Under high load, a dynamic page server running Apache with mod_php spends a lot of its time doing an operation called "lingering close", which amounts to waiting a second or two on a connection just to make sure it gets closed properly.
Lingerd, then, is a daemon (service) designed to take over this job. Unlike Apache, which uses one process per connection, lingerd can do thousands of lingers in parallel, from a single process. The end result is that Apache needs much fewer running processes to handle the same load, which greatly increases its scalability, and diminishes the load on the server.
For technical reasons, lingerd is only effective on servers where keep-alives are disabled. To get the most out of your servers, for a high traffic site, you need to have the dynamic webpages served from one Apache server, and the images and other static data from another. The dynamic page server can then use lingerd.
More testing is still needed under other platforms (eg BSD, Solaris). Lingerd is not portable to Windows.
The homepage for lingerd is at http://www.iagora.com/about/software/lingerd/
The daemon itself can be found at ftp://iagora.com/pub/software/lingerd/lingerd-0.93.tar.gz
Installing lingerd is fairly simple, but it does require recompiling Apache from source.
Here are some quick installation instructions for Apache with mod_php and lingerd (see http://www.phpbuilder.com/manual/install-unix.php for more details), under Unix or Linux:
1. tar zxvf apache_1.3.x.tar.gz 2. tar zxvf php-3.0.x.tar 3. tar zxvf lingerd-0.93.tar 4. cd lingerd-0.93 5. vi config.h (set the right directories for SOCKPATH and PID_FILE; these directories must be writeable by the userid that Apache runs as) 6. vi apache-1.3/ap_lingerd.h (set SOCKPATH here too) 7. make ; make install (also set the system up to start lingerd automatically, e.g by adding the line "/usr/local/sbin/lingerd" to your /etc/rc.d/rc.local file) 8. cp apache-1.3/ap_lingerd.* ../apache_1.3.x/src/main/ 9. cd ../apache_1.3.x 10. patch -p0 -d src/ < ../lingerd-0.93/apache-1.3/aplinger.diff 11. ./configure --prefix=/www 12. cd ../php-3.0.x 13. ./configure --with-mysql --with-apache=../apache_1.3.x --enable-track-vars 14. make ; make install 15. cd ../apache_1.3.x 16. ./configure --prefix=/www --activate-module=src/modules/php3/libphp3.a 17. make ; make install
To see if lingerd is running, and how many connections it has been handling, watch out for its messages in /var/log/messages.
Lingerd is not tightly tied to the dynamic programming language, so it can be used with PHP3, PHP4, and also with other scripting engines like mod_perl, mod_snake or mod_dtcl.
You can find more information about lingerd in its homepage at http://www.iagora.com/about/software/lingerd/ , and in the documentation files included in the distribution.