As real-time systems continue to grow, performance evaluation plays a critical role in the design of these systems since the computation time, the service time, and the responsive actions must satisfy the time constraints. One of these systems is the real-time distributed multimedia-on-demand (MOD) service system. The MOD system usually fails when it misses a task deadline. The main units of the MOD system usually communicate with each other and work concurrently under timing constraints. The MOD system is designed to store, retrieve, schedule, synchronize and communicate objects comprising of mixed data types including images, text, video and audio, in real-time. In the MOD system, such data types represent the main concept of movie files. Modeling of such concurrency, communication, timing, and multimedia service (e.g., store, retrieve) is essential for evaluating the real-time MOD system. To illustrate how to model and analyze the important multimedia aspects of the MOD system, we use the Real-net (R-net) modeling technique. We choose R-net as an extension of Time Petri Net due to its ability to specify hard real-time process interaction, represent the synchronization of multimedia entities, describe concurrent multimedia activities, and illustrate the inter-process timing relationships as required for multimedia presentation. Based on modular techniques, we build three R-net performance models for describing the dynamic behavior of the MOD service system. The first model adopts the Earliest Deadline First (EDF) disk scheduling algorithm. The other models adopt the Scan-EDF algorithm. These algorithms help us to illustrate how the real-time user requests can be satisfied within the specified deadline times. Since R-nets are amenable to analysis including Markov process modeling, the interesting performance measures of the MOD service system such as the quality of service, the request response time, the disk scheduling algorithm time, and the actual retrieval time can be easily computed. In the performance analysis of the MOD models, we use our R-NET package.
GSR: A global seek-optimizing real-time disk-scheduling algorithm
