Scheduling poiicies for an on-demand video server with batching

Abstract

In an on-demand video server environment, clients make requests for movies to a centralized video server. Due to the stringent response time requirements, continuous delivery of a video stream to the client has to be guaranteed by reserving sufficient resources required to deliver a stream. Hence there is a hard limit on the number of streams that can be simultaneously delivered by a server. The server can satisfy multiple requests for the same movie using a single disk I/O stream by sending the same data pages to multiple clients (using the multicast facility if present in the system). This can be achieved by batching requests for the same movie that arrive within a short duration of time. In this paper, we consider various policies for selecting the movie to be multicast. The choice of a policy depends very much on the customer waiting time tolerance before reneging. We show that an FCFS policy that schedules the movie with the longest outstanding request can perform better than the MQL policy that chooses the movie with the maximum number of outstanding requests. Additionally, if the user behavior can be influenced by guaranteeing maximum waiting time then it may be beneficial to pre-Allocate a fixed number of streams for popular movies. Finally, we demonstrate using empirical distribution for movie requests, that a substantial reduction (of the order of 60%) in required server capacity can be achieved by batching.