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Got different benchmarks from Programming Crystal

Tags: crystal benchmarking not-quite-errata programming

Finally reading Programming Crystal, by Ivo Balbaert and Simon St. Laurent. Good stuff. The Crystal language has advanced some since the book came out, but nearly all the code runs as-is.

Something that jumped out at me was the difference between their results and mine with the benchmarking example. Not the raw numbers. I’d be a little confused if those were exactly the same. The ratios caught my attention.

Given this source:

require "benchmark"


Benchmark.ips do |x|"Appending") do
  end"Using to_s") do
  end"Interpolation") do

def append
  IOM << 42

def to_s
  IOM << 42.to_s

def interpolation
  IOM << "#{42}"

Here’s what we’re told to expect.

Build the code for production using $ crystal build —release and execute that with: $ ./benchmarking

You’ll get results like this:

Appending    34.06M ( 29.36ns) (± 3.97%) fastest
Using to_s   12.67M ( 78.92ns) (± 7.55%) 2.69× slower
Interpolation  2.8M (356.75ns) (± 3.84%) 12.15× slower

But in Crystal 0.36.1 on Ubuntu 20.04, running on Windows WSL2:

$ ./benchmarking
    Appending 110.36M (  9.06ns) (± 3.70%)   0.0B/op        fastest
   Using to_s  18.52M ( 54.00ns) (± 5.36%)  16.0B/op   5.96× slower
Interpolation  19.19M ( 52.12ns) (± 2.99%)  16.0B/op   5.75× slower

Sure, my numbers are bigger than the book’s. That’s cool. But interpolation and to_s are so close to each other on my machine!

Maybe that’s WSL? After I get the day’s tasks done, I revisit on my computer’s Manjaro partition.

$ ./benchmarking
    Appending 123.54M (  8.09ns) (± 2.57%)   0.0B/op        fastest
   Using to_s  56.57M ( 17.68ns) (± 3.49%)  16.0B/op   2.18× slower
Interpolation  56.55M ( 17.68ns) (± 4.32%)  16.0B/op   2.18× slower

Well heck.

It’s faster on native Linux than WSL. That’s hardly surprising. But the differences between to_s and interpolation are now negligible. For that matter, both of them are closer to the speed of append than to_s was in the book’s example!

Is the difference because of changes in Crystal? Some dependency, like LLVM? My computer’s 40GB of RAM compared to whatever the authors used? My hard drive? GPU? Is Mercury in retrograde?

I don’t know! I just saw different numbers and thought it was curious.

My point isn’t that the book’s wrong. Heck no. The example’s supposed to remind you that testing your assumptions is important. All I’ve done is emphasized the validity of the lesson.


Good book. Fun language. Don’t forget to try out the example code. And if you need to care about performance? Don’t assume — benchmark.

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Added to vault 2024-01-15. Updated on 2024-01-26