Lecture 5: Process Scheduling Section 2'4

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Lecture 5 Process Scheduling(Section 2.4)
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Schematic Scheduling
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Process Life Cycle
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CPU and I/O bursts
Long CPU burst
Waiting for I/O
Short CPU burst
Interrupt back from I/O operation, ready to use
the CPU.
CPU not needed. Process goes to blocked/waiting
state.
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The Scheduler

• A module in OS to execute scheduling decisions.
• This is done by
• switching context
• switching to user mode
• jumping to the proper location in the user
  program to restart that program

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When is scheduling done?

• Scheduling absolutely required when
• A process exists
• A process becomes blocked (on I/O, etc)
• Scheduling may be required when
• New process created
• I/O interrupt
• Clock interrupt

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Terminology Preemptive vs. non-Preemptive

• Preemptive A Process can be suspended and
  resumed
• Non-preemptive A process runs until it
  voluntarily gives up the CPU (waiting on I/O or
  terminate).
• Most modern OSs use preemptive CPU scheduling,
  implemented via timer interrupts.
• Non-preemptive is used when suspending a process
  is impossible or very expensive e.g., cant
  replace a flight crew in middle of flight.

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Scheduling Performance Metrics

• All systems
• Fairness
• Policy enforcement
• Balance
• Batch Systems
• Throughput
• Turnaround time
• CPU utilization

• Interactive Systems
• Response time
• Proportionality
• Real-time Systems
• Meeting deadlines
• Predictability

Waiting time turnaround time minus solo
execution time
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Scheduling Policies

• Batch systems
• First Come First Served
• Shorted Job First
• Shortest Remaining Time Next
• Interactive systems
• Round Robin
• Priority Scheduling
• Multiple Queues
• Shortest Process Next
• Guaranteed Scheduling
• Lottery Scheduling
• Fair-share Scheduling
• Real-time systems
• Static vs. dynamic

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Example FCFS Scheduling

• First Come, First Served.
• Assume non-preemptive version
• Example Jobs are P1 (duration 24 units) P2
  (3) and P3 (3), arriving in this order at time 0
• Gantt Chart for FCFS schedule
• Average waiting time (0 24 27)/3 17

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FCFS Continued

• What if execution order is P2 , P3 , P1 ?
• Gantt chart
• Average waiting time (6 0 3)/3 3
• Much better!
• Convoy effect many short jobs stuck behind one
  long job

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Scheduling Policy SJF

• Shortest Job First
• Assume each job has a known execution time
• Non-preemptive version Schedule the shortest job
  first
• once CPU given to the process it cannot be
  preempted until completes its CPU burst.
• Preemptive version Shortest Remaining Time Next
• if a new process arrives with CPU burst length
  less than remaining time of current executing
  process, preempt.
• Problem?

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Preemptive Scheduling Round Robin

• Each process gets a small unit of CPU time (time
  quantum), usually 10-100 milliseconds.
• After this time has elapsed, the process is
  preempted and added to the end of the ready
  queue.
• Then the process at head of queue is scheduled.

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Round Robin Example

• Process Burst Time
• P1 53
• P2 17
• P3 68
• P4 24
• Gantt chart for time quantum 20
• Typically, higher average turnaround than SJF,
  but better response.

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Priority Scheduling

• Idea Jobs are assigned priorities.Always, the
  job with the highest priority runs.
• Note All scheduling policies are priority
  scheduling!
• Question How to assign priorities?

priority 1
priority 2
priority M
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Example for Priorities
Static priorities can lead to starvation!