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Combining Off-line Schedule Construction and Fixed Priority Scheduling in Real-Time Computer Systems



Publication Type:

Doctoral Thesis


Mälardalen University


Off-line scheduling and fixed priority scheduling (FPS) are often considered as complementing and incompatible paradigms. A number of industrial applications demand temporal properties (predictability, jitter constraints, end-to-end deadlines, etc.) that are typically achieved by using off-line scheduling. The rigid off-line scheduling schemes used, however, do not provide for flexibility. On the other hand, FPS has been widely studied and used in a number of industrial applications, mostly due to its simple run-time scheduling and small overhead. It provides more flexibility, but is limited with respect to predictability, as actual start and completion times of executions depend on run-time events.In this thesis we first show how off-line scheduling and FPS can be combined to get the advantages of both -- the capability to cope with complex timing constraints while providing run-time flexibility. The proposed approach assume that a schedule for a set of tasks with complex constraints has been constructed off-line. We present methods to analyze the off-line schedule and derive FPS attributes such that the runtime FPS execution matches the off-line schedule. In some cases, i.e., when the off-line schedule can not be expressed directly by FPS, we split tasks into instances (artifacts) to obtain a new task set with consistent task attributes. Our method keeps the number of newly generated artifact tasks minimal.At the same time, we investigate the behavior of the existing FPS servers to handle non-periodic events, while the complex constraints imposed on the periodic tasks are still fulfilled. In particular, we provide a solution to server parameter assignment to provide non-periodic events a good response time, while still fulfilling the original complex constraints on the periodic tasks.Secondly, we apply the proposed method to schedule messages with complex constraints on Controller Area Network (CAN). We analyze an off-line schedule constructed to solve complex constraints for messages, e.g., precedence, jitter or end-to-end deadlines, and we derive attributes, i.e., message identifiers, required by CAN's native protocol. At run time, the messages are transmitted and received within time intervals such that the original constraints are fulfilled.Finally, we propose a method to reduce the number of preemptions in legacy FPS systems consisting of tasks with priorities, periods and offsets. Unlike other approaches, our algorithm does not require modification of the basic FPS mechanism. Our method analyzes off-line a set of periodic tasks scheduled by FPS, detects the maximum number of preemptions that can occur at run-time, and reassigns task attributes such that the tasks are schedulable by the same scheduling mechanism while achieving a lower number of preemptions. In some cases, there is a cost to pay for achieving a lower number of preemptions, e.g., an increased number of tasks and/or reduced task execution flexibility. Our method provides for the ability to trade-off between the number of preemptions and the cost to pay.


author = {Radu Dobrin},
title = {Combining Off-line Schedule Construction and Fixed Priority Scheduling in Real-Time Computer Systems},
number = {18},
month = {September},
year = {2005},
school = {M{\"a}lardalen University},
url = {}