CPU%20Scheduling

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CPU Scheduling

• CSCI 4534

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Introduction

• By switching the CPU among processes, the O.S.
  can make the system more productive
• Some process is running at all times
• Several processes are kept in memory
• A process is executed until it must wait
• typically for I/O
• When this process waits, another runs
• running means the CPU is executing it

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CPU-I/O Burst Cycle

• Observed Property of Processes
• Processes alternate between a cycle of CPU
  execution and I/O wait
• CPU burst
• I/O burst (read from file/write to file)
• CPU burst
• CPU burst (to terminate)

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CPU-I/O Burst Cycle

• Durations of CPU bursts vary, but curve is
  similar to Figure 5.2 histogram, typically
• large number of short CPU bursts
• small number of long CPU bursts
• I/O bound programs have many short CPU bursts
• CPU bound programs have a few long CPU bursts

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CPU Scheduler

• When CPU is idle, the O.S. selects a process in
  the ready queue
• Done by short-term scheduler / CPU scheduler
• Ready Queue is set of PCBs in
• FIFO queue
• priority queue (actually a Ready State, not
  queue)
• tree
• unordered linked list

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CPU Scheduling Decisions

• CPU scheduling decisions are made when
• A process switches from running to waiting
• A process switches from running to ready
• A process switches from waiting to ready
• A process terminates
• What is preemptive?
• What are problems?
• What is nonpreemptive?

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Dispatcher

• The dispatcher gives control of CPU to the
  process selected by short-term scheduler
• It does context switching
• It switches to user mode
• It jumps to next executable statement in the user
  program
• The time taken to stop one process and start
  another is dispatch latency
• interval between stimulation and response

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Scheduling Criteria

• CPU utilization
• Throughput - processes completed per time unit
• Turnaround time - time from submission to
  completion
• Waiting time - in the ready queue
• Response time - time from submission to first
  response

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Scheduling Criteria

• Goals
• Maximize CPU Utilization and Throughput
• Minimize Turnaround, Waiting and Response Times
• May also want to minimize variance for response
  time for more predictability

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Scheduling Algorithms

• FCFS
• SJF
• Priority Scheduling
• Round-Robin
• Multilevel Queue
• Multilevel Feedback Queue