Page 87 - DCAP608_REAL TIME SYSTEMS

Real Time Systems

Notes
Did u know? Priority-driven scheduling is easy to implement. It does not require the
information on the release times and execution times of the jobs a priori.
Apart from the merit Priority-driven scheduling has demerits too. The timing behavior of a
priority driven system is non-deterministic.

!
Caution It is difficult to validate that all jobs scheduled in a priority-driven manner meet
their deadlines when the job parameters vary.

Validating Priority-Driven Systems is Much Harder than Time-Driven Systems

Timing characterization of Priority-Driven systems is difficult since these systems are not as
deterministic as the Time-Driven systems.
Validation of a system requires determination that all jobs meet their deadlines as specified by
the system requirements:
 Predictability of tasks

 Execution of jobs in a set of independent preempt-able jobs is predictable if priority-
driven scheduling is used on a processor
 Validation of a set of independent, preemptive, static, single processor priority-driven
systems are relatively easy since the execution of the jobs are predictable
 Maximum execution time is used to validate these systems

Self Assessment

Fill in the blanks:
1. In multiprocessor and distributed real-time systems, scheduling jobs ……………… on
processors can be used to achieve better performance.
2. Analytical and efficient ……………… methods for determining whether all the timing
constraints are met do not yet exist for systems using modern dynamic scheduling
strategies.
3. Exhaustive methods are often ……………… or ……………… since the execution time and
release time of each job may vary.
4. Priority-driven scheduling is ……………… to implement.
5. Priority-driven system or schedulers are ……………… schedulers.

6. In event triggered systems, events invoke an online scheduler, which takes a decision
based on a set of ……………… rules.

8.2 Off-line Versus On-line Scheduling

The heated debate on whether offline or online scheduling is preferable for real-time systems
has taken attention away from the question how the two are actually defined and what their
essential differences are. Results and arguments in the debate are typically based on a
conglomerate of assumptions, system design issues, and particular views of scheduling rather

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