# 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 only for specific pools
    poolOptions: {
      forks: {
        isolate: false,
      },
    },
  },
})
```
:::

:::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,
  },
})
```
:::

## 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 matchines 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 action 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

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

:::

:::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_THREADS or VITEST_MAX_FORKS depending on the pool:
VITEST_MAX_THREADS=7 vitest run --reporter=blob --shard=1/4 & \
VITEST_MAX_THREADS=7 vitest run --reporter=blob --shard=2/4 & \
VITEST_MAX_THREADS=7 vitest run --reporter=blob --shard=3/4 & \
VITEST_MAX_THREADS=7 vitest run --reporter=blob --shard=4/4 & \
wait # https://man7.org/linux/man-pages/man2/waitpid.2.html

vitest run --merge-reports
```

:::
