ITFN 2601 Introduction to Operating Systems

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ITFN 2601Introduction to Operating Systems

• Process Scheduling

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Agenda

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

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Processes

• A process is an executing Program
• Multiprogramming
• Consist of Program, input, output and a state

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Why Processes?

• Separation of Tasks
• Pseudo-Parallel Execution
• One Job, One Process

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Process Creation

• System Initialization
• System call by running process
• User request to create new process
• Initiation of batch job

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Process Termination

• Normal Exit
• Error Exit
• Fatal Error
• Killed by another process

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Process States

• Running
• Ready
• Blocked

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Scheduling

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

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Process Internals

• States
• Running
• Ready
• Blocking
• Process Control Block
• State
• Registers Memory
• PC, SP, Files

Running
Blocked
Ready
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Process Control

• Each Process must maintain its own PCB
• Information that Multiple Processes may use is
  duplicated

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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 kinds 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
  (CPU's, 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 the job arrives
  in the system and when it starts to produce
  output. For interactive jobs, response time might
  be more important than turnaround.
• Waiting 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 amount 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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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 SJF, called
  shortest-remaining-time-first (SRTF).
• Starvation is possible

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Three-Level Scheduling

• Admission Scheduler which jobs to admit to the
  system
• Memory Scheduler Which processes are kept in
  memory and which on disk
• CPU Scheduler Pick ready process to run

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Round-Robin

• Round-robin (RR). RR keeps all the burst in a
  queue and runs the first one, like FCFS. But
  after a length of time q (called a quantum), if
  the current burst hasn't completed, it is moved
  to the tail of the queue and the next burst 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.
• This process is moved to the ready state (the
  preempt arc in the diagram.
• The next job in the ready list (normally a queue)
  is selected to run

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Round Robin

• As q gets large, RR approaches FCFS
• As q gets small, RR approaches PS
• What q should we choose
• Tradeoff
• Small q makes system more responsive
• Large q makes system more efficient since less
  switching

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Round Robin, Example
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Priority Scheduling

• Always to run the highest priority burst
• 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 Next
• Estimation of Job 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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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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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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Summary

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