Improving Apache Web Server Performance

9th June, 2009 by

Even to most Internet professionals, the Apache web server software can be as perplexing as it is ubiquitous. Although not as comprehensive as the official 685-page documentation, this post outlines two useful features you can enable in the httpd.conf file to improve the performance of your web server: persistent connections and gzip compression.

Important note: An error in your Apache configuration can take all of your web sites offline. Please make a backup copy of the /etc/httpd/conf/httpd.conf file before you begin making modifications. Also remember that it is necessary to restart your account to apply either of the changes described below.

Persistent Connections (Keep-Alive)

In addition to your web page itself, web browsers must also download each stylesheet, script, image, and other file referenced by your page. Historically, this meant that browsers would connect to a server, download the page, and disconnect. The browser would then initiate a new connection, download the first referenced file, and disconnect, repeating the process to fetch each additional file. As web pages became increasingly complex, they began using more files and the slow connect/download/disconnect method became noticeably time-consuming. This problem was solved with the introduction of persistent connections, as described below in the Apache documentation:

The Keep-Alive extension to HTTP/1.0 and the persistent connection feature of HTTP/1.1 provide long-lived HTTP sessions which allow multiple requests to be sent over the same TCP connection. In some cases this has been shown to result in an almost 50% speedup in latency times for HTML documents with many images.

Under the default settings, persistent connections are enabled on your account, but the KeepAliveTimeout is set to 15 seconds (near line 86 of /etc/httpd/conf/httpd.conf). This means that Apache connections will stay active for 15 seconds after visitors have downloaded (and possibly even left) the web page. Apache connections use server resources and your account has a limited number of Apache connections, so this is generally undesirable.

  • For the best performance on most websites, leave KeepAlive On but reduce the KeepAliveTimeout to 1 or 2 seconds. This allows visitors to benefit from faster downloads through persistent connections, but helps prevent Apache from overloading the server and keeping too many connections open at once.
  • In rare cases, on websites with extremely heavy traffic, turn KeepAlive Off. Persistent connections guarantee faster page loads for your visitors but come at the cost of increased resource utilization by requiring more Apache connections to remain open concurrently on the server. If you are frequently reaching the maximum number of allowed Apache connections, it is probably necessary to upgrade your hosting package, but turning off persistent connections is one final step you can take to squeeze a little bit more out of your current plan.

Gzip Compression (mod_deflate)

HTML, CSS, and many other programming languages used on the web are highly redundant, allowing them to be compressed effectively. mod_deflate, Apache’s compression engine, is installed but not enabled by default. To enable automatic gzip compression, add the following lines of code to the end of your httpd.conf file:


SetInputFilter DEFLATE
AddOutputFilterByType DEFLATE text/html
AddOutputFilterByType DEFLATE text/css
AddOutputFilterByType DEFLATE text/xml
AddOutputFilterByType DEFLATE text/plain
AddOutputFilterByType DEFLATE text/rtf
AddOutputFilterByType DEFLATE text/javascript
AddOutputFilterByType DEFLATE application/x-javascript
AddOutputFilterByType DEFLATE application/xml
AddOutputFilterByType DEFLATE application/xhtml+xml
AddOutputFilterByType DEFLATE application/rss+xml
AddOutputFilterByType DEFLATE application/atom+xml
AddOutputFilterByType DEFLATE application/rdf+xml
AddOutputFilterByType DEFLATE application/postscript
AddOutputFilterByType DEFLATE application/msword
AddOutputFilterByType DEFLATE application/vnd.ms-excel
AddOutputFilterByType DEFLATE application/vnd.ms-powerpoint
AddOutputFilterByType DEFLATE image/svg+xml
AddType image/svg+xml .svg

After you have edited the httpd.conf file and restarted your account, test whether compression is working properly.

Persistent connections significantly improve loading multimedia-rich sites with large numbers of images and other files, while gzip compression reduces HTML, CSS, JavaScript, and other text by an average of 80 percent, but these two changes are just the beginning! Read Yahoo’s list of Best Practices for Speeding Up Your Web Site for even more ways to make your pages load faster.

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