. The response time of a task is the time it takes (as measured from the task arrival
time) for the task to produce its results.
.As already remarked, task instances get generated due to occurrence of events.
.These events may be internal to the system, such as clock interrupts, or external to
the system such as a robot encountering an obstacle
. A task is said to precede another task,if the first task must complete before the
second task can start.
.When a task Ti precedes Tj , then each instance of Ti precedes corresponding
instance of Tj .
. A precedence order defines a partial order among tasks.
. Tasks often need to share their results among each other when one task needs to
share the results produced by another task; clearly, the second task must precede
the first task.
. In fact, precedence relation between two tasks sometimes implies data sharing
between the two tasks (e.g., first task passing some results to the second task).
However, this is not always true.
. A task may be required to precede another even when there is no data sharing.
.E.g. in a chemical plant it may be required that the reaction chamber must be filled
with water before chemicals are introduced
. In this case, the task handling filling up the reaction chamber with water must
complete, before the task handling introduction of the chemicals is activated.
. It is, therefore, not appropriate to represent data sharing using precedence
. Data sharing among the tasks does not necessarily impose any particular ordering
among the tasks.
. Here, T2 uses the results of T3.T2 may even start executing first , after sometimes it
may receive some data from T3, and continues its execution.
TYPES OF REAL-TIME TASKS & THEIR CHARACTERISTICS:
Based on the way real-time tasks recur over a period of time, it is possible to classify
them into three main categories:
. Sporadic tasks
. Aperiodic tasks.
.A periodic task is one that repeats after a certain fixed time interval.
.Periodic tasks are sometimes referred to as clock-driven tasks.
.The fixed time interval after which a task repeats is called the period of the task.