scheduling

• Foreground (interactive) processes
• Background (batch) processes
• Highest priority
• System processes
• Interactive processes
• Interactive editing processes
• Batch processes
• Student processes
• Lowest priority

10
Multilevel Feedback Queue

• Most complex scheduling, has to define
• Number of queues
• The scheduling algorithm for each queue
• The method used to determine when to upgrade a
  process to a higher priority queue
• The method used to determine when to demote a
  process to a lower priority queue
• The method used to determine which queue a
  process will enter when that process needs
  service