I just ran a small test that started and ran one million green threads on Ruby 1.8.7, and it worked just fine. I won't make any claims about the performance, but it certainly works. 1.9+ supports both green threads (fibers) and system/kernel threads, and would be much better to test.

MRI is slow regardless, so unless you're bound by extremely slow IO from other components, you're going to run into other limits long before you reach those kind of numbers, though.