RTOS Concepts And Definitions

1
Chapter 3

• RTOS Concepts And Definitions
• Department of Computer Science
• Hsu Hao Chen
• Professor
• Hsung-Pin Chang

2
Outline(1/2)

• Priorities
• Ready threads and suspended threads
• Preemptive, priority-based scheduling
• Round-Robin scheduling
• Kernel? RTOS
• Context Switch

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Outline(2/2)

• Interrupt handling?Thread starvation
• Priority inversion
• Priority inheritance
• Preemption-threshold

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Priorities(1/2)

• Most real-time systems use a priority system as a
  means of establishing the relative importance of
  threads.
• Priorities include two classes
• Static priority
• Dynamic priority
• ThreadX provides priority values from 0 to 31,the
  value 0 represents the highest priority

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Priorities(2/2)
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Ready threads and suspended threads(1/3)

• ThreadX maintains to manage threads two classes
• Suspended Threads List
• Occur
• Wait for an unavailable resource
• Removed
• It is placed on the Ready Thread List
• It is terminated

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Ready threads and suspended threads(2/3)
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Ready threads and suspended threads(3/3)

• Ready Threads List
• Occur
• Thread is ready for execution
• Remove
• When ThreadX schedules a thread for execution
• If all the threads on the list have equal
  priority,ThreadX selects the thread that has been
  waiting the longest.

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Preemptive, priority-based scheduling

• A higher priority thread can interrupt and
  suspend a currently executing thread that has a
  lower priority

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Round-Robin scheduling

• Provide processor sharing in the case in which
  multiple threads have the same priority
• Two primary ways are supported by ThreadX
  Round-robin processing and cooperative
  multithreading

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Round-Robin scheduling
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Kernel? RTOS

• A kernel is minimal implementation of an RTOS
• Consists of at least a scheduler and a context
  switch handler
• Full-blown operating systems
• RTOS is dedicated to the control of hardware and
  must operate within specified time contraints

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Context Switch

• Context is the current execution state of a
  thread
• Consists of such items as the program counter
  ,register and stack pointer
• Context switch refers to the saving or restoring
  a different threads
• When a threads context is restored then the
  thread resumes execution at the stopping point

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Interrupt handling?Thread starvation

• Interrupt handling
• Interrupt vector
• Interrupt service routine (ISR)
• Thread starvation
• Priority-based scheduling
• Solution would be to gradually raise the priority
  of starved

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Priority inversion
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Preemption-threshold

• Disabling preemption
• Threads with priorities higher than 15 will be
  permitted
• Even though priorities 15 through 19 are higher
  than threads priority of 20,threads with those
  priorities will not be allowed to preempt this
  thread