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Clock Driven Approach and Priority Driven Approach | Real Time System

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clock driven and priority driven approachTime driven/clock driven approach and
Greedy/priority driven approach,
Real Time System Notes | Sixth Semester,
BSc.CSIT | Tribhuvan University (TU)

Clock driven/Time driven approach
When scheduling is clock-driven, decisions on what job executes at what time are made at specific time instants. These instants are chosen a prior before the system begins execution. All the parameters of hard real time jobs are fixed and known.

A schedule of the job is computed off-line and is stored for use at run-time. The scheduler schedules the jobs according to this schedule at each scheduling decision time, which reduces scheduling overhead during runtime. A frequently adopted solution is to make scheduling decision at regularly spaced time instants. One way to implement a scheduler that makes scheduling decision periodically is to use a hardware time, which is set to expire periodically without the intervention of the scheduler.

When the system is initialized, the scheduler selects and schedules the job(s) that will execute until the next scheduling decision time and then blocks itself waiting for the expiration of timer. When timer expires, the scheduler awakes and repeats these actions.

Priority Driven/Greedy approach/ Event-driven/ List-scheduling/ Work-conserving

  • As listed in title above, priority driven approach is also known as greedy approach, event-driven approach, list scheduling and work conserving scheduling.
  • They are a class of scheduling algorithm that never leaves any resource idle intentionally.
  • Scheduling decision is made when events such as release and completion of jobs occur.
  • It tries to make locally optimal decisions.
  • When a processor or resource is available and some job can use it to make progress, such an algorithm never makes the job wait.
  • Any priority driven algorithm can be implemented by assigning priorities to jobs.
  • Jobs ready for execution are placed in one or more queues ordered by the priority of the jobs.
  • At any scheduling decision time, the jobs with the highest priorities are scheduled and executed on the available processor.
  • Example: First In First Out (FIFO), Last In Last Out (LIFO), Shortest Execution Time First (SETF), Longest Execution Time First (LETF).
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