Task assignment policies in distributed server systems: A survey

Data intensive computing, in the Web environment, motivates the distributed designs of Web server systems (Web clusters) because of their scalability and cost-effectiveness instead of one Web server with high performance. The task assignment policy, in such systems, focuses on the manner of assigning the tasks that reach these systems (e.g. the case of intensive requests that reach the distributed Web server systems) in order to minimize the response time and thus, improve the performance. These tasks, generally, follow the ''heavy-tailed'' distribution which has the property that there is a tiny fraction (about 3%) of large tasks that makes half (50%) of the total load. Several policies were proposed in the literature to deal with the nature of this Web traffic. This paper presents a state-of-art of the existing task assignment policies. We classify these policies in two classes: policies which assume that the task size is known a priori, and policies which assume that the task size is not known a priori (like TAGS, TAPTF and TAPTF-WC). The first class of policies regroups policies which consider no knowledge of load information at the servers when assigning the incoming tasks, known as static policies (like Random, Round Robin, etc.) and, policies, known as dynamic policies (like Central Queue Policy, Least Loaded First ''LLF'', etc.) which use some load information (e.g. the processing capacity, the queue load, etc.) to process.