feat: add --no-isolate flag to improve performance, add documentation about performance

docs/config/index.md

@@ -2066,3 +2066,18 @@ Tells fake timers to clear "native" (i.e. not fake) timers by delegating to thei
 - **Version:** Since Vitest 1.1.0
 
 Path to a [workspace](/guide/workspace) config file relative to [root](#root).
+
+### isolate
+
+- **Type:** `boolean`
+- **Default:** `true`
+- **CLI:** `--no-isolate`, `--isolate=false` 
+- **Version:** Since Vitest 1.1.0
+
+Run tests in an isolated environment. This option has no effect on `vmThreads` pool.
+
+Disabling this option might improve [performance](/guide/performance) if your code doesn't rely on side effects (which is usually true for projects with `node` environment).
+
+::: note
+You can disable isolation for specific pools by using [`poolOptions`](#pooloptions) property.
+:::

docs/guide/cli.md

@@ -81,6 +81,7 @@ Run only [benchmark](https://vitest.dev/guide/features.html#benchmarking-experim
 | `--outputFile <filename/-s>` | Write test results to a file when the `--reporter=json` or `--reporter=junit` option is also specified <br /> Via [cac's dot notation] you can specify individual outputs for multiple reporters |
 | `--coverage` | Enable coverage report |
 | `--run` | Do not watch |
+| `--isolate` | Run every test file in isolation. To disable isolation, use --no-isolate (default: `true`) |
 | `--mode <name>` | Override Vite mode (default: `test`) |
 | `--workspace <path>` | Path to a workspace configuration file |
 | `--globals` | Inject APIs globally |

docs/guide/performance.md

@@ -0,0 +1,51 @@
+# Performance
+
+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`. If you are using several pools at once with `poolMatchGlobs`, you can also disable isolation for a specific pool you are using.
+
+::: 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,
+      },
+    },
+  },
+})
+```
+:::
+
+::: note
+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,
+  },
+})
+```
+:::

packages/vitest/src/defaults.ts

@@ -62,6 +62,7 @@ export const fakeTimersDefaults = {
 
 const config = {
   allowOnly: !isCI,
+  isolate: true,
   watch: !isCI,
   globals: false,
   environment: 'node' as const,

packages/vitest/src/node/cli.ts

@@ -30,6 +30,7 @@ cli
   .option('--run', 'Disable watch mode')
   .option('--mode <name>', 'Override Vite mode (default: test)')
   .option('--workspace <path>', 'Path to a workspace configuration file')
+  .option('--isolate', 'Run every test file in isolation. To disable isolation, use --no-isolate (default: true)')
   .option('--globals', 'Inject apis globally')
   .option('--dom', 'Mock browser API with happy-dom')
   .option('--browser [options]', 'Run tests in the browser (default: false)')

packages/vitest/src/node/core.ts

@@ -281,6 +281,7 @@ export class Vitest {
       'testNamePattern',
       'passWithNoTests',
       'bail',
+      'isolate',
     ] as const
 
     const cliOverrides = overridesOptions.reduce((acc, name) => {

packages/vitest/src/node/plugins/index.ts

@@ -51,6 +51,13 @@ export async function VitestPlugin(options: UserConfig = {}, ctx = new Vitest('t
         )
         testConfig.api = resolveApiServerConfig(testConfig)
 
+        testConfig.poolOptions ??= {}
+        testConfig.poolOptions.threads ??= {}
+        testConfig.poolOptions.forks ??= {}
+        // prefer --poolOptions.{name}.isolate CLI arguments over --isolate, but still respect config value
+        testConfig.poolOptions.threads.isolate = options.poolOptions?.threads?.isolate ?? options.isolate ?? testConfig.poolOptions.threads.isolate ?? viteConfig.test?.isolate
+        testConfig.poolOptions.forks.isolate = options.poolOptions?.forks?.isolate ?? options.isolate ?? testConfig.poolOptions.forks.isolate ?? viteConfig.test?.isolate
+
         // store defines for globalThis to make them
         // reassignable when running in worker in src/runtime/setup.ts
         const defines: Record<string, any> = deleteDefineConfig(viteConfig)
@@ -91,6 +98,9 @@ export async function VitestPlugin(options: UserConfig = {}, ctx = new Vitest('t
               allow: resolveFsAllow(getRoot(), testConfig.config),
             },
           },
+          test: {
+            poolOptions: testConfig.poolOptions,
+          },
         }
 
