http keep-alive in the modern age

Question

So according to the haproxy author, who knows a thing or two about http:

Keep-alive was invented to reduce CPU usage on servers when CPUs were 100

Answer 1

In the years since this was written (and posted here on stackoverflow) we now have servers such as nginx which are rising in popularity.

nginx for example can hold open 10,000 keep-alive connections in a single process with only 2.5 MB (megabytes) of RAM. In fact it's easy to hold open multiple thousands of connections with very little RAM, and the only limits you'll hit will be other limits such as the number of open file handles or TCP connections.

Keep-alive was a problem not because of any problem with the keep-alive spec itself but because of Apache's process-based scaling model and of keep-alives hacked into a server whose architecture wasn't designed to accommodate it.

Especially problematic is Apache Prefork + mod_php + keep-alives. This is a model where every single connection will continue to occupy all the RAM that a PHP process occupies, even if it's completely idle and only remains open as a keep-alive. This is not scalable. But servers don't have to be designed this way - there's no particular reason a server needs to keep every keep-alive connection in a separate process (especially not when every such process has a full PHP interpreter). PHP-FPM and an event-based server processing model such as that in nginx solve the problem elegantly.

Update 2015:

SPDY and HTTP/2 replace HTTP's keep-alive functionality with something even better: the ability not only to keep alive a connection and make multiple requests and responses over it, but for them to be multiplexed, so the responses can be sent in any order, and in parallel, rather than only in the order they were requested. This prevents slow responses blocking faster ones and removes the temptation for browsers to hold open multiple parallel connections to a single server. These technologies further highlight the inadequacies of the mod_php approach and the benefits of something like an event-based (or at the very least, multi-threaded) web server coupled separately with something like PHP-FPM.