A simple distributed scheduling policy for parallel interactive continuous media servers

In this paper we discuss the design of parallel interactive continuous media servers suitable for the implementation of scalable server-based media delivery services like Video-on-Demand or Teleshopping. The main design problems for the development of such servers is to ensure the just-in-time delivery of media elements in order to maximize the Quality of Service and to minimize the buffer size at the user site. Just-in-time delivery means that the media elements should be sent as late as possible to the users but early enough to ensure a continuous replay of the media. This is important because clients have to provide buffer space for data arriving to early. The Quality of Service measures the number of data elements arrived in time at the user side. Thus, the real-time properties of the internal communication network as well as the congestion arising at the disks are of highest importance. We present models for parallel media servers and a very simple scheduler that is fully distributed and can therefore easily be implemented on a scalable parallel continuous media server. For each requested data element the scheduler sends a request to the storage subsystems at a point of time only depending on the deadline of that request, i.e. the time the data has to be delivered to the user, and the length of the path the data has to be routed through the internal network of the parallel server. In order to minimize the buffer space at the user site, and to maximize the Quality of Service, we develop timing strategies for the scheduler using simulation results as well as analytical observations.