Operating Systems

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Operating Systems

• Scheduling (part 1)
• Lecture 8

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Why Multiprogramming

• OS is a process and must share the CPU with
  application processes
• Processes need to perform I/O

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Scheduler Organization

• Scheduling policy -- when is it time to remove a
  process from the CPU and which process should be
  next allocated
• Scheduling mechanism -- how the process manager
  knows when to switch running processes and how to
  allocate/deallocate a process from the CPU

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Scheduling Mechanism

• Enqueuer
• Dispatcher
• Context Switcher

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Scheduler Mechanism, cont.

• Saving Process Contexts
• Voluntary vs. involuntary sharing
• Performance issues

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Scheduling Policy

• External Priorities
• Maximum equitability
• favor short/long running processes
• resource utilization
• maximized throughput
• average/maximum turnaround times
• maximized system availability

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Scheduling Policy, cont.

• Deadlines
• Response time
• Fairness
• Optimal scheduling

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Notation

• P Pi 0 lt i lt n -- processes
• t(Pi) -- how much time a process requires
• W(Pi) -- how long a process needs to spend before
  its first transition to running
• S(Pi) running, blocked, ready -- states
• Ttrnd(Pi) -- turnaround time. Time from when
  process becomes ready until its done

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Strategy background

• Throughput rate -- jobs/unit time
• Response time
• Batch systems vs. time sharing systems
• Resources -- involvement of I/O
• t(pi) t1 d1 t2 d2 tk dk tk1

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Scheduling Strategies

• 2 types
• Non-preemptive -- once a process starts, it isnt
  removed until it finishes or yields the processor
• preemptive
• Analysing effectiveness through examining system
  load
• hypothetical load
• real load

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System Load

• l - mean arrival rate of new processes to the
  ready queue
• 1/l - mean time between arrivals
• m - mean service rate
• 1/m - mean service time of a process
• Load (fraction of time the CPU is busy
• R l 1/m l/m
• If larger than 1, we have a problem

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Non-preemptive strategies

• First Come First Served
• Shortest Job Next
• Priority Scheduling