# Improving Performance

## Test Isolation

By default Vitest runs every test file in an isolated environment based on the [pool](/config/pool):

- `threads` pool runs every test file in a separate [`Worker`](https://nodejs.org/api/worker_threads.html#class-worker)
- `forks` pool runs every test file in a separate [forked child process](https://nodejs.org/api/child_process.html#child_processforkmodulepath-args-options)
- `vmThreads` pool runs every test file in a separate [VM context](https://nodejs.org/api/vm.html#vmcreatecontextcontextobject-options), but it uses workers for parallelism

This greatly increases test times, which might not be desirable for projects that don't rely on side effects and properly cleanup their state (which is usually true for projects with `node` environment). In this case disabling isolation will improve the speed of your tests. To do that, you can provide `--no-isolate` flag to the CLI or set [`test.isolate`](/config/isolate) property in the config to `false`.

::: code-group
```bash [CLI]
vitest --no-isolate
```
```ts [vitest.config.js]
import { defineConfig } from 'vitest/config'

export default defineConfig({
  test: {
    isolate: false,
  },
})
```
:::

You can also disable isolation for specific files only by using `projects`:

```ts [vitest.config.js]
import { defineConfig } from 'vitest/config'

export default defineConfig({
  test: {
    projects: [
      {
        test: {
          name: 'Isolated',
          isolate: true, // (default value)
          exclude: ['**.non-isolated.test.ts'],
        },
      },
      {
        test: {
          name: 'Non-isolated',
          isolate: false,
          include: ['**.non-isolated.test.ts'],
        },
      },
    ],
  },
})
```

:::tip
If you are using `vmThreads` pool, you cannot disable isolation. Use `threads` pool instead to improve your tests performance.
:::

For some projects, it might also be desirable to disable parallelism to improve startup time. To do that, provide `--no-file-parallelism` flag to the CLI or set [`test.fileParallelism`](/config/fileparallelism) property in the config to `false`.

::: code-group
```bash [CLI]
vitest --no-file-parallelism
```
```ts [vitest.config.js]
import { defineConfig } from 'vitest/config'

export default defineConfig({
  test: {
    fileParallelism: false,
  },
})
```
:::

## Limiting Directory Search

You can limit the working directory when Vitest searches for files using [`test.dir`](/config/dir) option. This should make the search faster if you have unrelated folders and files in the root directory.

## Caching Between Reruns

In watch mode, Vitest caches all transformed files in memory, which makes reruns fast. However, this cache is discarded once the test run finishes. By enabling [`experimental.fsModuleCache`](/config/experimental#experimental-fsmodulecache), Vitest persists this cache to the file system so it can be reused across reruns.

This improvement is most noticeable when rerunning a small number of tests that depend on a large module graph. For full test suites, parallelization already mitigates the cost because other tests populate the in-memory cache while earlier tests are still running. For example, running one test file with a huge module graph (>900 modules):

```shell
# the first run
Duration  8.75s (transform 4.02s, setup 629ms, import 5.52s, tests 2.52s, environment 0ms, prepare 3ms)

# the second run
Duration  5.90s (transform 842ms, setup 543ms, import 2.35s, tests 2.94s, environment 0ms, prepare 3ms)
```

## Pool

By default Vitest runs tests in `pool: 'forks'`. While `'forks'` pool is better for compatibility issues ([hanging process](/guide/common-errors.html#failed-to-terminate-worker) and [segfaults](/guide/common-errors.html#segfaults-and-native-code-errors)), it may be slightly slower than `pool: 'threads'` in larger projects.

You can try to improve test run time by switching `pool` option in configuration:

::: code-group
```bash [CLI]
vitest --pool=threads
```
```ts [vitest.config.js]
import { defineConfig } from 'vitest/config'

export default defineConfig({
  test: {
    pool: 'threads',
  },
})
```
:::

## Sharding

Test sharding is a process of splitting your test suite into groups, or shards. This can be useful when you have a large test suite and multiple machines that could run subsets of that suite simultaneously.

