Chapter 15: RealTime Software Design

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Chapter 15 Real-Time Software Design

• Yonsei University
• 2nd Semester, 2009
• Sanghyun Park

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Topics Covered

• System design
• Real-time executives
• Monitoring and control systems
• Data acquisition systems

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Real-Time Systems

• A real-time system is a software system where the
  correct functioning of the system depends on the
  _____ produced by the system and the ____ at
  which these results are produced
• Soft real-time system
• Hard real-time system
• Time is critical.Real-time systems MUST respond
  within specified times

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Stimulus/Response Systems

• One way of looking at a real-time system is as a
  stimulus / response system
• Given a stimulus, the system must produce a
  response within a specified time
• Periodic stimuli
• Stimuli which occur at __________ time
  intervals(A temperature sensor may be polled 10
  times per second)
• Aperiodic stimuli
• Stimuli which occur at ___________ times(A
  system power failure may trigger an interrupt
  which must be processed by the system)

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Architectural Considerations

• A real-time system has to respond to stimuli that
  occurat different times
• Control should be transferred to the appropriate
  _______ for that stimulus as soon as it is
  received
• This is impractical in _________ programs
• Real-time systems are usually designed as a set
  of _________, cooperating processes with a
  real-time _________ controlling these processes

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A Real-Time System Model
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Sensor/Actuator Control Processes
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System Elements

• Sensor control
• Collect information from sensors.May buffer
  information collected from a sensor
• Data processor
• Carry out processing of collected information and
  compute the system response
• Actuator control
• Manage actuator operation

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Real-Time System Design Process

• Identify stimuli and associated responses
• Define the ______ constraints associated with
  each stimulus and response
• Aggregate the stimulus and response processing to
  _________ processes
• For each stimulus and response,design _________
  to carry out the required computations
• Design a _________ system which will ensure that
  processes are started in time to meet their
  deadlines
• Integrate the system under the control of a
  real-time _________

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Real-Time System Modeling

• The effect of a stimulus in a real-time system
  may trigger a transition from one state to
  another
• ______ state model is a good, language-independent
  wayof representing the design of a real-time
  system
• The UML includes notations for defining state
  machine models

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State Machine ModelOf A Microwave Oven
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Real-Time Programming

• Hard real-time systems may have to be programmed
  in ________ language to ensure that deadlines are
  met
• System-level languages such as C allow efficient
  programs to be written but do not have constructs
  to support concurrency or _______ resource
  management
• ___ as a language designed to support real-time
  system design includes a general purpose
  concurrency mechanism

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Java As A Real-Time Language

• Java supports lightweight concurrency (threads
  and synchronized methods) and can be used for
  some ____ real-time systems
• Java is not suitable for hard real-time
  programming or programming where precise control
  of timing is required
• Not possible to specify thread execution time
• Uncontrollable garbage collection
• Not possible to discover queue sizes for shared
  resources
• Variable virtual machine implementation
• Not allow for detailed run-time space or
  processor analysis

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Real-Time Executives

• Real-time executives are analogous to an ________
  system in a general-purpose computer
• Responsible for process management and resource
  (processor and memory) allocation
• Components
• Real-time clock (timer)
• Interrupt handler manages aperiodic requests for
  service
• Scheduler chooses the next process to be run
• Resource manager allocates memory and processor
  resources
• Dispatcher starts process execution

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ComponentsOf A Real-Time Executives
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Starting A Process
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Stimuli Priority

• The executive has to be able to manage at least
  two priority levels for the system processes
• ________ level priority highest priority which
  is allocated to processes requiring a very fast
  response
• ______ level priority allocated to periodic
  processes

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Scheduling Strategies

• Non pre-emptive scheduling
• Once a process has been scheduled for
  execution,it runs to _________ or until it is
  blocked for some reason(e.g., waiting for
  input)
• Pre-emptive scheduling
• The execution of an executing process may be
  stoppedif a ______ priority process requires
  service
• Scheduling algorithms
• Round-robin
• Rate monotonic
• Shortest deadline

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Monitoring and Control Systems

• Important standard/generic real-time system
  class
• Continuously check sensors and take actions
  depending on sensor values
• Monitoring systems take an actionwhen some
  _________ sensor value is detected
• Control systems ___________ control hardware
  actuators depending on the value of associated
  sensors

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Burglar Alarm System

• A system is required to monitor sensors on doors
  and windows to detect the presence of intruders
  in a building
• When a sensor indicates a break-in, the system
  switches on lights around the area and calls
  police automatically
• The system should include provision for operation
  without a main power supply

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Stimuli To Be Processed

• Two classes
• Power failureGenerated automatically by a
  circuit monitor.When received, the system must
  switch to backup power within 50 ms
• Intruder alarmStimulus generated by system
  sensors.Response is to call the police, switch
  on building lights and the audible alarm

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Stimulus/ResponseTiming Requirements
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Process Architectureof the Burglar Alarm System
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Control Systems

• The burglar alarm system is a _________ system
  rather than a control system as it does not
  include actuators which are directly affected by
  sensor values
• Control systems are similar but, in response to
  sensor values, the system sends control signals
  to actuators
• An example of a control system is a building
  _______ control system which monitors
  temperatures and switches heaters on and off

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Process Architectureof a Temperature Control
System
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Data Acquisition System

• Collect data from sensors for subsequent
  processing and analysis
• Data collection processes and processing
  processes may have different periods and
  deadlines
• Data collection may be faster than
  processinge.g., collection information about an
  explosion
• ________ or ring buffers are a mechanism for
  smoothing speed differences

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Reactor Data Collection

• A system collects data from a set of sensors
  monitoring the neutron flux from a nuclear
  reactor
• Flux data is placed in a ring buffer for later
  processing
• The ring buffer is itself implemented as a
  __________ process so that the collection and
  processing processes may be synchronized

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The Architecture Of AFlux Monitoring System
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A Ring Buffer For Data Acquisition
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Mutual Exclusion

• _______ processes collect data and add it to the
  buffer. ________ processes take data from the
  buffer and make elements available
• Producer and consumer processes must be mutually
  ________ from accessing the same element
• The buffer must stop producer processes from
  adding information to a full buffer and consumer
  processes from trying to take information from an
  empty buffer