Using Vitest bench to track performance regressions in your CI

One of the easiest ways to track performance regressions in your JavaScript applications is to use Vitest's bench API. It lets you write benchmarks as easily as unit tests:

import { bench, describe, test } from "vitest";

describe("fibonacci", () => {
  test("should return 55 when called with 10", () => {
    expect(fibonacci(10)).toBe(55);
  });

  bench("fibo 10", () => {
    fibonacci(10);
  });
});

Then you can run the benchmarks with vitest bench:

pnpm vitest bench

The problem that we now face is that the results are inconsistent between runs. Running the command multiple times in a CI/CD runner or on your machine will yield different results. Thus it will be hard to spot performance regressions with confidence.

This is where using CodSpeed comes in handy. CodSpeed provides a way to run benchmarks in a consistent manner, and thus detect performance regressions with confidence.

We are excited to announce that CodSpeed now has first-class support for Vitest!

How to use Vitest with CodSpeed

Install the @codspeed/vitest-plugin package and vitest as dev dependencies:

pnpm add -D @codspeed/vitest-plugin vitest

Create or update the vitest.config.ts file at the root of your project:

import codspeedPlugin from "@codspeed/vitest-plugin";
import { defineConfig } from "vitest/config";

export default defineConfig({
  plugins: [codspeedPlugin()],
  // ...
});

Let's create a fibonacci function and create some benchmarks on it. Create a new file src/fibonacci.ts:

export function fibonacci(n: number): number {
  if (n <= 1) return n;
  return fibonacci(n - 1) + fibonacci(n - 2);
}

Create a new file src/fibonacci.bench.ts:

import { bench, describe } from "vitest";
import { fibonacci } from "./fibonacci";

describe("fibonacci", () => {
  bench("fibo 10", () => {
    fibonacci(10);
  });

  bench("fibo 15", () => {
    fibonacci(15);
  });
});

Now we can run the benchmarks with vitest bench:

$ vitest bench

[CodSpeed] bench detected but no instrumentation found, falling back to default vitest runner
 RUN  v1.0.3 ...

 ✓ src/fibonacci.bench.ts (2) 1521ms
   ✓ fibonacci (2) 1520ms
     name                hz     min     max    mean     p75     p99    p995    p999     rme  samples
   · fibo 10   1,760,687.42  0.0005  1.0279  0.0006  0.0006  0.0007  0.0007  0.0016  ±0.50%   880344   fastest
   · fibo 15     173,932.15  0.0052  0.0594  0.0057  0.0059  0.0064  0.0070  0.0147  ±0.08%    86967


 BENCH  Summary

  fibo 10 - src/fibonacci.bench.ts > fibonacci
    10.12x faster than fibo 15

We can see that CodSpeed has been detected and that the benchmarks have been run. For the moment we can see the default output of Vitest's bench API.

Running the benchmarks in your CI

To generate performance reports, you need to run the benchmarks in your CI. This allows CodSpeed to detect the CI environment and properly configure the instrumented environment.

Enable the repository on CodSpeed

The first step is to log in to CodSpeed and import the repository you want to track. You can find more information about this in the CodSpeed documentation.

After installing the GitHub app and importing the repository in CodSpeed, you will be prompted a CODSPEED_TOKEN secret to copy. This token is used to authenticate the CodSpeed GitHub Actions workflow.

Add a CodSpeed GitHub Actions workflow

Open a Pull Request on your repo with the previous changes, and the following GitHub Actions workflow .github/workflows/codspeed.yml. It will run the benchmarks and report the results to CodSpeed on every push to the main branch and every pull request:

name: CodSpeed

on:
  push:
    branches:
      - "main"
  pull_request:

jobs:
  benchmarks:
    name: Run benchmarks
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: pnpm/action-setup@v2
      - uses: actions/setup-node@v3
        with:
          cache: pnpm
      - name: Install dependencies
        run: pnpm install
      - name: Run benchmarks
        uses: CodSpeedHQ/action@v3
        with:
          run: pnpm vitest bench
          # token retrieved from the CodSpeed app at the previous step
          token: ${{ secrets.CODSPEED_TOKEN }}

If everything went well, the workflow should run and after it finishes you should see a comment from the CodSpeed GitHub app on the Pull Request with the run report.

Visualizing the benchmarks on the CodSpeed app

By clicking on the CodSpeed comment in the Pull Request, you will be redirected to the CodSpeed report of the PR. You will see a list of the benchmarks, and when clicking on the fibo 10 benchmark its flame graph will appear:

Execution profile

BaseDiffHead

Execution profile not availableWe couldn't create an execution profile for this benchmark. Check the documentation for more details on enabling trace generation.

And that's it for the installation of CodSpeed with Vitest, now we can explore a bit more what CodSpeed offers.

Visualizing a performance change

One of the main features of CodSpeed is to detect and visualize the performance changes of a pull request.

Let's now say that we are on a more complex project where we have a parsePr function that parses a GitHub pull request. We created a pull request with some performance improvements, and the following is in the CodSpeed report:

Execution profile

BaseDiffHead

parsePr::short body

parsePr (92.89%)

prepareParsingBody (63.21%)

parseBody (57.32%)

parseIssueFixed (25.49%)

logMetrics (19.64%)

sendEvent (8.01%)

parseTitle (28.67%)

modifyTitle (6.49%) (new)

logMetrics (21.33%)

sendEvent (8.70%)

Slower

Faster

New

We observe a 25% improvement in the overall benchmark, driven by enhancements in the parsePr and parseIssueFixed functions. Although the logMetrics function has experienced regression, its impact on the overall benchmark is less pronounced compared to the observed improvements. Additionally, a new function, modifyTitle, is introduced and denoted by the distinctive blue color.

Thanks to the differential flame graph, it is now really easy to understand at a glance what code changes impact the performance.

Wrapping up

And there you have it—CodSpeed now supports Vitest to make performance testing a breeze for your JavaScript and TypeScript projects! With an easy-to-use bench API and the magic touch of CodSpeed's consistency, tracking regressions is as simple as adding a unit test.

Get ready to supercharge your CI with performance reports, flame graphs, and more!

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Resources

• Vitest integration in the CodSpeed documentation
• @codspeed/vitest-plugin on GitHub
• Trace Generation in the CodSpeed documentation
• Vitest bench API