Page 91 - DCAP608_REAL TIME SYSTEMS

Real Time Systems

Notes 8.3 Summary

 In multiprocessor and distributed real-time systems, scheduling jobs dynamically on
processors can be used to achieve better performance.

 Analytical and efficient validation methods for determining whether all the timing
constraints are met do not yet exist for systems using modern dynamic scheduling
strategies.
 Exhaustive methods are often infeasible or unreliable since the execution time and release
time of each job may vary.
 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.

 Priority-driven system or schedulers are online schedulers.
 Priority-driven scheduling is easy to implement.
 In event triggered systems, events invoke an online scheduler, which takes a decision
based on a set of pre-defined rules, e.g., represented as priorities.
 An offline schedulability test can be used to show that, if a set of rules is applied to a given
task set at runtime, all tasks will meet their deadlines.

 Major representative lines of such algorithms are based on fixed priorities, e.g., rate
monotonic or dynamic priorities.
 The scheduling table of offline scheduling provides a “proof by construction” that all
timing constraints will be met.
 When offline guarantees are required, task parameters have to be known offline.
 The terms offline” and online” scheduling cannot be seen as disjoint in general.

8.4 Keywords

Efficiency: It is the capacity to do any work.
On-line Scheduling Algorithm: In this form of scheduling, scheduler makes each scheduling
decision without knowledge about the jobs that will be released in the future the parameters of
each job become known to the on-line scheduler only after the job is released.
Validation Algorithm: It is the form of algorithm that allows us to determine whether all jobs
in a system indeed meet their timing constraints despite scheduling anomalies.

8.5 Review Questions

1. Explain the challenges in validating timing constraints in priority driven system.
2. What are priority-driven schedulers?
3. What is online scheduling?

4. What is offline scheduling?
5. Distinguish between off-line versus on-line scheduling.
6. Write a short note on validation algorithm.

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