Page 89 - DCAP608_REAL TIME SYSTEMS

Real Time Systems

Notes violated in any situation, an offline approach only needs to show that one situation exists,
i.e., the scheduling table, in which the timing constraints are met: instead of a “for all”
proof, considering even situations which may never occur during the runtime of a system,
a “there exists one” suffices.
Thus, the process follows the steps: task set with timing constraints-schedulability test and
determination of rules (for example, via directives priority or deadline) – execution of rules by
runtime scheduler-timing constraints met.

Looking closer, we can see that TT real-time scheduling work in the same way. Instead of a
definition of rules, e.g., earliest deadline first, “the decisions on which task to execute are
represented in the scheduling table, schedule next task as given table”.
While TT scheduling has to assume a periodic world and ET provides flexibility for tasks with
not fully known parameters, example, aperiodic, the difference concerns mostly runtime
execution without guarantees. When offline guarantees are required, task parameters have to be
known offline: without worst case execution, period or maximum arrival frequency, offline
guarantees cannot be given, independent of the scheduling paradigm used.
Hence, we can conclude that the terms “offline” and “online” scheduling cannot be seen as
disjoint in general. Real-time scheduling requires offline guarantees, which require assumptions
about online behavior at design time. At runtime, both offline and online execute according to
some (explicitly or implicitly) defined rules, which guarantee feasibility. The question “offline”
vs. “online” is thus less black and white, but more about how much of the decision process is
Thus, both offline and online are based on a substantial offline part. The question is then where
to set the tradeoff between determinism – all decisions offline - and flexibility – some decisions
online.

Lab Exercise Search through internet and make points of differences between the terms
“offline” and “online” scheduling.

Self Assessment

State whether the following statements are True or False:
7. Defined rules are represented as priorities.
8. 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.
9. Major representative lines of such algorithms are based on fixed priorities, e.g., rate
monotonic or dynamic priorities.

10. The scheduling table of offline scheduling provides a “proof by construction” that all
timing constraints will be met.
11. When offline guarantees are required, task parameters have to be known online.

12. The terms “offline” and “online” scheduling cannot be seen as disjoint in general.

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