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Difference between Data dependency and Temporal Dependency | Real Time System

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difference between data and temporal dependencyDifference between Data dependency and Temporal Dependency,
Real Time System Notes | Sixth Semester,
BSc.CSIT | Tribhuvan University (TU)

Difference between Data dependency and Temporal dependency is described as follows;

Data Dependency
In many real time system, jobs communicate via shared data. Often the designer chooses not to synchronize producer and consumer jobs. Rather, each producer places the data generated by it in a shared address space to be used by consumer at any time. They are not explicitly constrained to execute in turn.

Example: In an avionics system, the navigation job updates the location of the airplane periodically. These data are placed in shared space. Whenever the flight management job needs navigation data, it reads the most current data produced by the navigation job. There is no precedence constraint between them.

In a task graph, data dependencies among jobs are represented explicitly by data-dependency edge among jobs. There is a data-dependency edge from a vertex Ji to vertex Jk in the task graph if the job jk consumes data generated by Ji or job Ji sends message to jk. A parameter of an edge from Ji to Jk is the volume of data.

To ensure data integrity, some locking mechanism may be in place which allows only one job to access the shared data at a time.

Temporal Dependency
Some jobs may be constrained to complete within a certain amount of time relative to one another. The difference in completion time of 2 jobs is called temporal distance between them. Jobs are said to have temporal distance constraint if their temporal distance must be no more than some finite value. Jobs with temporal distance constraint may or may not have deadlines.

Example: Consider display of video frames and accompanying audio. To have lip synchronization the time between display of each frame and generation of corresponding audio segment must be no more than 160 msec.
These timing requirements can be stated more naturally in terms of deadline of jobs.

In a task graph, temporal distance constraints among job are represented by temporal dependency edge. There is a temporal dependency edge from vertex Ji to vertex Jk must be completed within a certain time after Ji completes. The temporal distance is given by the temporal distance parameter of the edge. The value of this parameter is infinite if the jobs have no temporal distance constraint.

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