One billion row challenge in Common Lisp

  Finished         Common Lisp         certainty/1brc

The one billion row challenge or 1brc for short has been all over my internet bubble lately. I first heard about it on hackernews and while I was not immediately interested, it quickly dawned on me that there are some interesting challenges lurking in this.

Since, I’m still having a lot of fun writing in this dead, useless programing language, called Common Lisp, I figured I want to give it a try. I didn’t expect to land anywhere near the highly optimized Java solutions, but I was very curious how close I get and if my Common Lisp-Fu is strong enough already, to tackle this.

In this short post, I just want to briefly introduce the project, and come back to a more elaborate treatment of this, some time in the future. This project allowed me to play around with lparallel, which I was keen to explore for a while.


My fastest solution managed to process the one billion rows in 28 seconds, on a relatively beefy Apple MacBook Pro M2. I did attempt to optimize further, but ultimately wasn’t successful.

I used mmap to optimize IO, lprallel to work on chunks in parallel, and tried to optimize the parsing and processing of the data. I didn’t go so far as to implement my own hash map.

Now I will say, that I know that SBCL is known to be quite a bit slower on ARM than on x86, so I imagine there are some gains to be made still. The same is true for mmap, which appears to be slower on macOS than on Linux / Unix. I would be curious to see, how fast it can be on the machines used for the Java solutions.


  • Solve the one billion row challenge in Common Lisp at all
  • Try to solve it with a runtime that feels acceptable


  • Use memory mapped files to speed up IO
  • Use multiple threads to process chunks of the input in parallel

January 27, 2024