To split Vitest tests on multiple different runs, use [`--shard`](/guide/cli#shard) option with [`--reporter=blob`](/guide/reporters#blob-reporter) option:

```sh
vitest run --reporter=blob --shard=1/3 # 1st machine
vitest run --reporter=blob --shard=2/3 # 2nd machine
vitest run --reporter=blob --shard=3/3 # 3rd machine
```

> Vitest splits your _test files_, not your test cases, into shards. If you've got 1000 test files, the `--shard=1/4` option will run 250 test files, no matter how many test cases individual files have.

Collect the results stored in `.vitest-reports` directory from each machine and merge them with [`--merge-reports`](/guide/cli#merge-reports) option:

```sh
vitest run --merge-reports
```

::: details GitHub Actions example
This setup is also used at https://github.com/vitest-tests/test-sharding.

```yaml
# Inspired from https://playwright.dev/docs/test-sharding
name: Tests
on:
  push:
    branches:
      - main
jobs:
  tests:
    runs-on: ubuntu-latest
    strategy:
      matrix:
        shardIndex: [1, 2, 3, 4]
        shardTotal: [4]
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: 20

      - name: Install pnpm
        uses: pnpm/action-setup@a7487c7e89a18df4991f7f222e4898a00d66ddda # v4.1.0

      - name: Install dependencies
        run: pnpm i

      - name: Run tests
        run: pnpm run test --reporter=blob --shard=${{ matrix.shardIndex }}/${{ matrix.shardTotal }}

      - name: Upload blob report to GitHub Actions Artifacts
        if: ${{ !cancelled() }}
        uses: actions/upload-artifact@v4
        with:
          name: blob-report-${{ matrix.shardIndex }}
          path: .vitest-reports/*
          include-hidden-files: true
          retention-days: 1

      - name: Upload attachments to GitHub Actions Artifacts
        if: ${{ !cancelled() }}
        uses: actions/upload-artifact@v4
        with:
          name: blob-attachments-${{ matrix.shardIndex }}
          path: .vitest-attachments/**
          include-hidden-files: true
          retention-days: 1

  merge-reports:
    if: ${{ !cancelled() }}
    needs: [tests]

    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: 20

      - name: Install pnpm
        uses: pnpm/action-setup@a7487c7e89a18df4991f7f222e4898a00d66ddda # v4.1.0

      - name: Install dependencies
        run: pnpm i

      - name: Download blob reports from GitHub Actions Artifacts
        uses: actions/download-artifact@v4
        with:
          path: .vitest-reports
          pattern: blob-report-*
          merge-multiple: true

      - name: Download attachments from GitHub Actions Artifacts
        uses: actions/download-artifact@v4
        with:
          path: .vitest-attachments
          pattern: blob-attachments-*
          merge-multiple: true

      - name: Merge reports
        run: npx vitest --merge-reports
```

If your tests create file-based attachments (for example via `context.annotate` or custom artifacts), upload and restore [`attachmentsDir`](/config/attachmentsdir) in the merge job as shown above.

:::

:::tip
Test sharding can also become useful on high CPU-count machines.

Vitest will run only a single Vite server in its main thread. Rest of the threads are used to run test files.
In a high CPU-count machine the main thread can become a bottleneck as it cannot handle all the requests coming from the threads. For example in 32 CPU machine the main thread is responsible to handle load coming from 31 test threads.

To reduce the load from main thread's Vite server you can use test sharding. The load can be balanced on multiple Vite server.

```sh
# Example for splitting tests on 32 CPU to 4 shards.
# As each process needs 1 main thread, there's 7 threads for test runners (1+7)*4 = 32
# Use VITEST_MAX_WORKERS:
VITEST_MAX_WORKERS=7 vitest run --reporter=blob --shard=1/4 & \
VITEST_MAX_WORKERS=7 vitest run --reporter=blob --shard=2/4 & \
VITEST_MAX_WORKERS=7 vitest run --reporter=blob --shard=3/4 & \
VITEST_MAX_WORKERS=7 vitest run --reporter=blob --shard=4/4 & \
wait # https://man7.org/linux/man-pages/man2/waitpid.2.html

vitest run --merge-reports
```

:::
