Scheduling Hierarchy, Scheduler and Schedules | Conditions to be valid Schedule

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Conditions to be valid Schedule,
Real Time System Notes | Sixth Semester,
BSc.CSIT | Tribhuvan University (TU)

Scheduling Hierarchy
Figure below shows the application system, represented by a task graph at the top, resource graph at the bottom and in between them scheduling algorithm and resource access control protocol used by an operating system. Scheduling and resource-access control graph gives the processor time and resource requirements of jobs, the timing constraints of each job, and the dependencies of jobs. The resource graph describes the amount of the resources available to execute the application system, the attributes of the resources, and rules governing their usage.

Scheduling hierarchy
Scheduler and Schedules
Jobs are scheduled and allocated resources according to a chosen set of scheduling algorithms and resource access-control protocols. The module which implements these algorithms is called the scheduler. The total amount of time assigned to a schedule is the total length of all the time intervals during which the job is scheduled on a processor. A Schedule is an assignment of all the jobs in the system on the available processors.

Conditions to be a valid schedule
If a scheduler produces only valid schedules then it is said to be the correctness of the schedule. A schedule is said to be valid if it satisfies the following conditions:

  • Every processor is assigned to at most one job at any time.
  • Every job is assigned at most one processor at any time.
  • No job is scheduled before its release time.
  • Depending on the scheduling algorithms used, the total amount of processor time assigned to every job is equal to its maximum or actual execution time.
  • All the precedence and resource usage constraints are satisfied.
  • One more assumptions here is that jobs do not run in parallel on more than one processor to speed up their execution.
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