REAL - TIME SCHEDULING

• Many real time systems run a known collection of tasks. The execution time of the tasks is frequently known ahead of time.

• Tasks have deadlines by which they must complete.

• If a task that runs for 3 time units must be done at time 10, it must start by time 7.

• If two tasks that runs for 3 time units each must be done at time 10, one must start by time 4.

• Hard real-time systems – required to complete a critical taskwithin a guaranteed amount of time.

• Soft real-time computing – requires that critical processesreceive priority over less fortunate ones.

Correctness of the system may depend not only on the logical result of the computation but alsoon the time when these results are produced,

example:

– Tasks attempt to control events or to react to eventsthat take place in the outside world

– These external events occur in real time andprocessing must be able to keep up

– Processing must happen in a timely fashion

• neither too late, nor too early

• EDF – Earliest Deadline First Scheduling

• Static table-driven
– Table determines at run time when a task begins execution
• Static priority-driven preemptive
– Traditional priority-driven scheduler is used
• Dynamic planning-based
– Feasibility determined at run time
• Dynamic best effort
– No feasibility analysis is performed