Scheduling for Embedded Real-Time Systems

1
Scheduling for Embedded Real-Time Systems

• Amit Mahajan and Haibo

2
Overview

• Real-Time Systems
• Scheduling problem in Real-Time Systems
• Control Dominated Systems
• Dataflow Systems
• Conclusions

3
Real-Time Systems

• Are composed of several distinct cooperating
  tasks
• Are usually non-terminating
• React faster than the rate of the environment
  i.e. the composing tasks have tight timing
  constraints

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Scheduling problem in Real-Time Systems

• Given
• A set processing units with known processing
  capabilities
• A set of tasks with known characteristics and
  constraints on the completion time
• Derive an order of execution of these tasks

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Overview

• Real-Time Systems
• Scheduling problem in Real-Time Systems
• Control Dominated Systems
• Dataflow Systems
• Conclusions

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Static Scheduling Policies

• Tasks execute in a fixed order determined offline
• Suited best to the cases where the time when the
  tasks become enabled are known well in advance
• Easy to debug but usually give low processor usage

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Static Scheduling Policies (contd.)

• Round Robin
• Tasks checked for readiness in a predetermined
  order and executed immediately if they are ready
• Easy to give bounds on the time between a request
  for an execution and the corresponding execution
• Cyclic Scheduling
• Like round robin except that tasks may appear
  more than once in one cycle
• Used in dataflow systems

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Dynamic Scheduling Policies

• Order of task execution decided at runtime
• Usually based on some sort of priority of tasks
• May or may not be pre-emptive
• Suited for systems with irregular task
  activations
• High processor usage but difficult to debug

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Dynamic Scheduling Policies (contd.)

• Work by Liu and Layland. Under strict assumptions
  
• RMS
• Static priorities
• Priority of a task is proportional to its rate
• EDF
• Dynamic priorities
• Priority of a task is inversely proportional to
  the time remaining to its deadline
• Under a set of relaxed conditions, Audsley et al.
  proposed an optimal priority assignment algorithm
  for static priority scheduling

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Schedulability Analysis

• To determine if a set of tasks meets its timing
  constraints
• Common approach worst case response time (WCRT)
  analysis
• Once WCRT is computed, checking whether a task
  meets its deadline is trivial in many cases
• With Liu and Layland assumptions of preemptive
  static priority scheduling and independent
  periodic tasks with fixed runtimes, WCRT analysis
  is easy

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WCRT analysis for realistic real-time embedded
systems

• Real-time embedded systems frequently flout the
  Liu layland assumptions
• state- dependent run-times, dependency between
  tasks and non-periodic events in the environment
• Balarin and Sangiovanni proposed a conservative
  extension to WCRT analysis for systems in which
• Scheduling is either preemptive or nonpreemptive
  static priority
• The environment only needs to respect the timing
  between successive occurrences of events
• FOR MOST SYSTEMS WITH STATE AND INPUT DEPENDENT
  RUNTIMES, TRACTABLE WCRT ANALYSIS IS A MYSTERY!

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Synchronous Scheduling

• E.g Esterel
• Synchronous assumption
• Given a set of communicating tasks, each
  represented by a FSM, the computation of the
  output as well as the communication among the
  FSMs takes place in zero time
• The synchronicity hypothesis reduces the set of
  FSMs to a single product machine
• For synchronous systems, the structure of the
  product machine is equivalent to the static
  scheduling of the component FSMs

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Synchronous Scheduling (contd.)

• Advantages
• Deterministic response
• Ease of debugging and verification
• Problems
• May lead to inefficient execution time as
  compared to task-based scheduling
• Some tasks in a system may not be representable
  as FSM
• Undelayed feedback loops are a cause of great
  trouble

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Overview

• Real-Time Systems
• Scheduling problem in Real-Time Systems
• Control Dominated Systems
• Dataflow Systems
• Conclusions

15
Dataflow Systems

• Dataflow graphs used for scheduling purposes in
  DSP systems
• Explicit representation of interaction between
  tasks
• More emphasis on the throughput of the system
• Two types of dataflow models
• SDF
• static compile time scheduling possible
• DDF or control DF
• more powerful than SDF but require some dynamic
  scheduling
• Most DSP algorithms dont require much decision
  making SDF is a good enough model

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SDF

• Used in many block-diagram based rapid
  prototyping and code generation for programmable
  DSPs
• For uniprocessor scheduling
• Code generator steps through the schedule and
  generates a sequence of actor invocations
• In-line code used instead of sub-routine calls
• Loops used to prevent explosion in code size due
  to multiple appearances of an actor in a schedule

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Research Issues in SDF scheduling

• Minimize code and buffer size used for actors.
  Approaches are
• Minimizing for one of the criterions. Among the
  existing solutions, choosing the solution that
  minimizes the second criterion
• Exploring solutions that optimize based on a
  tradeoff between the two constraints i.e.
  avoiding the extremes
• Minimizing activation schedules

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Conclusions

• Dataflow systems
• Regularity of inputs and control lends itself
  well to static scheduling
• Good techniques exist scheduling on one or
  several processors with tradeoff between
  code-size, buffer size, execution time etc
• Control-dominated systems
• Efficient scheduling schemes known for a very
  small class of systems
• Lack of a good model to capture inter-task
  dependencies
• Lack of good techniques to determine the WCRT on
  complex processors without making unrealistic
  assumptions