Static and dynamic scheduling algorithms for scalable Web server farm

Multiprocessor-based servers are often used for building popular Web sites which have to guarantee an acceptable Quality of Web Service. In common multi-node systems, namely Web server farms, a Web switch (say, Dispatcher) routes client requests among the server nodes. This architecture resembles a traditional cluster in which a global scheduler dispatches parallel applications among the server nodes. The main difference is that the load reaching Web server farms tends to occur in waves with intervals of heavy peaks. These heavy-tailed characteristics have motivated the use of policies based on dynamic state information for global scheduling in Web server farms. This paper presents an accurate comparison between static and dynamic policies for different classes of Web sites. The goal is to identify main features of architectures and load management algorithms that guarantee scalable Web services. We verify that a Web farm with a Dispatcher with full control on client connections is a very robust architecture. Indeed, we demonstrate that if the Web sire provides only HTML pages or simple database searches, the Dispatcher does not need to use sophisticated scheduling algorithms even if the load occurs in heavy bursts. Dynamic scheduling policies appears to be necessaly for scalability only when most requests are for Web services of three or more orders of magnitude higher than providing HTML pages with some embedded objects.