perf: avoid spawning extra workers if no tests will run there

packages/vitest/src/node/pool.ts

@@ -175,11 +175,11 @@ export function createPool(ctx: Vitest): ProcessPool {
     const groupedSpecifications: Record<string, TestSpecification[]> = {}
     const groups = new Set<number>()
 
-    const factories: Record<LocalPool, () => ProcessPool> = {
-      vmThreads: () => createVmThreadsPool(ctx, options),
-      vmForks: () => createVmForksPool(ctx, options),
-      threads: () => createThreadsPool(ctx, options),
-      forks: () => createForksPool(ctx, options),
+    const factories: Record<LocalPool, (specs: TestSpecification[]) => ProcessPool> = {
+      vmThreads: specs => createVmThreadsPool(ctx, options, specs),
+      vmForks: specs => createVmForksPool(ctx, options, specs),
+      threads: specs => createThreadsPool(ctx, options, specs),
+      forks: specs => createForksPool(ctx, options, specs),
       typescript: () => createTypecheckPool(ctx),
     }
 
@@ -241,7 +241,7 @@ export function createPool(ctx: Vitest): ProcessPool {
 
           if (pool in factories) {
             const factory = factories[pool]
-            pools[pool] ??= factory()
+            pools[pool] ??= factory(specs)
             return pools[pool]![method](specs, invalidate)
           }
 

packages/vitest/src/node/pools/forks.ts

@@ -5,6 +5,7 @@ import type { ContextRPC, ContextTestEnvironment } from '../../types/worker'
 import type { Vitest } from '../core'
 import type { PoolProcessOptions, ProcessPool, RunWithFiles } from '../pool'
 import type { TestProject } from '../project'
+import type { TestSpecification } from '../spec'
 import type { SerializedConfig } from '../types/config'
 import EventEmitter from 'node:events'
 import * as nodeos from 'node:os'
@@ -65,6 +66,7 @@ function createChildProcessChannel(project: TestProject, collect = false) {
 export function createForksPool(
   vitest: Vitest,
   { execArgv, env }: PoolProcessOptions,
+  specifications: TestSpecification[],
 ): ProcessPool {
   const numCpus
     = typeof nodeos.availableParallelism === 'function'
@@ -75,12 +77,16 @@ export function createForksPool(
     ? Math.max(Math.floor(numCpus / 2), 1)
     : Math.max(numCpus - 1, 1)
 
+  const recommendedCount = vitest.config.watch
+    ? threadsCount
+    : Math.min(threadsCount, specifications.length)
+
   const poolOptions = vitest.config.poolOptions?.forks ?? {}
 
   const maxThreads
-    = poolOptions.maxForks ?? vitest.config.maxWorkers ?? threadsCount
+    = poolOptions.maxForks ?? vitest.config.maxWorkers ?? recommendedCount
   const minThreads
-    = poolOptions.minForks ?? vitest.config.minWorkers ?? Math.min(threadsCount, maxThreads)
+    = poolOptions.minForks ?? vitest.config.minWorkers ?? Math.min(recommendedCount, maxThreads)
 
   const worker = resolve(vitest.distPath, 'workers/forks.js')
 

packages/vitest/src/node/pools/threads.ts

@@ -5,6 +5,7 @@ import type { ContextTestEnvironment } from '../../types/worker'
 import type { Vitest } from '../core'
 import type { PoolProcessOptions, ProcessPool, RunWithFiles } from '../pool'
 import type { TestProject } from '../project'
+import type { TestSpecification } from '../spec'
 import type { SerializedConfig } from '../types/config'
 import type { WorkerContext } from '../types/worker'
 import * as nodeos from 'node:os'
@@ -46,6 +47,7 @@ function createWorkerChannel(project: TestProject, collect: boolean) {
 export function createThreadsPool(
   vitest: Vitest,
   { execArgv, env }: PoolProcessOptions,
+  specifications: TestSpecification[],
 ): ProcessPool {
   const numCpus
     = typeof nodeos.availableParallelism === 'function'
@@ -56,12 +58,16 @@ export function createThreadsPool(
     ? Math.max(Math.floor(numCpus / 2), 1)
     : Math.max(numCpus - 1, 1)
 
