Promise and Peril for Alternative Ruby Impls

My how things have changed in a couple short years.

Two years ago, in 2006, there were essentially two viable Ruby implementations: Matz's Ruby 1.8.x codebase, and JRuby. At the time, JRuby was just barely starting to run Rails. I consider that a sort of "singularity" in the lifetime of an implementation, the inflection point at which it becomes more than a toy. (OT: these days, I consider the ability to run Rails faster than Matz's Ruby a better inflection point, but we've had the Rails thing going for two years). Cardinal (Ruby on Parrot) was mostly dead, or at least on its way to being dead. YARV, eventually to become the Ruby 1.9 VM, was perhaps only half completed and was not yet officially marked to be the next Ruby. Rubinius had not really been started, or at least had not officially been named and could not be considered anywhere near viable. IronRuby was still Wilco Bauer's IronRuby, a doomed codebase and project name eventually to be adopted by Microsoft's later Ruby implementation effort. So there was very little competition, and most people still considered JRuby to be a big joke. Ha ha.

Fast forward to Spring 2008. Ruby 1.8.x has mostly been put in maintenance mode, but remains by far the most widely-deployed Ruby implementation, despite its relatively poor performance. Largely, this is because only Ruby 1.8.x is 100% compatible with Ruby 1.8.x, and because it's already packaged and shipped on a number of OSes, including Rubyist favorite OS X. But the rest of the field has become a lot more muddled. There are now around six implementations that have past the Rails inflection point or will soon, a handful of others likely to fade into obscurity (after contributing their own genetics to the Ruby ecosystem, surely), a few mysterious up-and-comers...and Cardinal is still dead.

Let's review the promise, peril, and status of all the implementations. Note, this is largely a mix of facts and my opinions. Corrections for the facts are welcome. Corrections for the opinions...well...let's take it offline.

Ruby 1.8

Matz's venerable code base (usually called MRI for "Matz's Ruby Interpreter", MatzRuby or Ruby 1.8 in this article) still has a death-grip on the Ruby world. For 99% of Ruby developers, MatzRuby is still the king of the hill. This is in the face of poor relative performance (slower than JRuby and 1.9 for sure, and slower than most of the others for many cases), poor memory management (conservative, non-compacting GC), and many upstart implementations which solve these problems. Why is this?

It's not hard to answer. It's compatibility and status quo. If all my apps run fine on MatzRuby, and MatzRuby is already installed, and I'm satisfied with the performance characteristics of MatzRuby...why would I run anything else? Because MatzRuby is largely not *that bad* for most uses, especially as relates to simple system scripting work, it's unlikely to go away any time soon. And since all but one of the alternative impls is targeting Ruby 1.8 features and compatibility, the still-in-development Ruby 1.9 is not gaining much traction yet (which is probably a good thing).

You should all know the specs and status for MatzRuby. Current official release is 1.8.6 patchlevel 114. There's a 1.8.7 preview 2 out now that backports a whole bunch of features from 1.9 and breaks some compatibility. The jury's still out on whether it will go final as-is. MatzRuby is a simple AST-walking interpreter, with a conservative GC, minimal/cumbersome Unicode support, and a large library of third-party native extensions. MatzRuby is still the gold standard for what Ruby 1.8 "is".

The peril for Ruby 1.8 right now involves keeping that 99% of Ruby users interested in Ruby while 1.9 bakes without breaking compatibility. Ruby 1.8.7 pre1 introduced some Ruby 1.9 features but also broke a bunch of stuff (most notably Rails), and pre2 doesn't pass the specs 1.8.6 does. We had a design meeting last week where it was decided that we folks working on the Ruby specs need to help the Ruby core team get involved, so they can start running the full suite as part of their development process. That's going to happen soon, but until it does 1.8 releases can break basically anything from day today and nobody will know about it.

Beyond compatibility and keeping the masses happy, Ruby 1.8 could use a little performance, scaling, and memory lovin' too. Unfortunately almost all that effort is going toward Ruby 1.9 now, leaving the vast majority of Ruby users stuck on one of the slowest implementations. That's good for us alternative implementers, since it means we're gaining users every day; but it's not good for the MatzRuby lineage because they're losing mindshare. It's hard to deny that the future of Ruby lies with the "excellent" implementations, if not with the "best" ones, and the definition of "excellent" is moving forward every day. Ruby 1.8 is not.

Ruby 1.9

Ruby 1.9 is the merging of the Ruby 1.8 class library and memory model with a large number of new features and a bytecode-based execution engine. It represents the work of Koichi Sasada, who first announced his YARV ("Yet Another Ruby VM") project at RubyConf 2004. YARV took a bit longer than he and many others expected to be completed, but as a result of his tireless efforts it is now the official Ruby 1.9 VM.

Ruby 1.9 introduces many new features, like a character-aware (Unicode and any other encoding) String implementation, Enumerator/Enumerable enhancements, and numerous refinements and additions to the rest of the class library I won't attempt to list here. Ruby 1.9 is defining, in essence, what the rest of the implementations will soon have to implement. For all the debates, most of the additions in Ruby 1.9 have been well-received by the community, though the exposure level is still extremely low.

Ruby 1.9 has, I believe, reached the Rails singularity. With some work over the past few months, Rails has moved closer and closer to running on Ruby 1.9. Last I heard, there was only one bug that needed patching in the 1.9 codebase for Rails trunk to run unmodified. Expect to see an announcement about Ruby 1.9 and Rails at RailsConf next month.

