Page 88 - DCAP608_REAL TIME SYSTEMS

Unit 8: Working of Real-time Scheduling

than the actual cores of the ways to schedule themselves. We do not wish to get involved into the Notes
“TT’ vs. ET” debate; rather have a look the related scheduling paradigms to determine what
their differences are. We conclude they are much smaller than perpetuated clichees.
One of the central choices for scheduler design is that of the activation paradigm, i.e., when are
events recognized, who initiates activities, when are these decisions taken? In conventional
systems, the event triggered approach is prevalent, in which occurrences of events initiate
activities in the system immediately. In time triggered systems, activities are initiated at
predefined points in time.
Offline Scheduling: We start with the time triggered approach, contrasting some properties
already here to those of event triggered. Initiating activities in the system with the progression
of time requires thorough, complete understanding of the system and the environment it will
operate in. Scheduling for TT is usually carried out via a scheduling table, which lists tasks and
their activation times. An offline algorithm takes complete information about the system
activities, which reflect the knowledge about anticipated environmental situations and
requirements, and creates a single table, representing a feasible solution to the given
requirements. As the algorithm is performed offline, fairly complex task sets can be handled,
For example, precedence constraints, distribution and communication over networks, task
allocation, mutual exclusion, separation of tasks, etc. Should a feasible solution not be found,
retries are possible, for example, by changing the parameterization of the algorithm or the
properties of the task set. At runtime, a very simple runtime dispatcher executes the decisions
represented in the table, i.e., which (portion of a) task to execute next. Typically, a minimum
granularity of time is assumed for the invocations of the runtime scheduler, so called slots.
Online Scheduling: 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.

Task Write the procedural steps to use the online scheduling.
Fundamentally Different

Given the different assumptions, approaches, and properties, offline and online scheduling
could be perceived as very different or even opposing paradigms. We will take a closer look.
The ET real-time scheduling process takes sets of tasks with timing constraints and performs a
test if these constraints can be met if a given algorithm is used at runtime. The algorithm may
take properties of tasks, notably priority or deadline, as input, or determine them as directives,
artifacts for the online scheduling algorithm then, the task properties, “priority” are separated
from the importance of a task, “deadline” from the timing constraint. Rather, they both serve
only to direct the online scheduling algorithm to execute the proper rules for schedulability.

Notes Scheduling Table
The scheduling table of offline scheduling provides a “proof by construction” that all
timing constraints will be met. In contrast to a proof that no timing constraints could be
Contd...

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