Page 55 - DCAP608_REAL TIME SYSTEMS
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Real Time Systems
Notes Temporal Dependency
Some jobs may be constrained to complete within a certain amount of time relative to one
another. Jobs are said to have temporal distance constraint, if their distance must be no more
than some finite value. Consider the display of video frames and the accompanying audio of a
person speaking.
Did u know? To have lip synchronization, the time between the display of each frame and
the generation of the corresponding audio segment must be no more than 160 msec.
In a task graph, temporal distance constraints among jobs are represented by the temporal
dependency edges.
AND/OR Precedence Constraints
In classical model, a job with more than one immediate predecessor must wait until its immediate
predecessors have been completed before its execution can begin. We can call such jobs AND
jobs and dependencies among them AND precedence constraints. AND jobs are represented by
unfilled circles in the task graph. For example, a job transmits message to user, its immediate
predecessors are jobs that set up a connection for message, encrypt the message and check the
user’s account for properties like quality of delivery service. OR job is one which can begin
execution at or after its release time provided one or some of its immediate predecessors has
been completed. It is represented as square vertices.
Conditional Branches
Similarly in the classical model, all the immediate successors of a job must be executed; an
outgoing edge from every vertex expresses an AND constraint.
This convention makes it inconvenient for us to represent conditional execution of jobs.
For every second do the following
Process radar returns
Generate track records
Perform track association
for the target T on each of the established tracks do
If the target T is within distance D from self, do the following
Analyze the trajectory of T.
If T is on collision course with self, sound alarms
Else
Compute the current distance of T from self
If the current distance is larger than previous distance
Drop the track of T
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