Where Does Rails Spend Its Time?

Last updated 3 years ago by Noah Gibbs


You may know that I run Rails Ruby Bench and write a fair bit about it. It’s intended to answer performance questions about a large Rails app running in a fairly real-world configuration.

Here’s a basic question I haven’t addressed much in this space: where does RRB actually spend most of its time?

I’ve used the excellent StackProf for the work below. It was both very effective and shockingly painless to use. These numbers are for Ruby 2.6, which is the current stable release in 2019.

(Disclaimer: this will be a lot of big listings and not much with the pretty graphs. So expect fairly dense info-dumps punctuated with interpretation.)

About Profiling

It’s hard to get high-quality profiling data that is both accurate and complete. Specifically, there are two common types of profiling and they have significant tradeoffs. Other methods of profiling fall roughly into these two categories, or a combination of them:

  • Instrumenting Profilers: insert code to track the start and stop points of whatever it measures; very complete, but distorts the accuracy by adding extra statements to the timing; usually high overhead; don’t run them in production

  • Sampling Profilers: every so many milliseconds, take a sample of where the code currently is; statistically accurate and can be quite low-overhead, but not particularly complete; fast parts of the code often receive no samples at all; don’t use them for coverage data; fast ones can be run in production

StackProf is a sampling profiler. It will give us a reasonably accurate picture of what’s going on, but it could easily miss methods entirely if they’re not much of the total runtime. It’s a statistical average of samples, not a Platonic ideal analysis. I’m cool with that - I’m just trying to figure out what bits of the runtime are large. A statistical average of samples is perfect for that.

I’m also running it for a lot of HTTP requests and adding the results together. Again, it’s a statistical average of samples - just what I want here.

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