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CprE 458/558: Real-Time Systems PowerPoint PPT Presentation


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CprE 458/558: Real-Time Systems. Resource Reclaiming. Dynamic Scheduling Model. Schedulability checking (on-line) - by the scheduler Schedule construction (on-line) - by the scheduler Dispatching, Task execution, & Resource reclaiming - by the processors. Resource Reclaiming.

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CprE 458/558: Real-Time Systems

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Cpre 458 558 real time systems l.jpg

CprE 458/558: Real-Time Systems

Resource Reclaiming

CprE 458/558: Real-Time Systems (G. Manimaran)


Dynamic scheduling model l.jpg

Dynamic Scheduling Model

  • Schedulability checking (on-line) - by the scheduler

  • Schedule construction (on-line) - by the scheduler

  • Dispatching, Task execution, & Resource reclaiming - by the processors

CprE 458/558: Real-Time Systems (G. Manimaran)


Resource reclaiming l.jpg

Resource Reclaiming

Resource reclaiming refers to the problem of utilizing resources left unused by a task when:

  • it executes less than its worst case computation time. The actual computation time of a task can be less than its worst case computation time due to

    • data dependent loops and conditional statements in the task code.

    • architectural features of system such as cache hits and branch predictions.

CprE 458/558: Real-Time Systems (G. Manimaran)


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Resource Reclaiming (Contd.)

  • it is deleted from the current schedule. Deletion of a task takes place when extra tasks are initially scheduled to account for fault tolerance.

  • it is dropped to make room for more critical tasks.

CprE 458/558: Real-Time Systems (G. Manimaran)


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Properties of Reclaiming Algorithms

  • Correctness: No run-time anomaly.

  • Inexpensive: The overhead cost of a resource reclaiming should be very low compared to tasks’ computation times.

  • Bounded complexity: The complexity of a resource reclaiming algorithm should be independent of the number of tasks in the schedule.

  • Effectiveness: A resource reclaiming algorithm is effective only when its performance is better than that of when no resource reclaiming is used.

CprE 458/558: Real-Time Systems (G. Manimaran)


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Resource Reclaiming Algorithms

  • Basic Algorithm: If there are “simultaneous holes,” in the schedule, it can be reclaimed.

  • Early Start Algorithm: reclaims resources with the condition that if two tasks are non-overlapping in the pre-run schedule, then they should not overlap in the post-run schedule.

  • RV Algorithm: Based on a data structure, called Restriction Vector.

  • RV with Migration: Allows task migration from one processor to another.

CprE 458/558: Real-Time Systems (G. Manimaran)


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Reclaiming -- Example

CprE 458/558: Real-Time Systems (G. Manimaran)


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Reclaiming – Example (contd.)

CprE 458/558: Real-Time Systems (G. Manimaran)


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