feat(config): allow percentage value for workers option

packages/vitest/rollup.config.js

@@ -67,6 +67,7 @@ const external = [
   'worker_threads',
   'node:worker_threads',
   'node:fs',
+  'node:os',
   'node:stream',
   'node:vm',
   'inspector',

packages/vitest/src/node/config.ts

@@ -1,3 +1,4 @@
+import { cpus } from 'node:os'
 import { resolveModule } from 'local-pkg'
 import { normalize, relative, resolve } from 'pathe'
 import c from 'picocolors'
@@ -52,6 +53,13 @@ function parseInspector(inspect: string | undefined | boolean | number) {
   return { host, port: Number(port) || defaultInspectPort }
 }
 
+function getWorkersCountByPercentage(percent: string) {
+  const maxWorkersCount = cpus().length
+  const workersCountByPercentage = Math.round((Number.parseInt(percent) / 100) * maxWorkersCount)
+
+  return Math.max(1, Math.min(maxWorkersCount, workersCountByPercentage))
+}
+
 export function resolveApiServerConfig<Options extends ApiConfig & UserConfig>(
   options: Options,
   defaultPort: number,
@@ -176,11 +184,21 @@ export function resolveConfig(
   }
 
   if (resolved.maxWorkers) {
-    resolved.maxWorkers = Number(resolved.maxWorkers)
+    if (typeof options.maxWorkers === 'string' && options.maxWorkers.trim().endsWith('%')) {
+      resolved.maxWorkers = getWorkersCountByPercentage(options.maxWorkers)
+    }
+    else {
+      resolved.maxWorkers = Number(resolved.maxWorkers)
+    }
   }
 
   if (resolved.minWorkers) {
-    resolved.minWorkers = Number(resolved.minWorkers)
+    if (typeof options.minWorkers === 'string' && options.minWorkers.trim().endsWith('%')) {
+      resolved.minWorkers = getWorkersCountByPercentage(options.minWorkers)
+    }
+    else {
+      resolved.minWorkers = Number(resolved.minWorkers)
+    }
   }
 
   resolved.browser ??= {} as any

packages/vitest/src/types/config.ts

@@ -343,13 +343,13 @@ export interface InlineConfig {
   poolOptions?: PoolOptions
 
   /**
-   * Maximum number of workers to run tests in. `poolOptions.{threads,vmThreads}.maxThreads`/`poolOptions.forks.maxForks` has higher priority.
+   * Maximum number or percentage of workers to run tests in. `poolOptions.{threads,vmThreads}.maxThreads`/`poolOptions.forks.maxForks` has higher priority.
    */
-  maxWorkers?: number
+  maxWorkers?: number | string
   /**
-   * Minimum number of workers to run tests in. `poolOptions.{threads,vmThreads}.minThreads`/`poolOptions.forks.minForks` has higher priority.
+   * Minimum number or percentage of workers to run tests in. `poolOptions.{threads,vmThreads}.minThreads`/`poolOptions.forks.minForks` has higher priority.
    */
-  minWorkers?: number
+  minWorkers?: number | string
 
   /**
    * Should all test files run in parallel. Doesn't affect tests running in the same file.
@@ -1009,6 +1009,9 @@ export interface ResolvedConfig
     enabled: boolean
   }
   runner?: string
+
+  maxWorkers: number
+  minWorkers: number
 }
 
 export type ProjectConfig = Omit<

test/config/fixtures/workers-option/vitest.config.js

@@ -0,0 +1 @@
+export default {}

test/config/test/workers-option.test.ts

@@ -0,0 +1,21 @@
+import { type UserConfig, expect, test, vi } from 'vitest'
+
+import * as testUtils from '../../test-utils'
+
+vi.mock('node:os', async importOriginal => ({
+  ...(await importOriginal() as Record<any, any>),
+  cpus: vi.fn(() => Array.from({ length: 10 })),
+}))
+
+function runVitest(config: UserConfig) {
+  return testUtils.runVitest({ ...config, root: './fixtures/workers-option' })
+}
+
+test.each([
+  { option: 'minWorkers' },
+  { option: 'maxWorkers' },
+] as const)('$option=50% should set resolved.$option=5', async ({ option }) => {
+  const { ctx } = await runVitest({ [option]: '50%' })
+
+  expect(ctx?.config[option]).toBe(5)
+})