Make Your Jest Tests up to 20% Faster by Changing a Single Setting

March 21, 2021

TL;DR

When you have Jest as your test runner, passing the --maxWorkers=50% option will make the tests faster in most cases. For watch mode, use --maxWorkers=25%, and for CI disable Jest workers with --runInBand. You can experiment with the percentage and fine-tune for your particular setup.

// package.json
{
  "scripts": {
    // standalone Jest
    "test": "jest --maxWorkers=50%",
    "test:watch": "jest --watch --maxWorkers=25%",
    "test:ci": "jest --runInBand",

    // or with Create React App
    "test": "react-scripts test --watchAll=false --maxWorkers=50%",
    "test:watch": "react-scripts test --maxWorkers=25%",
    "test:ci": "react-scripts test --watchAll=false --runInBand"
  }
}

How Jest selects the number of workers to use

The Jest test runner—that is also supplied by default with Create React App—does not run optimally out of the box.

By default, Jest will run on all available CPU threads, using one thread for the cli process and the rest for test workers. When in watch mode, it will use half the available CPU threads.

This however results in sub-optimal performance on all systems I tested on.

We can adjust --maxWorkers by either providing a number of threads, or a percentage of the available system threads. I prefer using percentage, as it’s usually easy to find a value that works across multiple systems with different CPUs.

Benchmarking Jest with and without —maxWorkers=50%

These are the stats for the testsuite used. It’s a React app with mostly unit tests:

Test Suites: 43 passed, 43 total
Tests:       1 skipped, 258 passed, 259 total
Snapshots:   2 passed, 2 total

Here are the results on an Intel i9-9900KS (5GHz / 8 cores 16 threads):
A 21% speedup.

$ hyperfine 'npm test' 'npm test -- --maxWorkers=50%'
Benchmark #1: npm test
  Time (mean ± σ):      4.763 s ±  0.098 s    [User: 49.334 s, System: 5.996 s]
  Range (min … max):    4.651 s …  4.931 s    10 runs

Benchmark #2: npm test -- --maxWorkers=50%
  Time (mean ± σ):      3.925 s ±  0.044 s    [User: 27.776 s, System: 4.028 s]
  Range (min … max):    3.858 s …  3.973 s    10 runs

Summary
  'npm test -- --maxWorkers=50%' ran
    1.21 ± 0.03 times faster than 'npm test'

And here are the results on a 2016 13” MacBook Pro (3.3GHz / 2 cores 4 threads):
A 14% speedup.

$ hyperfine 'npm test' 'npm test -- --maxWorkers=50%'
Benchmark #1: npm test
  Time (mean ± σ):     14.380 s ±  0.230 s    [User: 22.869 s, System: 3.689 s]
  Range (min … max):   14.049 s … 14.807 s    10 runs

Benchmark #2: npm test -- --maxWorkers=50%
  Time (mean ± σ):     12.567 s ±  0.213 s    [User: 19.628 s, System: 3.290 s]
  Range (min … max):   12.258 s … 12.942 s    10 runs

Summary
  'npm test -- --maxWorkers=50%' ran
    1.14 ± 0.03 times faster than 'npm test'

And finally, a 2020 M1 MacBook Air:
A 12% speedup.

$ hyperfine 'npm test' 'npm test -- --maxWorkers=50%'
Benchmark #1: npm test
  Time (mean ± σ):      9.782 s ±  0.115 s    [User: 10.874 s, System: 1.548 s]
  Range (min … max):    9.686 s …  9.908 s    3 runs

Benchmark #4: npm test --maxWorkers=4
  Time (mean ± σ):      5.216 s ±  0.060 s    [User: 19.301 s, System: 3.523 s]
  Range (min … max):    5.179 s …  5.285 s    3 runs

Summary
  'npm test -- --maxWorkers=50%' ran
    1.12 ± 0.01 times faster than 'npm test'

What about running alongside other programs?

Measuring this is harder, but I have noticed that running with --maxWorkers=25% performs the best for my use cases. This gives the best performance for test:watch alongside code watch/hot reloading, and for running husky commit hooks in parallel.

What about CI?

In my and other’s experience, --runInBand can be the fastest option for CI runs.

What does --runInBand do? From the official docs:

Run all tests serially in the current process, rather than creating a worker pool of child processes that run tests. This can be useful for debugging.

Turns out, it’s also useful in resource-constrained environments like CI, where the overhead of worker processes is higher than the speedup of running tests in parallel.

Finding the optimal number of threads for a given testsuite/system

It’s easy to write a small script to find the optimal number of threads for your particular usecase:

export MAX_WORKERS=15; hyperfine --parameter-scan num_threads 1 $MAX_WORKERS 'npm run test -- --maxWorkers={num_threads}' -m 3 -w 1

Here are the results on an Intel i9-9900KS (5GHz / 8 cores 16 threads):

Summary
  'npm run test:jest -- --maxWorkers=7' ran
    1.01 ± 0.01 times faster than 'npm run test:jest -- --maxWorkers=8'
    1.02 ± 0.02 times faster than 'npm run test:jest -- --maxWorkers=6'
    1.04 ± 0.02 times faster than 'npm run test:jest -- --maxWorkers=5'
    1.05 ± 0.02 times faster than 'npm run test:jest -- --maxWorkers=9'
    1.08 ± 0.03 times faster than 'npm run test:jest -- --maxWorkers=10'
    1.11 ± 0.02 times faster than 'npm run test:jest -- --maxWorkers=11'
    1.11 ± 0.02 times faster than 'npm run test:jest -- --maxWorkers=4'
    1.18 ± 0.02 times faster than 'npm run test:jest -- --maxWorkers=13'
    1.19 ± 0.02 times faster than 'npm run test:jest -- --maxWorkers=14'
    1.21 ± 0.04 times faster than 'npm run test:jest -- --maxWorkers=12'
    1.23 ± 0.02 times faster than 'npm run test:jest -- --maxWorkers=15'
    1.25 ± 0.02 times faster than 'npm run test:jest -- --maxWorkers=3'
    1.58 ± 0.02 times faster than 'npm run test:jest -- --maxWorkers=2'
    2.55 ± 0.04 times faster than 'npm run test:jest -- --maxWorkers=1'

As you can see, the optimal number of workers in this case is 7, not the 8 that 50% would give us. However the difference between the two is within the margin of error, and 50% is more flexible.

Conclusion

Jest performance out of the box can be easily improved by tweaking maxWorkers. If you decide to test this for yourself, hyperfine makes it very easy.

Hope this was helpful! Feel free to reach out to me on Twitter @VanTanev.

Happy hacking!


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Written by Ivan Tanev
Twitter / GitHub