Practical mod_perl

Chapter 14. Defensive Measures for Performance Enhancement

If you have already worked with mod_perl, you have probably noticed that it can be difficult to keep your mod_perl processes from using a lot of memory. The less memory you have, the fewer processes you can run and the worse your server will perform, especially under a heavy load. This chapter presents several common situations that can lead to unnecessary consumption of RAM, together with preventive measures.

Controlling Your Memory Usage

When you need to control the size of your httpd processes, use one of the two modules, Apache::GTopLimit and Apache::SizeLimit, which kill Apache httpd processes when those processes grow too large or lose a big chunk of their shared memory. The two modules differ in their methods for finding out the memory usage. Apache::GTopLimit relies on the libgtop library to perform this task, so if this library can be built on your platform you can use this module. Apache::SizeLimit includes different methods for different platforms—you will have to check the module’s manpage to figure out which platforms are supported.

Defining the Minimum Shared Memory Size Threshold

As we have already discussed, when it is first created, an Apache child process usually has a large fraction of its memory shared with its parent. During the child process’s life some of its data structures are modified and a part of its memory becomes unshared (pages become “dirty”), leading to an increase in memory consumption. You ...