mod_rewrite

One of the most powerful and complicated aspects of Apache is the mod_rewrite module. As its name implies, mod_rewrite provides the ability to rewrite URIs.

Although this feature does not necessarily represent a significant performance problem, for each rule that matches and each URI that must be rewritten, system resources are expended. The key is to try to avoid processing URIs too many times and to keep away from the URIs that you don't need to process.

One way of addressing this problem is to use the [L] option. This option tells Apache that it has reached the last rule it should apply to the given URI. That is, no further rules are processed. Unfortunately this is not as easy as it seems.

If the rewrite rule does in fact cause a redirection, the new URI is itself processed, and everything starts over again. In many cases you can avoid this with the use of re-write conditions (RewriteCond), so that if the URI matches a given condition, you won't need to process it further.

A related option is the [S] or skip flag. This flag will skip over a given number of rewrite rules. For example, if the URI is not a file (e.g., a directory) you can tell Apache to skip over rules that only apply to files. In other words, you don't avoid the rewrite rules completely, but you reduce the number you need to process.

Conclusion

The Pareto principle (80:20 rule) applies to web server performance tuning. Implementing many of the concepts described in this article can increase your performance relatively quickly; however, to squeeze out the last bit of advantage, you probably need to spend a lot more time. Depending on the kind of server you run, your mileage will vary.