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Chapter 3

Chapter 3. RTOS Concepts And Definitions Department of Computer Science Hsu Hao Chen Professor Hsung-Pin Chang. Outline(1/2). Priorities Ready threads and suspended threads Preemptive, priority-based scheduling Round-Robin scheduling Kernel 、 RTOS Context Switch. Outline(2/2).

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Chapter 3

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  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

  3. Outline(2/2) • Interrupt handling、Thread starvation • Priority inversion • Priority inheritance • Preemption-threshold

  4. 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

  5. Priorities(2/2)

  6. 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

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

  8. 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.

  9. Preemptive, priority-based scheduling • A higher priority thread can interrupt and suspend a currently executing thread that has a lower priority

  10. 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

  11. Round-Robin scheduling

  12. 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

  13. 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 thread’s • When a thread’s context is restored then the thread resumes execution at the stopping point

  14. 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

  15. Priority inversion

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

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