1. Periodic Real-time Tasks :
The real-time task that is repeated after a certain time interval is known as periodic real-time task. Basically the periodic real-time tasks are controlled by the clock interrupts. Therefore, the periodic real-time tasks are also called clock-driven tasks. The time interval after which the task repeats itself is known as period of the task.
For example, when a flight is detected by the radar and until the radar exists, the radar signal zone is an example of periodic real-time task.
2. Aperiodic real-time Tasks :
The real-time task that occurs at any random time is known as aperiodic real-time task. Between two aperiodic real-time tasks the time interval may be even zero. Soft real-time tasks are generally aperiodic real-time tasks. It is also possible that these tasks may occur frequently or there might be a large time interval between two aperiodic real-time tasks.
For example, typing on the key-board is an aperiodic real-time task.
Difference between Periodic and Aperiodic Real-time Tasks:
|PERIODIC TASK||APERIODIC TASK|
|It repeats itself after a certain time interval.||It can occur at random instants.|
|These tasks are controlled by clock interrupts.||These tasks are not controlled by clock interrupts.|
|The time interval between occurrence of two consecutive tasks can’t be zero.||The time interval between occurrence of two consecutive tasks can be zero.|
|Periodic tasks generally include soft and hard real-time tasks both.||Aperodic tasks generally include soft real-time tasks.|
|Deadline of all instances of periodic task can be meet easily.||To meet deadline of all instances of an aperodic task is quite difficult.|
|It includes vast majority of internal tasks.||It includes interactive task with users.|
|Example: Taking information from sensor at a time interval.||Example: Logging task in a distributed system.|
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