Professional LAMP: Linux®, Apache, MySQL®, and PHP5 Web Development

9.4. Web Server Improvements

By its very nature, Apache is highly configurable, with many settings that can be tweaked to optimize one aspect or another of its behavior. Some reduce the amount of memory Apache requires, while others have the aim of reducing the amount of work carried out by each process.

When you start Apache, the first thing it does is start up half-a-dozen (the exact figure is, of course, configurable) instances of httpd. These are the processes that actually handle HTTP requests. All the server itself does is manage these children, starting new ones and stopping existing ones as needed. Each instance uses up about a megabyte of RAM (it's hard to be more exact, but this is a reasonable heuristic), and sits around waiting for a request. If there are more requests being made than there are instances, then new ones will be started up, and if things go quiet later, then the excess will be shut down. Occasionally an instance of httpd will be shut down even if there is sufficient work available. This allows the operating system to reclaim memory that may have leaked from either the httpd process itself or any of the modules it was using. And of course it is possible for an httpd process to simply crash, in which case that particular request will fail; but another instance will be on standby in case the request is tried again. This is a key reason why Apache operates in this way: it enables the server to keep running even after it has crashed.

The configuration options ...