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Server performance has become a crucial issue for improving the overall performance of the World-Wide Web. This paper describes Webmonitor, a tool for evaluating and understanding server performance, and presents new results for a realistic workload.Webmonitor measures activity and resource c onsumption, both within the kernel and in HTTP processes running in user space. Webmonitor is implemented using an e cient combination of sampling and event-driven techniques that exhibit low overhead. Our initial implementation is for the Apache World-Wide Web server running on the Linux operating system. We demonstrate the utility of Webmonitor by measuring and understanding the performance of a Pentium-based PC acting as a dedicated WWW server. Our workload uses a le size distribution with a h e avy tail. This captures the fact that Web servers must concurrently handle some requests for large audio and video les, and a large number of requests for small documents, containing text or images.Our results show that in a Web server saturated by client requests, over 90% of the time spent handling HTTP requests is spent in the kernel. Furthermore, keeping TCP connections open, as required b y TCP, causes a factor of 2-9 increase in the elapsed time required to service a n H T T P r equest. Data gathered f r om Webmonitor provide insight into the causes of this performance penalty. Speci cally, we observe a signi cant increase in resource consumption along three dimensions: the number of HTTP processes running at the same time, CPU utilization, and memory utilization. These results emphasize the important role of operating system and network protocol implementation in determining Web server performance.