ITFN 2601 Introduction to Operating Systems

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ITFN 2601Introduction to Operating Systems

• Lecture 4
• Scheduling

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Agenda

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

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Scheduling When

• New Process is Created
• Parent Process
• Child Process
• Process Exits
• When a process blocks
• I/O Interrupt occurs
• Clock Interrupts
• Non-preemptive
• Preemptive

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Objectives of a Good Scheduling Policy

• Fairness
• Efficiency
• Low response time (important for interactive
  jobs)
• Low turnaround time (important for batch jobs)
• High throughput

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Scheduling

• Throughput
• The amount of useful work accomplished per unit
  time. This depends, of course, on what
  constitutes useful work. One common measure of
  throughput is jobs/minute (or second, or hour,
  depending on the kind of job)
• Utilization
• For each device, the utilization of a device is
  the fraction of time the device is busy. A good
  scheduling algorithm keeps all the devices (CPUs,
  disk drives, etc.) busy most of the time.

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Scheduling

• Turnaround
• The length of time between when the job arrives
  in the system and when it finally finishes
• Response Time
• The length of time between when a job arrives in
  the system and when it starts to produce output.
  For interactive jobs, response time might be more
  important than turnaround.
• Wait Time
• The amount of time the job is ready (runnable
  but not running). This is a better measure of
  scheduling quality than turnaround since the
  scheduler has no control of the mount of time the
  process spends computing or blocked waiting for
  I/O.

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Preemption

• Needs a clock interrupt (or equivalent)
• Needed to guarantee fairness
• Found in all modern general purpose operating
  systems
• Without preemption, the system implements run to
  completion (or yield)

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Scheduling Algorithms(Batch)

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

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Scheduling Algorithms(Batch, cont)

• Shortest Remaining
• 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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First-Come First-Served

• The simplest possible scheduling discipline is
  called First-come, first-served (FCFS). The ready
  list is a simple queue (first-in/first-out). The
  scheduler simply runs the first job on the queue
  until it blocks, then it runs the new first job,
  and so on. When a job becomes ready it is simply
  added to the end of the queue.

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FCFS

• Main advantage of FCFS is that it is easy to
  write and understand
• No starvation
• If one process gets into an infinite loop, it
  will run forever and shut out all the others
• FCFS tends to excessively favor long bursts.
  CPU-bound processes

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Shortest Job First (SJF)

• Whenever the CPU has to choose a burst to run, it
  chooses the shortest one
• Non-preemptive policy
• Preemptive version of the SJN, called shortest
  remaining time first
• Starvation is possible

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Scheduling Algorithms(Interactive)

• Round Robin
• Fairest overall
• Response time variable but finite
• Priority Scheduling
• Fair
• More Fair for users with higher priorities
• Response time inverse to priority

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Round-Robin

• Round-robin (RR). RR keeps all the processes in a
  queue and runs the first one, like FCFS. After a
  length of time q (called quantum), if the current
  burst hasnt completed, it is moved to the tail
  of the queue and the next process is started

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Round Robin

• An important preemptive policy
• Essentially the preemptive version of FCFS
• The key parameter is the quantum size q
• When a process is put into the running state, a
  timer is set to q
• If the timer goes off and the process is still
  running, the OS preempts the process.
• The process is moved to the ready state
• The next job in the queue is selected to run

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Round Robin

• Quantum cant be too large
• Quantum cant be too small
• What quantum should we choose
• Tradeoff
• Small q makes system more responsive
• Large q makes system more efficient since theres
  less switching

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Round Robin, Example
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Priority Scheduling

• Always run the highest priority process
• Preemptive or non-preemptive
• Priorities can be assigned externally to
  processes based on their importance
• Assigned (and changed) dynamically

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Other Interactive Scheduling

• Multiple Queues
• Shortest Process Next
• Guaranteed Scheduling
• Lottery Scheduling
• Fair-Share Scheduling

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Scheduling Algorithms(Interactive, cont)

• Multi-Quantized
• Response time proportionate to quanta
• More bookkeeping
• Shortest Process Next
• Estimation of process length
• Unfair for large jobs
• Fast response for small jobs

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Scheduling Algorithms(Interactive, cont)

• Guaranteed Scheduling
• Lottery Scheduling
• Alotted time proportional to Job Size/Importance
• Sharing
• Fair by user, not necessarily fair by job
• Responses become disproportionate

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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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Scheduler Mechanism

• Some processes are intrinsically more important
  at some times than others
• Time-dependent response
• High-priority request
• How can a process raise its scheduling priority?

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Thread Scheduling

• User-level threads
• Process has a Thread Scheduler
• Same concept as Process Scheduler
• Conflicts with Process Scheduling
• Kernel Level Threads
• Kernel has Thread and Process Scheduler
• Two Quantas
• Thread Quanta
• Process Quanta

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Summary

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