Online Response Time Optimization of Apache Web Server

Abstract

Abstract — Properly optimizing the setting of configuration parameters can greatly improve performance, especially in the presence of changing workloads. This paper explores approaches to online optimization of the Apache web server, focusing on the MaxClients parameter (which controls the maximum number of workers). Using both empirical and analytic techniques, we show thatMax-Clients has a concave upward effect on response time and hence hill climbing techniques can be used to find the optimal value of MaxClients. The underlying intuition is thatMaxClients controls the trade-off between delays in the TCP Accept Queue and delays due to contention for operating system resources. We investigate two optimizers that employ hill climbing—one based on Newton’s Method and the second based on fuzzy control. A third technique is a heuristic that exploits relationships between bottleneck utilizations and response time minimization. In all cases, online optimization reduces response times by a factor of 10 or more compared to using a static, default value. The trade-offs between the online schemes are as follows. Newton’s method is well known but does not produce consistent results for highly variable data such as response times. Fuzzy control is more robust, but converges slowly. The heuristic works well in our prototype system, but it may be difficult to generalize because it requires knowledge of bottleneck resources and an ability to measure their utilizations. Index Terms — web server; Apache; response time; optimization; queueing; fuzzy control I.