Overload Control Mechanisms for Web Servers

Web servers often experience overload situations due to the extremely bursty nature of Internet traffic, popular online events or malicious attacks. Such overload situations significantly affect performance and may result in lost revenue as reported by the recent denial of service attacks. Overload control schemes are well researched and understood in telecommunication systems. However, their use in web servers is currently very limited. Our focus in this paper is to propose effective overload control mechanisms for web servers. An important aspect in overload control is to minimize the work spent on a request which is eventually not serviced due to overload. This paper studies three simple schemes for controlling the load effectively. The first scheme selectively drops incoming requests as they arrive at the server using an intelligent network interface card (NIC). The second scheme provides feedback to a previous node (proxy server or ultimate client) to allow a gapping control that reduces offered load under overload. The third scheme is simply a combination of the two. The experimental results show that even these simple schemes are effective in improving the throughput of the web server by 40% and response time by 70% under heavy overloads, as compared with the case without any overload control.