There has been increasing interest in the real-time community for Quality of Service (QoS) based systems, such as multimedia and telecommunication systems. In this paper, we deal with scheduling components integrating new QoS functionalities under Linux/RTAI (Real-Time Application Interface) [Rta04]. The work relies on a periodic task model which allows occasional deadline violations, together with soft aperiodic tasks. This model discards selected task instances in such a way that the overall performance of the real-time system remains acceptable even in case of overload. The key objective is to exploit the skips in order to minimize the response time of soft aperiodic requests, using the EDL (Earliest Deadline as Late as possible) server, which is a dynamic slack stealing algorithm. The level of QoS, i.e. the skip factor, is fixed by the application programmer. Simulation results show the performance of two algorithms, namely EDL-RTO and EDL-BWP. Both the mean reponse time of aperiodic requests and the ratio of periodic tasks completions are considered. We also provide results to estimate the preemption cost induced by these algorithms. Finally, integration of these QoS scheduling strategies has been done in the open-source library of CLEOPATRE, a patch to Linux/RTAI.

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