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CprE 458/558: Real-Time Systems

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

  2. 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)

  3. 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)

  4. 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)

  5. 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)

  6. 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)

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

  8. Reclaiming – Example (contd.) CprE 458/558: Real-Time Systems (G. Manimaran)

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