Chapter 6 RealTime Embedded Multithreading

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Chapter 6 Real-Time Embedded Multithreading

• The Thread The Essential Component

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OUTLINE

• Thread Control Block
• Summary of Thread Services
• Execution Overview
• Thread States
• Thread Design
• Thread Internals

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Thread Control Block (cont.)

• The Thread Control Block (TCB) is a structure
  used to maintain the state of a thread during
  run-time.
• A TCB can be located anywhere in memory, but it
  is most common to make the Control Block a global
  structure by defining it outside the scope of any
  function.

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Thread Control Block (cont.)

• In most case, the developer need not know the
  contents of the TCB.

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Thread Control Block (cont.)

• Two useful members of the Thread control Block
• tx_run_count
• This member contains a count of how many times
  the thread has been scheduled.
• tx_state
• This member contain the state of the
  associated thread.

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Thread Control Block (cont.)
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Thread Control Block

• The develop may inspect the members of the TCB,
  but is strictly prohibited from modifying them.
• The value of tx_state for an executing thread is
  TX_READY.(0x00)

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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)

• Thread Creation

• A thread is declared with the TX_THREAD
  data type and is defined with the
  tx_thread_create service.
• Each thread must have its own stack
• The stack size is crucial, it must be large
  enougth to accommodate worst-case function call
  nesting, local variable allocation, and saving
  the threads last execution context

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Summary of Thread Services (cont.)

• The predefined minimum stack size constant,
  TX_MINIMUM_STACK, is probably too small for most
  application.
• It is important to design applications that
  create a reasonable number of thread and that
  avoid excessive stack usage within threads.
• Developers should generally avoid recursive
  algorithms and large local data structure.

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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)

• What happens when a stack area is too small?
• An important feature of multithreading is that
  the same C function can be called from multiple
  threads.
• A reentrant function is one that can be safely
  called while it is already being executed.

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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)

• If a thread is placed in completed state, it
  cannot be executed again.

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Summary of Thread Services (cont.)

• Thread Deletion

• A thread can be deleted only if it is in a
  terminated or completed state.
• This service cannot be called from a thread
  attempting to delete itself.

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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)

• It is the responsibility of the application to
  manage the memory area used by the deleted
  threads stack, which is available after the
  thread has beendeleted.

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Summary of Thread Services (cont.)

• Identify Thread

• The tx_thread_identify service returns a pointer
  to the currently executing thread.
• If this service is called from an ISR, then the
  return value represents the thread that was
  running prior to the executing interrupt handler.
• My_thread_ptr tx_thread_identify()

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Summary of Thread Services (cont.)

• Get Thread Information

• The tx_thread_info_get service obtain such
  information about a thread.

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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)

• Preemption-Threshold Change

• The service tx_thread_preemption_change changes
  the preemption-thres_hold of an existing thread.

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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)

• Priority Change

• When this service is called, the
  preemption-threshold of the specified thread is
  automatically set to the new priority.

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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)

• Relinquish

• A thread may voluntarily relinquish control to
  another thread by using the tx_thread_relinquish
  service.

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Summary of Thread Services (cont.)

• Speedy_thread 5,5 slow_thread 10,10

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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)

• Speedy 5,5
• Slow 10,10

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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)

• Resume Thread Execution

• When a thread is created with the TX_DONT_START
  option, it is placed in a suspended state.
• When a thread is suspended because of a call to
  tx_thread_suspend, it is also placed in a
  suspended state.
• The only way such threads can be resumed is when
  another thread calls the tx_thread_resume service.

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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)

• Thread Sleep

• On some occasions, a thread needs to be suspended
  for a specific amount of time .
• This is achieved with the tx_thread_sleep
  service, which causes the calling thread to
  suspend for the specified number of timer-ticks
• Status tx_thread_sleep(100)

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Summary of Thread Services (cont.)

• Suspend thread Execution

• A specified thread can be suspended by calling
  the tx_thread_suspend service.
• A thread can suspend itself , it can suspend
  another thread , or it can be suspended by
  another thread.
• This type of suspension is called unconditional
  suspension.
• Condition suspension in which a thread is
  suspended because it is waiting for a resource
  that is not available, or a thread is sleeping
  for a specific period of time.

