ECE 695 Embedded Systems Winter 2006

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ECE 695 Embedded SystemsWinter 2006

• Session 7

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Summary to Date

• Foreground/Background Schedules
• Clock driven
• Deterministic
• Frame based
• Static
• Advantages
• Hard deadline management
• Best periodic performance
• Disadvantages
• Manual schedule development
• Difficult to develop for large number of tasks
• Difficult to integrate aperiodic task handling

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Timing Constraints

• Release Time
• Time when task becomes available for execution
• Foreground/background approach
• Available at start of minor frame (when scheduled
  in minor frame)
• E.g. task 1 executes in every minor frame
• T1k k tm
• That is, the release time for the kth iteration
  of task 1(T1) is obtained by multiplying the
  minor frame time (tm) by k.
• Deadline
• Time when task must be completed
• In the example above, the absolute deadline is
  the start of the next minor frame (k1) tm
• In reality, it is the time the next scheduled
  task must startktme1
• Where e1is the execution time of task 1

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Timing Constraints (cont)

• Response time
• For aperiodic tasks, may be more natural to talk
  about response time
• Define as length of time between release time and
  completion
• Maximum allowable response time called relative
  deadline
• In our class examples, aperiodic events were
  detected and the task was placed in a queue to
  run when there was time
• Release time is the time it is placed in the
  queue
• We have not been concerned with response time so
  far.

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Clock Driven Scheduling

• Examples drawn from clock driven,
  foreground/background schdules
• Terms not very useful for scheduling purposes
• Period and execution time more important
• Constraints applicable for event driven
  scheduling

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Consider Event Driven Scheduling

• Periodic Tasks
• Assumptions
• Tasks are independent
• No aperiodic tasks for now
• Schedule based on priority
• Assume static priorities
• Most current real-time systems use static
  priorities
• Priority may be assigned apriori or when a task
  is released

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Periodic ScheduleFrame Based
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Timing Constraints

• Release Timetasks in minor frame 5
• T45 5 tm
• T25 5 tm e4
• T15 5 tm e1
• T155 5 tm e15

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Priority Scheduling of Periodic Tasks

• Rate Monotonic Scheduling
• Priorities assigned based on task period
• Shorter the period, the higher the priority
• Since rate is inverse of period, higher rate
  means higher priority

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Consider the Example
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Exercise

• Construct a rate monotonic schedule from this
  example

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What does all this mean

• Event driven systems require scheduling software
• Schedules built on line (as events occur)
• Under certain conditions, can run periodic
  systems
• This is usually the realm of the Real-Time
  Operating System (RTOS)
• We will consider the RTOS next time