APERIODIC TASK SCHEDULING

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APERIODIC TASK SCHEDULING
Notation
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Earliest Due Date (EDD) - Jacksons Rule
Set of tasks
Problem
Algorithm
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Earliest Due Date (EDD) - Jacksons Rule
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Earliest Due Date (EDD) Example 1
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Earliest Due Date (EDD) Example 2
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Earliest Due Date (EDD) Guaranteed Feasibility
Order tasks by increasing deadlines. Then
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Earliest Deadline First (EDF) Horns Algorithm
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Earliest Deadline First (EDF) Horns Algorithm
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Earliest Deadline First (EDF) Example
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Earliest Deadline First (EDF) Guarantee of
Schedualability

• Dynamic Scheduling
• Assume Schedulable
• Need to Guarantee that

Assuming all tasks are ordered by increasing
deadlines
Worst case finishing time
For Guaranteed Schedulability
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EDF - Non-Preemptive Scheduling
The problem is NP hard
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Non-Acyclic Search Tree Scheduling
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Bratleys Algorithm
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Jack Stankovics Spring Algorithm

• This does not yield an optimal schedule, but the
  general problem is NP hard. This does lend
  itself to artificial intelligence and learning.
• The objective is to find a feasible schedule when
  tasks are have different types of constraints,
  such as
• precedence relations,
• resource constraints,
• arbitrary arrivals,
• non-preemptive properties, and
• importance levels.
• A heuristic function H is used to drive the
  scheduling toward a plausible path.
• At each level of the search, function H is
  applied to each of the remaining tasks. The task
  with the smallest value determined by the
  heuristic function H is selected to extend the
  current schedule. If a schedule is not looking
  strongly feasible, a minimal amount of
  backtracking is used.

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Jack Stankovics Spring Algorithm
Precedence constraints can be handled by adding
a term E 1 if the task is eligible and E
infinity if it is not.
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Jack Stankovics Spring Algorithm
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Scheduling with Precedence ConstraintsLatest
Deadline First - Optimizes max Lateness
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Latest Deadline First
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EDF with Precedence Constraints

• The problem of scheduling a set of n tasks with
  precedent constraints and dynamic activations can
  be solved if the tasks are preemptable.
• The basic ideas is transform a set of
  dependent tasks into a set of independent tasks
  by adequate modification of timing parameters.
  Then, tasks are scheduled by the Earliest
  Deadline First (EDF) algorithm, iff is
  schedulable. Basically, all release times and
  deadlines are modified so that each task cannot
  start before its predecessors and cannot preempt
  their successors.

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EDF with Precedence Constraints
Modifying the release time
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EDF with Precedence Constraints
Modifying the Deadlines
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Summary
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