Introduction to RealTime Scheduling

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Introduction to Real-Time Scheduling

• Fred Kuhns
• Applied Research Laboratory
• Computer Science
• Washington University

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Introduction

• Considerations
• schedulability analysis performed
• static or dynamic schedulability analysis
• analysis result includes schedule
• Scheduling Paradigms
• static table-driven - static analysis and
  schedule
• static priority driven - static analysis, no
  (explicit) schedule, priorities.
• dynamic planning based - feasibility check at
  run-time, schedule produced
• dynamic best effort - No feasibility check,
  attpemts to meet deadlines but no guarantee.

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Scheduling Algorithms

• Most instances, scheduling problem is intractable
• Resource constrained scheduling is NP-complete.
  Precedence constraints agravate the problem.
• Heuristics and approximation techniques used to
  handle tasks with complex requirements.

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Static Table Driven

• Typically applied to systems with safety critical
  requirements.
• Resources preallocated to tasks
• Periodic tasks feasible schedule if and only if
  there exists a feasible schedule for the
  Hyperperiod (least common multiple of the
  periods)
• RM or EDF used for relatively simple systems
  (simple characteristics)

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Priority-driven Preemptive

• Most famous analysis of the problem is Liu and
  Layland where they described RM and EDF.
• single processor, periodic tasks, preemption
• RM static priorities based on period. Optimal
  among static-priority schemes.
• EDF dynamic priority based on deadline.
• May assign priority based on laxity - LLF Least
  Laxity First
• May assign priority based on any of the tasks
  parameters

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Priority Driven

• Preemptive scheduling may simplify analysis but
  overhead needs to be considered.
• scheduling algorithm
• task dispatcher
• context switch overhead

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Dynamic Planning-based

• Focus on dynamic feasibility checks
• Must consider worst case execution times,
  resource requirements, timing constraints,
  periodic tasks, preemption, precedence
  constraints, importance levels and fault
  tolerance
• If task arrival times are not known a priori,
  cannot guarantee performance
• Various heuristic algorithms for scheduling tasks
  in a distributed environment.

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Dynamic Best-Effort

• Used by many systems system computes task
  priorities and schedules accordingly. Extensive
  simulations are used to gain confidence in
  behavior
• May compensate for other priority driven
  algorithms (EDF and RM) which perform poorly in
  overload conditions
• May use a value function to schedule during
  overload maximize sum of task values. May also
  discard low value tasks
• Dynamic scheduling is optimal if it always
  produces a feasible schedule whenever a
  clairvoyant algorithm does.

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Other Issues

• Fault tolerance
• static primary and alternate tasks.
• static precomputed contingency schedules to
  speed recovery
• fault-tolerance vs timeliness
• Combine dynamic scheduling with imprecise
  computations to effect tradeoffs

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Resource Reclaiming

• Utilizing unused time variance in task execution
  time may result in unused time.
• Run time anomalies when actual computation times
  of nonpreemptive tasks differ from expected
  causing unexpected behavior.
• Resource reclamation algorithms can improve
  schedulability of dynamically arriving tasks.

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RTOS

• Primary function areas
• Process management and synchronization
• Memory management
• Interprocess communication
• I/O
• Categories of RTOS
• small proprietary
• RT extensions to commercial timesharing systems
• research operating systems

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Small Proprietary OSes

• Typically, kernel provides
• fast context switch, small size, quick interrupt
  response time (predictable?), minimize interrupt
  disable times, memory partitions (no virtual
  memory, why?), special sequential files (why?)
• Timing
• bounded execution time of most primitives
• real-time clock
• priority scheduling mechanism
• special alarms and timeout

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RT Extensions to Commercial OSes

• Advanced SW development environments
• Problems
• optimized for average case
• assigns resources on demand
• ignore application specific information
• independent CPU scheduling and resource allcoation

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Research Operating System

• Develop extended or new process models
• RT synchronization mechanisms
• facilitate timing analysis
• fault tolerance
• multiprocessor or distributed support
• real-time micro-kernel

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Clock Driven

• Scheduling time instants When scheduling
  decisions are made
• clock-driven - instants predefined offline
• table-driven - table precomputed offline

Scheduling time instants
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Weighted Round Roben

• Round Robin Scheduling - time-sharing systems
• Weighted Round Robin associate weight with each
  job.
• weight w number of time slices
• adjusting weight allocates CPU shares

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Priority Driven

• Work-conserving The highest priority, runnable
  job is always dispatched to available CPU
• resources are never idle while runnable jobs
• Priority list, preemptions and other rules
  describe algorithm completly
• Example algorithms EDF, FIFO, LIFO etc.
• preemption versus non-preemption