Last week the Phusion guys, gave a talk at Google about the implementation of Ruby Enterprise Edition, and it's now available on YouTube.

It's good stuff if you are a VM geek, like me. The cover two major topics

1. How they made the Ruby 1.8 garbage collector Copy on Write (CoW) friendly, which allows multiple Ruby processes to share memory for unchanged objects. This greatly reduces the footprint of a typical Rails Deployment for example. They talks about a series of attempts to change the Ruby GC to move the mark bits from the objects themselves to a separate memory structure. The result is significant memory savings, a slightly slower GC, but overall faster Ruby performance because they replaced malloc with a faster allocator (from Google apparently) which more than offsets the GC performance
2. How a contribution from the Event Machine developers improves thread context switching for Ruby 1.8's user space threads. Ruby 1.8 handles thread context switches by copying the execution stack to and from the heap, which can result in significant time being taken up by memcopy calls. Ruby Enterprise edition now has an optional feature which instead switches the base stack pointer on a thread switch. This is processor specific, and is only available right now for Intel 32 and 64 bit processors.

So if this kind of stuff interests you, I'd recommend spending the half hour or so that it takes to watch the video