Splitting your CI build jobs between multiple machines running in parallel is a great way to make the process fast, which results in more time for building features. Github Actions allows running parallel jobs easily. In a previous article, we explained how you can use  Knapsack Pro to  split your RSpec test files efficiently between parallel jobs on GitHub Actions. Today we’d like to show how to address the problem of slow test files negatively impacting the whole build times.

Consider the split

Imagine you have a project with 30 RSpec spec files. Each file contains multiple test examples (RSpec “

its”). Most of the files are fairly robust, fast unit tests. Let’s say there are also some slower files, like feature specs. Perhaps one such feature spec file takes approximately 5 minutes to execute.

When we run different spec files on different parallel machines, we strive for similar execution time on all of them. In a described scenario, even if we run 30 parallel jobs (each one running just one test file), the 5 minute feature spec would be a bottleneck of the whole build. 29 machines may finish their work in a matter of seconds, but the whole build won’t be complete until the 1 remaining node finishes executing its file.

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