src: set UV_THREADPOOL_SIZE based on available parallelism

When UV_THREADPOOL_SIZE is not set, Node.js will auto-size it based on
uv_available_parallelism(), with a minimum of 4 and a maximum of 1024.

Author: RafaelGSS

src/node.cc

@@ -1220,6 +1220,24 @@ InitializeOncePerProcessInternal(const std::vector<std::string>& args,
 #endif  // HAVE_OPENSSL
   }
 
+  // