Connection Scheduling in Web Servers

Under high loads, a Web server may be servicing many hundreds of connections concurrently. In traditional Web servers, the question of the order in which concurrent connections are serviced has been left to the operating system. In this paper we ask whether servers might provide better service by using non-traditional service ordering. In particular, for the case when a Web server is serving static les, we examine the costs and bene ts of a policy that gives preferential service to short connections. We start by assessing the scheduling behavior of a commonly used server (Apache running on Linux) with respect to connection size and show that it does not appear to provide preferential service to short connections. We then examine the potential performance improvements of a policy that does favor short connections (shortest-connectionrst). We show that mean response time can be improved by factors of four or ve under shortest-connectionrst, as compared to an (Apache-like) size-independent policy. Finally we assess the costs of shortest-connectionrst scheduling in terms of unfairness (i.e., the degree to which long connections su er). We show that under shortest-connectionrst scheduling, long connections pay very little penalty. This surprising result can be understood as a consequence of heavytailed Web server workloads, in which most connections are small, but most server load is due to the few large connections. We support this explanation using analysis.