Introduction to RealTime Systems

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Introduction to Real-Time Systems

• Fred Kuhns
• Computer Science Dept.
• Washington University

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What is a Real-Time System?

• Real-time systems have been defined as "those
  systems in which the correctness of the system
  depends not only on the logical result of the
  computation, but also on the time at which the
  results are produced"
• J. Stankovic, "Misconceptions About Real-Time
  Computing," IEEE Computer, 21(10), October 1988.

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Real-Time Characteristics

• Real-time systems often are comprised of a
  controlling system, controlled system and
  environment.
• Controlling system acquires information about
  environment using sensors and controls the
  environment with actuators.
• Timing constraints derived from physical impact
  of controlling systems activities. Hard and soft
  constraints.
• Periodic Tasks Time-driven recuring at regular
  intervals.
• Aperiodic event-driven.

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Typical Real-Time System
Con trolled System
Controlling System
Environment
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Some Definitions

• Timing constraint constraint imposed on timing
  behavior of a job hard or soft.
• Release Time Instant of time job becomes
  available for execution. If all jobs are
  released when the system begins execution, then
  there is said to be no release time
• Deadline Instant of time a job's execution is
  required to be completed. If deadline is
  infinity, then job has no deadline. Absolute
  deadline is equal to release time plus relative
  deadline
• Response time Length of time from release time
  to instant job completes.

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Hard versus Soft

• Hard failure to meet constraint is a fatal
  fault. Validation system always meets timing
  constraints.
• Deterministic constraints
• Probabilistic constraints
• Constraints in terms of some usefulness function.
• Soft late completion is undesirable but
  generally not fatal. No validation or only
  demonstration job meets some statistical
  constraint. Occasional missed deadlines or
  aborted execution is usually considered
  tolerable. Often specified in probabilistic terms

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Validating Constraints

• Validation Demonstration by a provably correct,
  efficient procedure or by exhaustive simulation
  and testing. Involves three steps
• timing constraints of each application and
  corresponding components are consistent,
• each component can meet its timing constraints if
  executed alone and required resources are
  available,
• The underlying scheduling algorithm(s), all
  timing constraints are met

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Developing a Reference Model

• Modeling the system to focus on timing properties
  and resource requirements. Composed of three
  elements
• workload model - describes applications supported
  by system
• Temporal parameters
• Precedence constraints and dependencies
• Functional parameters
• resource model - describes system resources
  available to applications
• Modeling resources (Processors and Resources)
• Resource parameters
• algorithms - defines how application uses
  resources at all times.
• Scheduling Hierarchy

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Tasks and Jobs Definitions

• Task (Ti) Set of related jobs jointly provide
  function.
• Job (Jij) Unit of work, scheduled and executed
  by system. characterized by the following
  parameters
• Temporal parameters timing constraints and
  behavior
• Functional parameters intrinsic properties of
  the job.
• Resource parameters resource requirements.
• Interconnection parameters how it depends on
  other jobs and how other jobs depend on it

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Resources

• Resources can be divided into passive and active
• Active resources Processors (Pi) they execute
  jobs.
• Every job must have one or more processors
• Same type if functionally identical and used
  interchangeably.
• Passive resource Resource (Ri)
• job may require Resources in addition to
  processor.
• reusable resources are not consumed

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Job/Task Temporal Parameters

• Hard real-time number and parameters of tasks
  are known at all time.
• for Job Ji
• ri - release time, may know range r-, r
  (jitter). For aperiodic/sporadic release or
  interrelease time is a random variable.
• di - absolute deadline
• Di - relative deadline
• (ri, di - feasible interval
• ei - Execution time. May know range e-, e.
  Most deterministic models use e.

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Periodic Task Model

• Jobs repeated at regular or semi-regular
  intervals modeled as periodic.
• Task Ti is a serious of periodic Jobs Jij.
• pi - period, minimum interrelease interval
  between jobs in Task Ti. Must be bounded from
  below.
• ei - maximum execution time for jobs in task Ti.
• rij - release time of the jth Job in Task i (Jij
  in Ti).
• ?i - phase of Task Ti, equal to ri1.
• H - Hyperperiod Least Common Multiple of pi for
  all i H lcm(pi), for all i.
• ui - utilization of Task Ti.
• U - Total utilization Sum over all ui.

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Aperiodic and Sporadic Tasks

• A periodic or Sporadic task is a stream of
  aperiodic or sporadic jobs.
• Jobs with a task have similar statistical
  behavior and timing requirements
• Assumed system is stationary within the
  hyperperiod
• Aperiodic jobs have soft or no deadlines. Want
  responsiveness.
• Sporadic jobs have hard deadlines

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Precedence Constraints and Task Graphs

• Jobs are either precedence constrained or
  independent.
• Precedence relation partial ordering operator lt
  
• Ji lt Jk Ji is predecessor of Jk, Jk is
  successor of Ji
• directed graph G (J,lt)
• Task graph is an extended precedence graph.

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Functional Parameters

• Preemptivity
• Preemption suspend job then dispatch different
  job to processor. Cost includes context switch
  overhead.
• Non-preemptable task - must be run from start to
  completion.
• Criticalness - positive integer indicating the
  relative importance of a job. Useful during
  overload.
• Optional Executions - jobs or portions of jobs
  may be declared optional. Useful during overload.
• Laxity - Laxity type gt hard or soft timing
  constraints. Supplemented by a usefulness
  function. Useful during overload.

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Resource Parameters

• Job resource parameters indicate processor and
  resource requirements.
• Preemptivity of resources
• non-preemptive serial access, typical case.
• preemptive - jobs can interleave access.
• Resource Graph
• vertex for processors (Pi) and resources (Ri)
• is-a-part-of edge Ri -gt Rk, Rk is a part of Ri
• accessibility edge Pi -gt Pk, cost of Pi
  accessing Pk

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Schedules and Scheduling

• Jobs scheduled and allocated resources based on a
  set of scheduling algorithms and access control
  protocols.
• Scheduler Module implementing scheduling
  algorithms
• Schedule assignment of all jobs to available
  processors, produced by scheduler.
• Valid schedule
• every processor assigned to at most one job at a
  time
• every job assigned to at most one processor at a
  time
• no job scheduled before its release time
• Total amount of processor time assigned to every
  job is equal to its maximum or actual execution
  time

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Definitions

• Feasible schedule Every job starts at or after
  release time and completes by deadline
• Schedulable set of jobs schedulable according to
  an algorithm if the it always produces a feasible
  schedule.
• Optimal Scheduling algorithm optimal if it
  always produces a feasible schedule if such a
  schedule exists
• Tardiness Zero if completion time lt deadline,
  otherwise gt 0 (complete - deadline).
• Lateness difference between completion time and
  deadline, can be negative if early.

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Performance Measures

• Miss rate percentage of jobs executed but
  completed late
• Loss rate percentage of jobs discarded
• Invalid rate sum of miss and loss rate.
• makespan If all jobs have same release time and
  deadline, then makespan response time of the
  last job to execute.
• Max or average response times
• Max or average tardiness/lateness