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Summary of Thread Services (cont.)

• If the specified thread is already suspended
  conditionally, the unconditional suspension is
  held internally until the prior suspension is
  lifted.
• When the prior suspension is lifted, the
  unconditional suspension of the specified thread
  is then performed.

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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)

• Terminate Application Thread

• This service terminates the specified application
  thread, regardless of whether or not that thread
  is currently suspended.
• A terminated thread cannot be executed again.
• If you need to execute a terminated thread, then
  you must delete it and then create it again.

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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)

• Time-Slice Change

• This service permits a thread to change its own
  time-slice or that of another thread.
• Note that if a preemption-threshold has been
  specified, then time-slicing for that thread is
  disabled.

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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)
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Summary of Thread Services (cont.)

• Abort Thread suspension

• This service aborts sleep or any wait-related
  suspension of the specified thead.
• This service does not release explicit suspension
  that is made by the tx_thread_suspend service.

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Summary of Thread Services (cont.)
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Summary of Thread Services
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Execution Overview (cont.)

• There are four types of program execution within
  a threadX aplication initialization, thread
  execution, interrupt service routines(ISRs), and
  application timers.

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Execution Overview (cont.)
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Execution Overview (cont.)

• Initialization includes all program execution
  between processor reset and the entry point of
  the thread scheduling loop.
• The scheduling loop looks for an application
  thread that is ready for execution.
• When a ready thread is found, ThreadX transfer
  control to it.
• Without interrupts it would be extremely
  difficult to respond to changer in the external
  world in a timely manner.

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Execution Overview

• What happen when an interrupt occurs?
• Interrupt may also occur inside an executing ISR
  or an application timer.
• Application timers are very similar to ISRs,
  except actual hardware implementation is hidden
  from the application.
• Like ISRs, application timers most often
  interrupt thread execution.
• Unlike ISRs, application timers cannot interrupt
  each other

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Thread States
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Thread Design (cont.)

• Minimize the number of thread in the application
  system
• Choose priorities carefully
• Minimize the number of priorities
• Consider preemption-threshold
• Consider priority inheritance when mutexes are
  employed
• Consider round-robin scheduling
• Consider time-slicing

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Thread Design (cont.)

• Minimize the number of thread in the application
  system

• The number of threads in an application
  significantly affects the amount of system
  overhead.

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Thread Design (cont.)

• Choose priorities carefully

• Misuse of thread priorities can starve other
  thread, create priority inversion, reduce
  processing bandwidth, and make the applications
  run-time behavior difficult to understand.

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Thread Design (cont.)

• Minimize the number of priorities

• An application that has many different thread
  priorities inherently requires more system
  overhead than one with a smaller number of
  priorities.
• If distinct priorities for these thread are
  required, then the ThreadX preemption-threshold
  mechanism can prevent these extra context
  switches.

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Thread Design (cont.)

• Consider preemption-threshold

• There are three primary methods of preventing
  priority inversion in ThreadX.
• First, the developer can select application
  priorities and design run-time behavior in a
  manner that prevents the priority inversion
  problem
• Second, lower-priority threads can utilize
  preemption-threshold to block preemption from
  intermediate threads while they share resources
  with higher-priority threads.

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Thread Design (cont.)

• Finally, threads using ThreadX mutex objects to
  protect system resources may utilize the optional
  mutex priority inheritance to eliminate
  nondeterministic priority inversion.

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Thread Design (cont.)

• Consider Priority Inheritance

• Note that priority inheritance is available only
  with a mutex but not with a counting semaphore.

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Thread Design (cont.)

• Consider Round-Robin Scheduling

• ThreadX supports round-robin scheduling of
  multiple thread that have the same priority.
• This is accomplished through cooperative calls to
  the tx_thread_relinquish service.

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Thread Design

• Consider Time-Slicing

• Time-slicing provides another form of round-robin
  scheduling for threads with the same priority .

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Thread Internals
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