Page 140 - DCAP608_REAL TIME SYSTEMS

Unit 14: Advance Priority-driven Scheduling of Periodic Tasks

Notes
Example: Total utilization of T= {(3, 1), (5, 1.5), (7, 1.25), (9, 0.5)} is 0.85, which is larger
than URM(4) = 0.756, but this system is schedulable according to the RM algorithm!

14.1.3 Enhanced Schedulable Utilization

When some of the task parameters are known, this information allows us to improve the
schedulable utilization of the RM algorithm:

 The Utilization of Individual Tasks
 The Number n of Disjoint Subsets Each Containing Simply Periodic Tasks
h
 Some Functions of the Periods of the Tasks

14.1.4 Schedulable Utilization of the RM Algorithm for Multi-frame
Tasks

Consider a task that models the transmission of an MPEG compressed video over a network
link.

 Jobs in this task, modelling the transmissions of individual video frames, are released
periodically.
 The size of I-frames can be very large compared with that of Band P- frames, the execution
times of the jobs can vary widely.
 When modelled as a periodic task, the execution time of the task is equal to the transmission
time of an I-frame.
If we were to determine whether a system of such tasks is schedulable based on the schedulability
tests we have learned, we would surely underutilized the processor.

Notes The multi-frame task model is a more accurate model and leads to more accurate
schedulability tests.

Self Assessment

Fill in the blanks:
1. ……………… analysis method requires the periods and execution times of all the
tasks.
2. As long as the total utilization of a system satisfies, it will never miss any ………………
3. The multi-frame task model is a more ……………… model.
4. ……………… is the schedulable utilization of the RM algorithm when Di= pi for all.

5. A system of n independent, preemptible periodic tasks with relative deadlines equal to
their respective periods can be feasibly scheduled on a ………………

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