JRuby uses exactly the same in-memory format as Ruby, byte[]. We also support the Encoding logic from Ruby 1.9, again atop byte[]. JRuby actually has better String performance and encoding support than Java itself in many cases.

Rubinius boots negligibly faster than JRuby; for simple scripts, JRuby takes around 0.5s on my system versus 0.35s for Rubinius. All implementations that JIT to get to full speed will have startup issues.

JRuby does not use any of Rubinius's standard library. We do both share most of MRI's standard library, however.

As for RubySpec; Rubinius started it, and JRuby is a very heavy contributor to the project. However JRuby also runs many other Ruby test suites, so we have not been as dependent on filling out RubySpec.

We do both share implementation ideas and advocate for each other in debates with ruby-core. Hopefully in the future we can work together on adding new standard APIs as well (rather than unilaterally adding them to one implementation or the other).

I shouldn't be surprised, but my knowledge of the ruby world seems to be about two years out of date. I suppose any remaining additional memory usage is probably due to inheriting space/speed tradeoffs straight from upstream hotspot.

It's great to hear that the startup time has improved so much.

Actually a lot of the process size in JRuby is the fact that we need to set JVM's max size high for rare cases, but then the JVM happily grows to fill much of that size. If you choke it down to a smaller size, we're competitive with at least generational-GC impls like Rubinius (but still much larger than conservative-GC impls like MRI).