Real time scheduling G.Anuradha Ref:- Stallings

Real time computing Correctness of the system depends not only on the logical result of computation, but also on the time at which the results are produced Every task is associated with a deadline which is either start time or end time. Based on this concept we have – Hard real-time task – Soft real-time task Whether the task is periodic or not – Aperiodic:- deadline by which it must finish or start – Periodic:- requirement is once per period T

Characteristics of RTOS Determinism Responsiveness User Control Reliability Fail-soft operation

Scheduling of real-time process Unacceptable for real time applications

Real time task that is ready is scheduled as soon as the current process blocks or runs to completion

1.Combine priorities with clock based interrupts 2.Preemption points occur at regular intervals. 3.When a critical job is waiting to get executed after a long low priority job, deadline might occur.

Immediate Preemptive scheduler

Real-Time Scheduling-Classes of Algorithms Static table-driven approaches: determines when a task must begin execution Static priority-driven preemptive approaches: used to assign priorities to tasks Dynamic planning-based approaches: Feasibility is determined at run time Dynamic best effort approaches: system tries to meet all deadlines and aborts process with missed deadlines.

Deadline Scheduling Based on the objective of starting real-time tasks as rapidly as possible and emphasize rapid interrupt handling and task dispatching Ready time Starting deadline Completion deadline Processing time Resource requirements Priority Subtask structure

Execution profile of two periodic tasks

Execution profile of five aperiodic tasks