Detail Description of Offline Scheduling and Online Scheduling | Real Time System

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offline scheduling and online schedulingOffline Scheduling and Online Scheduling – detail explanation,
Real Time System Notes | Sixth Semester,
BSc.CSIT | Tribhuvan University (TU)

Offline Scheduling
A clock driven scheduler makes use of a pre-computed schedule of all hard real time jobs. This schedule is computed offline before the system begins to execute and the computation is based on the knowledge of the release times and processor-time/resource requirement of all jobs for all times. When the operation mode of the system changes, the new schedule specifying when each job in the new mode executes is also pre-computed and stored for use. In this case we say that scheduling is done offline.

Offline scheduling has got disadvantage and that is inflexibility because it is possible only when system is deterministic, meaning that the system provides some fixed set(s) of functions and that the release times and processor time/resource demands of all its jobs are known and do not vary or vary only slightly.

For deterministic system, offline scheduling has many advantages, the deterministic timing behavior of the resultant system being one of them. Because the computation of the schedules is done off-line, the complexity of scheduling algorithm used for this purpose is not important.

Online scheduling
Scheduling is done online when we use an online scheduling algorithm. If the scheduler makes each scheduling decision without knowledge about the jobs that will be released in the future, then scheduling is said to be done on-line. In this scheduling parameters of each job become known to online scheduler only after job is released. It is the only option in a system whose future workload is unpredictable.

Online scheduling can accommodate dynamic variation in user demands and resource availability but it cannot make the best use of system resource. Without prior knowledge about future jobs, the scheduler can’t make optimal scheduling decision.

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