[pjscott]

Shared libraries are just that: shared. They get memory-mapped, and the code is shared between processes. When a program has many threads open, something similar happens: the code is shared between the threads, but they get different stacks. Those stacks are a problem if you're spawning tens of thousands of threads, one for each connection you have open concurrently. You may find yourself running out of memory, or at least using way too much.

What Tornado does instead is serve a large number of connections from a single thread, using epoll to block on a group of threads until one or more are ready for I/O. This has much lower memory overhead, and does not require switching among threads.

This kind of I/O model is at its best when you have a huge number of connections, idle most of the time, and there are very few CPU-intensive operations. Long polling usually fits that description.

[wisty]

IIRC, another thing which keeps processes open is database access.

I'm not sure, but I think that with Apache + Django, you get something like: Apache gets request, gives it to Django, Django asks MySQL for a value, waits, waits, waits (taking up lots of memory, but not too much CPU, but your host charges for RAM), waits a little more, gets a value from MySQL, puts it in a template, hands the response to Apache, which then sends the response to the client.