Because of some technical complications in TCP/IP, at the end of each client connection, it is not enough for Apache to close the socket and forget about it; instead, it needs to spend about one second lingering (waiting) on the client.
More details can be found at http://httpd.apache.org/docs/misc/fin_wait_2.html.
lingerd is a daemon (service) designed to take over the job of properly closing network connections from an HTTP server such as Apache and immediately freeing it to handle new connections.
lingerd can do an effective job only if HTTP KeepAlives are turned off. Since Keep-Alives are useful for images, the recommended setup is to serve dynamic content with mod_perl-enabled Apache and lingerd, and static content with plain Apache.
With a lingerdsetup, we don't have the proxy (we don't want to use lingerd on our httpd_docs server, which is also our proxy), so the buffering chain we presented earlier for the proxy setup is much shorter here (see Figure 12-8).
Hence, in this setup it becomes more important to have a big enough kernel send buffer.
With lingerd, a big enough kernel send buffer, and KeepAlives off, the job of spoonfeeding the data to a slow client is done by the OS kernel in the background. As a result, lingerd makes it possible to serve the same load using considerably fewer Apache processes. This translates into a reduced load on the server. It can be used as an alternative to the proxy setups we have seen so far.
For more information about lingerd, see http://www.iagora.com/about/software/lingerd/.
Eric Cholet (Logilune) and
Stas Bekman (StasoSphere & Free Books).