The interesting thing about Ruby 1.9 is that it will mark only the second non-MatzRuby implementation to run Rails, JRuby being the first. This is due in large part to the massive effort required to implement a bytecode VM for Ruby (1.9 was only just released this past December), and to the fact that Ruby 1.9 is still very much a moving target. APIs are being added and refined, optimizations are being tossed about at the VM level, and memory and GC improvements are being considered. So while it's unlikely that anyone will be moving Rails apps to Ruby 1.9 in the near future, Ruby 1.9 is certainly viable...and represents the most-likely future evolution of the Ruby language itself.

What worries me about Ruby 1.9 is that its performance doesn't seem "better enough" to change the future of Ruby. While Koichi's own benchmarks show it's much faster than Ruby 1.8, on general application benchmarks it's usually less than a 50% improvement. Many times, JRuby is able to exceed Ruby 1.9 performance, even without similar optimizations and feature removals. It is certainly a "better" performance story than Ruby 1.8, but is it enough?

Ruby 1.9 also took the first steps toward concurrency by making threads native, but it encumbered them with a giant lock a la Python. That means you still can't get concurrent execution of Ruby code on Ruby 1.9, something JRuby's been able to do all along, and you can't scale Ruby 1.9 any better on wide systems than you could with Ruby 1.8. There's plans to solve this by adding fine-grained locks to most internal data structures, but that's a hard problem to solve on par with JRuby 2.0 challenges I'll talk about in a minute. And without something like the JVM to really optimize that locking, performance will take a hit.

It's also unclear if people really *want* all of what Ruby 1.9 has to offer. Sure, people love the idea of a real encoding-aware String, but response to the rest of what Ruby 1.9 offers--including performance--has been a collective "meh." People are not flocking to Ruby 1.9 in droves, and many are contemplating whether their future Ruby work will simply be a lateral move to one of the 1.8-compatible implementations. And on the JRuby project, we've received almost no requests to implement Ruby 1.9 features, so we've only added a few tiny ones. Whither Ruby 2.0?

JRuby

Ahh, JRuby. How you have changed my life.

JRuby is a Java-based implementation of Ruby, or if you prefer not to speak the word "Java", it's Ruby for the JVM. JRuby was started in 2002 by Jan Arne Petersen, and though it had a couple good years of activity it never really got to a compatible-enough level to run real Ruby applications. Jan Arne moved on at some point and efforts were largely picked up by Thomas Enebo, current co-lead of JRuby. He was especially active when JRuby was being updated from Ruby 1.6 compatibility to Ruby 1.8.4 compatibility, a task which today is largely complete. I joined the project in fall 2004 after attending RubyConf 2004, and at the time I did not know Ruby. Now I know Ruby in a deeper way than I ever really wanted to...but that's a discussion for another day. I wasn't a hugely active contributor until late 2005, when I started working on a new interpreter and refactoring JRuby internals. I presented JRuby for the first time at RubyConf 2005, and then in early 2006 milestones started dropping like flies: IRB ran, then RubyGems, then Rake...and then Rails. We were still dog slow at the time, but we were viable.

JRuby reached the Rails singularity in time for JavaOne 2006, an event that led Sun to hire Tom and me in fall 2006. We demonstrated on stage JRuby running a simple Rails application. It was cobbled together, running under either Tomcat or simply WEBrick at the time, but we had proved it was possible to have an alternative Ruby implementation compatible enough to run Rails. JRuby was no longer a joke.

Over the past two years (man, has it really been two years?) we've essentially rewritten almost all of JRuby a piece at a time. We've been through three interpreters, one prototype compiler and one complete compiler, multiple Regexp engines, and at least two implementations of the key core classes. I wrote the compiler for JRuby during summer 2007, completing it around RailsConf EU 2007. My first compiler! We now run faster than Ruby 1.8 in both interpreted and compiled modes, with interpreted being perhaps 15-20% faster and compiled being at least a few times faster, generally on par with Ruby 1.9. Of course since we're based on the JVM, we share its object model, garbage collector, binary representation. So JRuby is certainly a "mini-VM" but we leave the nasty bits to JVM implementers to handle. Pragmatism, friends, pragmatism.

Perhaps the most notable result of JRuby's existence is that there are now so many Ruby implementations. If we had not shown the promise, many of the others might not have risked the peril. Oh, and we've ended up with a cracker-jack implementation of Ruby on the JVM too...I suppose that's worth a little something.

Perils...always perils. JRuby has managed to surmount most of the perils that await other implementations. And being on the other side of the chasm, I can tell you now it doesn't get easier.

Compatibility is *hard*. I'm not talking a little hard, I'm talking monumentally hard. Ruby is a very flexible, complicated language to implement, and it ships with a number of very flexible, complicated core class implementations. Very little exists in the way of specifications and test kits, so what we've done with JRuby we've done by stitching together every suite we could find. And after all this time, we still have known bugs and weekly reports of minor incompatibilities. I don't think an alternative implementation can ever truly become "compatible" as much as "more compatible". We're certainly the most compatible alternative impl, and even now we've got our hands full fixing bugs. Then there's Ruby 1.9 support, coming up probably in JRuby 1.2ish. Another adventure.

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