Page 67 - DCAP608_REAL TIME SYSTEMS

Real Time Systems

Notes Self Assessment

Fill in the blanks:
1. Round-robin is commonly used for scheduling ……………… applications.
2. ……………… Scheduling assigns priorities to jobs.

3. In ……………… scheduling, decisions on what jobs execute at what times are made at
specific time instants.
4. ……………… are chosen a priori before the system begins execution.

5. Usually all the parameters of hard real-time jobs are ……………… and known.
6. The ……………… of a round is equal to the sum of the weights of all ready jobs.

6.3 Priority Driven Approach

It refers to a large class of scheduling algorithms that never leave any resource idle intentionally.
A resource is idle only when no job requiring the resource is ready for execution. Scheduling
decisions are made when events such as releases and completions of jobs occur. Hence these are
also called Event-Driven. It is also called Greedy, list and work-conserving scheduling. When a
processor or resource is available and some job can use it to make progress, such an algorithm
never makes the job wait. Jobs ready for execution are placed in one or may more queues
ordered by the priorities of the jobs. At the scheduling time, the jobs with the highest priorities
are scheduled and executed on the available processors. As we can dynamically change the
priorities of jobs, even round robin scheduling can be thought of as priority-driven. The Figure
below shows a classical precedence graph. Number next to jobs is their execution time. J5 is
released at time 4. All other jobs are released at time 0. We want to schedule and execute the jobs
on two shared memory processors P1 and P2 (cost of communication is negligible). The schedulers
on the processors keep one common priority queue of ready jobs.

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