So far in our series of Web Performance articles, we've addressed the three majors types of caching that Web server application developers can employ: server file caching, application caching, and data caching. We've looked at additional performance enhancements that Web server administrators can can activate, such as HTTP compression, file consolidation, and connection pipelining.

In this latest installment of our series, we're going a little deeper and focusing on Apache. The world's most popular Web server, Apache currently powers over 63% of sites on the World Wide Web. While Apache runs decently out of the box, development teams and system administrators should combine frequent load testing with the following tuning recommendations to ensure their applications perform well under heavy traffic.

Memory Models: Prefork vs. Worker

A Web server must be able to respond to anywhere from dozens to thousands of concurrent user requests. Each request, obviously, must be fulfilled by either a separate application process, or a separate thread running within a single process.

Apache can handle concurrent requests in two different ways, depending on which Multi-Processing Module (MPM) was selected at compile time. The default MPM on Unix is prefork, which handles each separate user request in a separate instance of Apache. The other MPM module available for Unix systems is worker, which launches fewer instances of Apache, and handles multiple requests in separate threads within each process.

Worker is the preferred MPM module for Apache under Unix, as it is faster and uses less memory. So why is prefork the default? Because any team that deploys worker must ensure that any modules that run under Apache are thread-safe. The thread-safe requirement extends to any libraries used by these modules, which makes running a programming environment such as PHP under worker tricky: while PHP is itself thread-safe, there is no guarantee that its various extensions are. Some developers, such as Brian Moon, have reported success in running PHP under worker using a minimal set of extension libraries. Your mileage, however, may vary.