+  const recommendedCount = vitest.config.watch
+    ? threadsCount
+    : Math.min(threadsCount, specifications.length)
+
   const poolOptions = vitest.config.poolOptions?.threads ?? {}
 
   const maxThreads
-    = poolOptions.maxThreads ?? vitest.config.maxWorkers ?? threadsCount
+    = poolOptions.maxThreads ?? vitest.config.maxWorkers ?? recommendedCount
   const minThreads
-    = poolOptions.minThreads ?? vitest.config.minWorkers ?? Math.min(threadsCount, maxThreads)
+    = poolOptions.minThreads ?? vitest.config.minWorkers ?? Math.min(recommendedCount, maxThreads)
 
   const worker = resolve(vitest.distPath, 'workers/threads.js')
 

packages/vitest/src/node/pools/vmForks.ts

@@ -5,6 +5,7 @@ import type { ContextRPC, ContextTestEnvironment } from '../../types/worker'
 import type { Vitest } from '../core'
 import type { PoolProcessOptions, ProcessPool, RunWithFiles } from '../pool'
 import type { TestProject } from '../project'
+import type { TestSpecification } from '../spec'
 import type { ResolvedConfig, SerializedConfig } from '../types/config'
 import EventEmitter from 'node:events'
 import * as nodeos from 'node:os'
@@ -72,6 +73,7 @@ function createChildProcessChannel(project: TestProject, collect: boolean) {
 export function createVmForksPool(
   vitest: Vitest,
   { execArgv, env }: PoolProcessOptions,
+  specifications: TestSpecification[],
 ): ProcessPool {
   const numCpus
     = typeof nodeos.availableParallelism === 'function'
@@ -82,12 +84,16 @@ export function createVmForksPool(
     ? Math.max(Math.floor(numCpus / 2), 1)
     : Math.max(numCpus - 1, 1)
 
+  const recommendedCount = vitest.config.watch
+    ? threadsCount
+    : Math.min(threadsCount, specifications.length)
+
   const poolOptions = vitest.config.poolOptions?.vmForks ?? {}
 
   const maxThreads
-    = poolOptions.maxForks ?? vitest.config.maxWorkers ?? threadsCount
+    = poolOptions.maxForks ?? vitest.config.maxWorkers ?? recommendedCount
   const minThreads
-    = poolOptions.maxForks ?? vitest.config.minWorkers ?? Math.min(threadsCount, maxThreads)
+    = poolOptions.maxForks ?? vitest.config.minWorkers ?? Math.min(recommendedCount, maxThreads)
 
   const worker = resolve(vitest.distPath, 'workers/vmForks.js')
 

packages/vitest/src/node/pools/vmThreads.ts

@@ -5,6 +5,7 @@ import type { ContextTestEnvironment } from '../../types/worker'
 import type { Vitest } from '../core'
 import type { PoolProcessOptions, ProcessPool, RunWithFiles } from '../pool'
 import type { TestProject } from '../project'
+import type { TestSpecification } from '../spec'
 import type { ResolvedConfig, SerializedConfig } from '../types/config'
 import type { WorkerContext } from '../types/worker'
 import * as nodeos from 'node:os'
@@ -49,6 +50,7 @@ function createWorkerChannel(project: TestProject, collect: boolean) {
 export function createVmThreadsPool(
   vitest: Vitest,
   { execArgv, env }: PoolProcessOptions,
+  specifications: TestSpecification[],
 ): ProcessPool {
   const numCpus
     = typeof nodeos.availableParallelism === 'function'
@@ -59,12 +61,16 @@ export function createVmThreadsPool(
     ? Math.max(Math.floor(numCpus / 2), 1)
     : Math.max(numCpus - 1, 1)
 
+  const recommendedCount = vitest.config.watch
+    ? threadsCount
+    : Math.min(threadsCount, specifications.length)
+
   const poolOptions = vitest.config.poolOptions?.vmThreads ?? {}
 
   const maxThreads
-    = poolOptions.maxThreads ?? vitest.config.maxWorkers ?? threadsCount
+    = poolOptions.maxThreads ?? vitest.config.maxWorkers ?? recommendedCount
   const minThreads
-    = poolOptions.minThreads ?? vitest.config.minWorkers ?? Math.min(threadsCount, maxThreads)
+    = poolOptions.minThreads ?? vitest.config.minWorkers ?? Math.min(recommendedCount, maxThreads)
 
   const worker = resolve(vitest.distPath, 'workers/vmThreads.js')