         // chokidar fsevents is unstable on macos when emitting "ready" event

packages/vitest/src/types/config.ts

@@ -280,6 +280,15 @@ export interface InlineConfig {
    */
   environmentMatchGlobs?: [string, VitestEnvironment][]
 
+  /**
+   * Run tests in an isolated environment. This option has no effect on vmThreads pool.
+   *
+   * Disabling this option might improve performance if your code doesn't rely on side effects.
+   *
+   * @default true
+   */
+  isolate?: boolean
+
   /**
    * Pool used to run tests in.
    *

test/config/test/resolution.test.ts

@@ -0,0 +1,104 @@
+import type { UserConfig } from 'vitest'
+import { describe, expect, it } from 'vitest'
+import { createVitest } from 'vitest/node'
+
+async function config(cliOptions: UserConfig, configValue: UserConfig = {}) {
+  const vitest = await createVitest('test', { ...cliOptions, watch: false }, { test: configValue })
+  return vitest.config
+}
+
+describe('correctly defines isolated flags', async () => {
+  it('prefers CLI poolOptions flags over config', async () => {
+    const c = await config({
+      isolate: true,
+      poolOptions: {
+        threads: {
+          isolate: false,
+        },
+        forks: {
+          isolate: false,
+        },
+      },
+    })
+    expect(c.poolOptions?.threads?.isolate).toBe(false)
+    expect(c.poolOptions?.forks?.isolate).toBe(false)
+    expect(c.isolate).toBe(true)
+  })
+
+  it('override CLI poolOptions flags over isolate', async () => {
+    const c = await config({
+      isolate: false,
+      poolOptions: {
+        threads: {
+          isolate: true,
+        },
+        forks: {
+          isolate: true,
+        },
+      },
+    }, {
+      poolOptions: {
+        threads: {
+          isolate: false,
+        },
+        forks: {
+          isolate: false,
+        },
+      },
+    })
+    expect(c.poolOptions?.threads?.isolate).toBe(true)
+    expect(c.poolOptions?.forks?.isolate).toBe(true)
+    expect(c.isolate).toBe(false)
+  })
+
+  it('override CLI isolate flag if poolOptions is not set via CLI', async () => {
+    const c = await config({
+      isolate: true,
+    }, {
+      poolOptions: {
+        threads: {
+          isolate: false,
+        },
+        forks: {
+          isolate: false,
+        },
+      },
+    })
+    expect(c.poolOptions?.threads?.isolate).toBe(true)
+    expect(c.poolOptions?.forks?.isolate).toBe(true)
+    expect(c.isolate).toBe(true)
+  })
+
+  it('keeps user configured poolOptions if no CLI flag is provided', async () => {
+    const c = await config({}, {
+      poolOptions: {
+        threads: {
+          isolate: false,
+        },
+        forks: {
+          isolate: false,
+        },
+      },
+    })
+    expect(c.poolOptions?.threads?.isolate).toBe(false)
+    expect(c.poolOptions?.forks?.isolate).toBe(false)
+    expect(c.isolate).toBe(true)
+  })
+
+  it('isolate config value overrides poolOptions defaults', async () => {
+    const c = await config({}, {
+      isolate: false,
+    })
+    expect(c.poolOptions?.threads?.isolate).toBe(false)
+    expect(c.poolOptions?.forks?.isolate).toBe(false)
+    expect(c.isolate).toBe(false)
+  })
+
+  it('if no isolation is defined in the config, fallback ot undefined', async () => {
+    const c = await config({}, {})
+    expect(c.poolOptions?.threads?.isolate).toBe(undefined)
+    expect(c.poolOptions?.forks?.isolate).toBe(undefined)
+    // set in configDefaults, so it's always defined
+    expect(c.isolate).toBe(true)
+  })
+})

test/run-once/vitest.config.ts

@@ -2,6 +2,6 @@ import { defineConfig } from 'vite'
 
 export default defineConfig({
   test: {
-    poolOptions: { threads: { isolate: false } },
+    isolate: false,
   },
 })

test/stacktraces/fixtures/vite.config.ts

@@ -41,12 +41,8 @@ export default defineConfig({
     },
   }],
   test: {
+    isolate: false,
     pool: 'forks',
-    poolOptions: {
-      forks: {
-        isolate: false,
-      },
-    },
     include: ['**/*.{test,spec}.{imba,?(c|m)[jt]s?(x)}'],
     setupFiles: ['./setup.js'],
   },