JIT Performance with a Simpler Benchmark

There have been some JIT performance improvements in Ruby 2.7, which is still prerelease. And lately I’m using a new, simpler benchmark lately for researching Ruby performance.

Hey - wasn’t JIT supposed to be easier to make work on simpler code? Let’s see how JIT, including the prerelease code, works with that new benchmark.

(Just wanna see graphs? These are fairly simple graphs, but graphs are always good. Scroll down for the graphs.)

The Setup - Methodology

You may remember that Rails Simpler Bench currently uses “hello, world”-type very simple routes that just return a static string. That’s probably the best possible Rails use case for JIT. I’m starting with no concurrency, just a single request at once. That doesn’t show JIT’s full speedup, but it’s the most accurate and more reproducible to measure… And mostly, we want to know if JIT speeds things up at all rather than showing the largest possible speedup. I’m also measuring in both Rails and plain Rack, with Puma, on a dedicated-tenancy AWS EC2 m4.2xlarge instance. There’s no networking happening outside the instance itself, so this should give us nice low-noise results.

I wound up running one set of tests (everything Ruby 2.6.2) on one instance and the other set (everything with new prerelease Ruby) on another - so don’t treat this as an apples-to-apples comparison of prerelease Ruby’s speedup over 2.6.2. That’s okay, there’s all sorts of reasons that’s not a good idea to do anyway. Instead, we’re just checking the relative performance of JIT to no-JIT for each Ruby.

“New prerelease Ruby 2.7” is going to be accurate for a lot of different commits before the release around Christmastime. For this article, I’m using commit 025206d0dd29266771f166eb4f59609af602213a, which was new on May 9th. It’s what “git pull” got when I was getting ready to write this post.

Each of these runs is done with 10 batches of 4 minutes of HTTP requests, after 2 minutes of warmup for the server. I’m using Puma for the app server and wrk as the HTTP load generator. This should sound a lot like the setup for several of my recent blog posts. You can find the benchmark code here, based on a variation of this config file.

The Results

Let’s start with Rails - it’s what gets asked the most often. How does JIT do?

Takashi has made it clear that JIT isn’t expected to be faster for Rails… and that has been my experience as well. But he says the new JIT does better than in 2.6.

So let’s try. How does new prerelease JIT do compared to the released 2.6? First I’ll show you the graph, then I’ll give a bit of interpretation.

That thick line toward the bottom is the X axis, or “rate == 0.”

That thick line toward the bottom is the X axis, or “rate == 0.”

Those pink bars are an indication of the 10th, 50th and 90th percentile from lowest to highest. It’s like a box plot that way.

On the left, for Ruby 2.6.2, the JIT and no-JIT plots are pretty far apart. The medians are 1280 (No JIT) versus 1060 (w/ JIT), for instance. JIT is substantially slower, though not as much slower as for Rails Ruby Bench. That should make sense. JIT has an easier time on simpler code with shorter methods so Rails Ruby Bench is a terrible case for it. Rails Simpler Bench isn’t as bad.

Better yet, on the right you can see that they’re getting quite close for Ruby 2.7 prerelease - only around 5% slower, give or take.

What About Rack?

What should we expect for Rack? Well, if simpler is better for JITting, Rack should have better JIT-versus-not performance. That is, JIT should do relatively better compared to non-JIT by some amount in 2.7 than 2.6.

And that’s roughly what we see:

JIT is still slower than non-JIT, but it’s getting closer. These numbers are much higher because a raw Rack “hello, world” route is very fast compared to Rails.

JIT is still slower than non-JIT, but it’s getting closer. These numbers are much higher because a raw Rack “hello, world” route is very fast compared to Rails.


What you’re seeing above is pretty much what Takashi Kokubun said - while JIT is still slower on Rails (and Rack) than no JIT, the newer changes in 2.7 look promising… And JIT is catching up. We have around a year and a half before Ruby 3x3 is tentatively scheduled for release. This definitely looks like JIT could be a plus for Rails instead of a minus by then, but I wouldn’t expect it to be, say, 30% faster. But Takashi may prove me wrong!