Page 49 - DCAP608_REAL TIME SYSTEMS

Real Time Systems

Notes when neither J <J nor J <J A job with predecessors is ready for execution when the time is at or
i k k i.
after its release time and all of its predecessors is completed. A task graph, which gives us a
general way to describe the application system, is an extended precedence Graph. The vertices in
a task graph represent jobs.
A task Graph is shown below:

The task represented in first row has phase 0, period 2 and relative deadline 7 and all jobs are
independent. The jobs released in later periods are ready for execution as soon as they are
released even though some job released earlier is not yet complete. The second row represents
a periodic task has phase 2, period 3 and relative deadline 3 and jobs are interdependent. The
nd
rd
nd
first job is predecessor of 2 and 2 is predecessor of 3 job. The last two tasks are not periodic.
4.4.2 Data Dependency
Data dependency cannot be captured by a precedence graph. In many real-time systems, jobs
communicate via shared data. Each producer can place the data generated by it in a shared
address space to be used by the consumer at any time so precedence graphs shows them as
independent.
What if consumer and producer need to be synchronized?
In an avionics system, the navigation job updates the location of the air plane periodically.
Whenever the flight management job needs navigation data, it read the most current data
produced by the navigation data from the shared space. Data dependencies among jobs are
represented explicitly by data dependency edges among jobs.

Self Assessment

State whether the following statements are True or False:
7. Each computation or data transmission that is executed repeatedly at regular or semi-
regular time intervals in order to provide a function of the system is modelled as a period
task.

8. A periodic task occurs at irregular intervals.
9. A task is said to be periodic if the jobs in it have either soft deadlines or no deadlines.
10. The jobs are said to have precedence constraints if they are constrained to execute in some
order.
11. If the jobs can execute in any order, they are said to be dependent.
12. Data dependency cannot be captured by a precedence graph.

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