Schematic Task Execution Example

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Schematic Task Execution Example
Key AD Task execution time AF Task period AE
Jitter BC Transport lag
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More About Deadlines

• Usually defined as
• Hard
• If missed, usually result in some sort of system
  catastrophic failure
• Firm
• If missed, usually cause the system to produce
  useless (or senseless) results
• Soft
• If missed, usually deter from the usefulness of
  the results in direct proportion to the extent by
  which the soft deadline is missed

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Performance of R-T Systems

• Main players are
• Response time
• Context switching
• Interrupt latency
• Computation speed
• Data storage access speed
• Data transfer rate
• Loads of hardware stats (I/O throughput, bus
  inertia, disk performance, memory/buffer size,
  controller performance, etc.)

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Interrupt Handling

• In non-R-T systems interrupts may be handled
  whenever deemed appropriate by the system
• In R-T systems its the external world which
  dictates when the system is to respond.
• Interrupts need not be sequential - meaning the
  allocation of priorities to different interrupts

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Multi-priority Interrupts
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Important Interrupt Modelling Facts

• More than one real-world interrupt can arrive at
  any time
• Interrupts can be interrupted by others with a
  higher priority
• Too many interrupts can hinder overall R-T system
  performance
• Devise ways by which to minimise the impact of
  multiple interrupts on the system

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Dynamic performance calculation

• Take into account
• Average interrupt frequency
• Average time required to service a serviceable
  interrupt
• Other parts of the software that can interrupt
  their servicing

It is of system-vital importance that the system
remains capable of meeting all its time
constraints even in the face of interrupt
onslaught. Failure to do so will result in
system collapse.
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R-T Task Classification

• Periodic
• Execution starts at pre-defined intervals
• Aperiodic
• Random execution start times
• Server
• Resource sharing and synchronisation

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Task Class Properties
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Example of R-T System Requirements(Based on
example in Software Engineering Fundamentals by
A. Behforooz et al.)
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Execution Time Estimation

• Values required for calculation are
• Average instruction execution time
• Interrupt latency
• Very detailed description of task
• I/O instruction execution time
• Interrupt latency is analysed in the following
  slide ...

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Interrupt Latency Composition
Interrupt latency
Interrupt occurs
Save state of current task
Decode interrupt
Start and complete interrupt actions
Save state of interrupt
Restore state of interrupted task
Continue execution of interrupted task
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Execution Time Estimation Example (1)

• Known quantities are
• Task rate (30 s-1) ? period of 33.33 ms
• Longest exec. path (2900 sloc)
• HLL-to-assembly expansion (61)
• Interrupt latency (0.1 ms)
• Av. int. trans. delay (0.85 ms)
• Av. instruction exec. time (300 ns)
• Assume a cushioning of 20

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Execution Time Estimation Example (2)

• Max task execution time
• 2900 sloc x 6 17,400 tloc
• 1 tloc execution time 0.0003 ms
• Total tloc execution time
• 17,400 tloc x 0.0003 ms 5.22 ms
• Total execution time for 1 task
• (total tloc exe. int. trans. delay int.
  latency)
• 5.22 ms 0.85 ms 0.1 ms 6.17 ms
• Total periodic execution time
• 6.17 ms x 30 tasks/s 185.1 ms
• Include cushioning (20)
• 185.1 ms 10.8 ms 196 ms