“What this means for the Ruby developer is that any thread which does I/O will cause the entire ruby process (the ruby interpretter and all ruby green threads) to block.”

Sorry, gotta disagree with you here. The MRI core I/O implementation is essentially non-blocking. If it detects that an I/O operation will block, it will simply set the thread's state to 'blocking on I/O' and schedule another green thread instead.

Consider this:
require 'thread'
require 'socket'

thread = Thread.new do
while true
puts rand
sleep 1
end
end

TCPSocket.new('www.google.com', 80).read

Look! The thread keeps running even though we're blocked on a read operation.

What is true however, is that native extensions that:
1. use blocking I/O
and
2. aren't explicitly programmed to support green threads (i.e. to cause a context switch when it detects a potentially blocking condition)
will block all green threads. The MySQL library is one such example.

http://www.elliterate.com/files/ruby_1.8.5_sigvtalrm_fix.patch

Any reason why this couldn’t work? It doesn’t seem to be causing any problems (yet) on our staging server.