I haven't used it myself, but you may want to take a look at Mike Perham's Phat. It modifies Rails to use asynchronous I/O and several cooperating Fibers per-thread/process to speed up Rails significantly.

http://www.mikeperham.com/2010/04/03/introducing-phat-an-asy...

http://news.ycombinator.com/item?id=1240609

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