src: set an appropriate thread pool size if given --v8-pool-size=0

doc/api/cli.md

@@ -1570,11 +1570,12 @@ added: v5.10.0
 
 Set V8's thread pool size which will be used to allocate background jobs.
 
-If set to `0` then V8 will choose an appropriate size of the thread pool based
-on the number of online processors.
+If set to `0` then Node.js will choose an appropriate size of the thread pool
+based on an estimate of the amount of parallelism.
 
-If the value provided is larger than V8's maximum, then the largest value
-will be chosen.
+The amount of parallelism refers to the number of computations that can be
+carried out simultaneously in a given machine. In general, it's the same as the
+amount of CPUs, but it may diverge in environments such as VMs or containers.
 
 ### `--watch`
 

src/node_platform.cc

@@ -45,6 +45,13 @@ static void PlatformWorkerThread(void* data) {
   }
 }
 
+static int GetActualThreadPoolSize(int thread_pool_size) {
+  if (thread_pool_size < 1) {
+    thread_pool_size = uv_available_parallelism() - 1;
+  }
+  return std::max(thread_pool_size, 1);
+}
+
 }  // namespace
 
 class WorkerThreadsTaskRunner::DelayedTaskScheduler {
@@ -340,6 +347,8 @@ NodePlatform::NodePlatform(int thread_pool_size,
   // current v8::Platform instance.
   SetTracingController(tracing_controller_);
   DCHECK_EQ(GetTracingController(), tracing_controller_);
+
+  thread_pool_size = GetActualThreadPoolSize(thread_pool_size);
   worker_thread_task_runner_ =
       std::make_shared<WorkerThreadsTaskRunner>(thread_pool_size);
 }

test/parallel/test-v8-flag-pool-size-0.js

@@ -0,0 +1,10 @@
+// Flags: --v8-pool-size=0 --expose-gc
+
+'use strict';
+
+require('../common');
+
+// This verifies that V8 tasks scheduled by GC are handled on worker threads if
+// `--v8-pool-size=0` is given. The worker threads are managed by Node.js'
+// `v8::Platform` implementation.
+globalThis.gc();