There are five types of task classes:
(i) Periodic and aperiodic tasks
(ii) Sporadic task
(iii) Critical task
(iv) Non-critical task
(1) Periodic Task: There are many tasks in real — time systems that are done repetitively.
For example one may wish to monitor the speed altitude and attitude of an aircraft every 100 ms. This sensor information will be used by periodic tasks that control surfaces of the aircraft
in order to maintain stability and other desired characteristics. The periodicity of these tasks is known to the designer, and much tasks can be pre-scheduled.
(2) Aperiodic Task: There are many other tasks that are aperiodic, that occur occasionally.
For instance, when the pilot wishes to execute a turn a large number of subtasks. Associated
with that action are self-off aperiodic tasks cannot be predicted and sufficient completing power must be held in a reserve to execute them in a timely fashion.
(3) Critical Tasks: Critical tasks are those whose timely executions is critical; if deadlines are missed, catastrophes occur. Example include life – support systems and the stability control of
air craft. If critical tasks are executed at a higher frequency then it is absolutely necessary.
(4) Non critical Tasks: Non critical tasks are real times tasks. As the name implies, they are not critical to the application. However they do deal with time varying data and hence they are useless if not completed within a deadline. The goal in scheduling these tasks is to maximize the percentage of jobs successfully executed within their deadlines.