ITFN 2601 Introduction to Operating Systems

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ITFN 2601Introduction to Operating Systems

• Lecture 4/5
• Scheduling

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Agenda

• Scheduling
• Batch
• Interactive
• Real-Time
• Threads

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Scheduling

• Process Conditions
• Processor Bound
• I/O Bound
• Scheduling how?
• Pre-emptive
• Non-pre-emptive

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Scheduler Goals

• Generic Goals
• Fairness of processor allocation
• Enforcement of Scheduling Policies
• Balance of utilization
• Batch-based Goals
• Maximize throughput of jobs
• Minimize turnaround on jobs

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Scheduler Goals II

• Interactive System Goals
• Minimize response time for user I/O
• User expectations should be met
• Real-time System Goals
• Deadlines must be met for Process Completion
• System Performance must be predictable

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Scheduling Algorithms(Batch)

• FIFO (First In First Out) NON-PREEMPTIVE
• Fairest
• Low throughput
• High Turnaround
• Shortest First NON-PREEMPTIVE
• High Throughput
• Low Turnaround
• Unfair for Large Jobs

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Scheduling Algorithms(Batch, cont)

• Shortest Remaining - PREEMPTIVE
• High Turnaround on Long Jobs
• Unfair for Large Jobs
• Multi-Scheduling (CPU or Memory Limited)
• HIGH Turnaround (disk swaps)
• Throughput highly variable, probably low
• Fairness highly variable

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Scheduling Algorithms(Interactive)

• Round Robin - PREEMPTIVE
• Fairest overall
• Response time variable but finite
• Priority Scheduling - PREEMPTIVE
• Fair
• More Fair for users with higher priorities
• Response time inverse to priority
• Windows/Unix typically implement this

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Round Robin, Example
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Scheduling Algorithms(Real-Time)

• Small Jobs
• High Priority
• Periodic/Aperiodic
• Schedulable?
• Iff the sum of the ratios CPU Time to Period time
  is less than one
• Sum(CPU/Period) lt 1
• Static/Dynamic?

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Summary

• Scheduler responsible for many goals
• Scheduling algorithms complex
• Know your math!