Profiling Test Performance

When you run Vitest it reports multiple time metrics of your tests:

bash

RUN  v2.1.1 /x/vitest/examples/profiling

✓ test/prime-number.test.ts (1) 4517ms
  ✓ generate prime number 4517ms

Test Files  1 passed (1)
     Tests  1 passed (1)
  Start at  09:32:53
  Duration  4.80s (transform 44ms, setup 0ms, collect 35ms, tests 4.52s, environment 0ms, prepare 81ms)
  # Time metrics ^^

Transform: How much time was spent transforming the files. See File Transform.
Setup: Time spent for running the setupFiles files.
Collect: Time spent for collecting all tests in the test files. This includes the time it took to import all file dependencies.
Tests: Time spent for actually running the test cases.
Environment: Time spent for setting up the test environment, for example JSDOM.
Prepare: Time Vitest uses to prepare the test runner.

Test runner

In cases where your test execution time is high, you can generate a profile of the test runner. See NodeJS documentation for following options:

WARNING

The --prof option does not work with pool: 'threads' due to node:worker_threads limitations.

To pass these options to Vitest's test runner, define poolOptions.<pool>.execArgv in your Vitest configuration:

ForksThreads

import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    pool: 'forks',
    poolOptions: {
      forks: {
        execArgv: [
          '--cpu-prof',
          '--cpu-prof-dir=test-runner-profile',
          '--heap-prof',
          '--heap-prof-dir=test-runner-profile',
        ],

        // To generate a single profile
        singleFork: true,
      },
    },
  },
});

import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    pool: 'threads',
    poolOptions: {
      threads: {
        execArgv: [
          '--cpu-prof',
          '--cpu-prof-dir=test-runner-profile',
          '--heap-prof',
          '--heap-prof-dir=test-runner-profile',
        ],

        // To generate a single profile
        singleThread: true,
      },
    },
  },
});

After the tests have run there should be a test-runner-profile/*.cpuprofile and test-runner-profile/*.heapprofile files generated. See Inspecting profiling records for instructions how to analyze these files.

See Profiling | Examples for example.

Main thread

Profiling main thread is useful for debugging Vitest's Vite usage and globalSetup files. This is also where your Vite plugins are running.

TIP

See Performance | Vite for more tips about Vite specific profiling.

We recommend vite-plugin-inspect for profiling your Vite plugin performance.

To do this you'll need to pass arguments to the Node process that runs Vitest.

bash

$ node --cpu-prof --cpu-prof-dir=main-profile ./node_modules/vitest/vitest.mjs --run
#      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                  ^^^^^
#               NodeJS arguments                                           Vitest arguments

After the tests have run there should be a main-profile/*.cpuprofile file generated. See Inspecting profiling records for instructions how to analyze these files.

File transform

In cases where your test transform and collection time is high, you can use DEBUG=vite-node:* environment variable to see which files are being transformed and executed by vite-node.

bash

$ DEBUG=vite-node:* vitest --run

 RUN  v2.1.1 /x/vitest/examples/profiling

  vite-node:server:request /x/vitest/examples/profiling/global-setup.ts +0ms
  vite-node:client:execute /x/vitest/examples/profiling/global-setup.ts +0ms
  vite-node:server:request /x/vitest/examples/profiling/test/prime-number.test.ts +45ms
  vite-node:client:execute /x/vitest/examples/profiling/test/prime-number.test.ts +26ms
  vite-node:server:request /src/prime-number.ts +9ms
  vite-node:client:execute /x/vitest/examples/profiling/src/prime-number.ts +9ms
  vite-node:server:request /src/unnecessary-file.ts +6ms
  vite-node:client:execute /x/vitest/examples/profiling/src/unnecessary-file.ts +4ms
...

This profiling strategy is a good way to identify unnecessary transforms caused by barrel files. If these logs contain files that should not be loaded when your test is run, you might have barrel files that are importing files unnecessarily.

You can also use Vitest UI to debug slowness caused by barrel file. The example below shows how importing files without barrel file reduces amount of transformed files by ~85%.

File treeexample.test.ts

├── src
│   └── utils
│       ├── currency.ts
│       ├── formatters.ts  <-- File to test
│       ├── index.ts
│       ├── location.ts
│       ├── math.ts
│       ├── time.ts
│       └── users.ts
├── test
│   └── formatters.test.ts
└── vitest.config.ts

import { expect, test } from 'vitest';
import { formatter } from '../src/utils'; 
import { formatter } from '../src/utils/formatters'; 

test('formatter works', () => {
  expect(formatter).not.toThrow();
});

Vitest UI demonstrating barrel file issues

To see how files are transformed, you can use VITE_NODE_DEBUG_DUMP environment variable to write transformed files in the file system:

bash

$ VITE_NODE_DEBUG_DUMP=true vitest --run

[vite-node] [debug] dump modules to /x/examples/profiling/.vite-node/dump

 RUN  v2.1.1 /x/vitest/examples/profiling
...

$ ls .vite-node/dump/
_x_examples_profiling_global-setup_ts-1292904907.js
_x_examples_profiling_test_prime-number_test_ts-1413378098.js
_src_prime-number_ts-525172412.js

Inspecting profiling records

You can inspect the contents of *.cpuprofile and *.heapprofile with various tools. See list below for examples.

Profiling Test Performance ​

Test runner ​

Main thread ​

File transform ​

Inspecting profiling records ​

Profiling Test Performance

Test runner

Main thread

File transform

Inspecting profiling records