Pros and Cons of Keep-Alive from Web Server Side

Question

Keep-Alive connection feature in HTTP protocol is meant to reduce TCP connection hits to web server. It should be able to improve web server performance. However, I found that some web servers deliberately disable KeepAlive feature from server side.

In my understanding, some reverse proxy, i.e. HAProxy, disables HTTP keep-alive in order to reduce memory usage which is more critical than CPU usage in some situation.

Is there any other reason why Web server disables Keep-Alive?

Answer 1

Actually, Keep-Alive is meant to improve HTTP performance, not server performance (though for SSL connections it does reduce the cost on the server of re-negotiating the encryption). The big win is in the number of round trips the browser has to make to get the content. With Keep-Alive the browser gets to eliminate a full round trip for every request after the first, usually cutting full page load times in half.

Keep-Alive increase server load which is why some shared hosting providers disable it. Each open connection consumes memory as well as a file descriptor (linux) and in extreme cases (some Apache configs) it may have a 1:1 mapping from connections to processes.