Page 98 - DCAP608_REAL TIME SYSTEMS

Unit 9: Concept of Clock-driven Scheduling

completed and the aperiodic job queues is nonempty, it lets the job at the head of the aperiodic Notes
job queue execute.

Lab Exercise Consider a system that contains four independent periodic tasks. They are T1
= (3, 2), T2 = (4, 1.8), T3 = (10, 2), and T4 = (15, 2), construct a static schedule for the first hyper
period of the task.

Self Assessment

State whether the following statements are True or False:

7. A straight forward way to implement the scheduler is to store the precomputed schedule
as a table.
8. A constraint enforced by this structure is that scheduling decisions are made periodically,
rather than at arbitrary times.
9. The scheduling decision times partition the timeline into intervals called frames.
10. Each frame has length f which is the frame size.

11. Scheduling decisions are made only at the end of every frame so there is no preemption in
a frame.
12. Phase is always a multiple of frame size.



Case Study The MARS Kernel

ime driven real-time systems are of increasing importance in the field of critical
computer control applications. Because of their predictable behaviour they are
Twell suited for systems whose correct operation in the time domain must be
guaranteed already in the design phase of an application. Time driven systems allow the
proof of the correct timing behaviour of an application by construction of a feasible
schedule. In the MARS system the time driven approach is realized. The structure of the
MARS operating system kernel differs significantly from that of others because of the
specific demands which a distributed time driven system imposes on its underlying
operating system. Based on the experiences with the first prototype of the MARS operating
system (MARS-1), a new operating system kernel, MARS-2, has been developed from
scratch. There have been some motivations for the development of MARS-2:
New processor boards (‘MARS components’) have been developed to fully support the
MARS concepts. These boards provide mechanisms to achieve a high self-checking coverage
and a highly predictable timing behaviour.

 The introduction of new concepts and mechanisms into the MARS system (e.g.
membership protocol, time redundant process execution, shadow component)
requires support by the runtime system.

 A predictable timing behaviour should be achieved by the new kernel. Although
the system overhead caused by the old implementation was bondable in principle,

Contd...

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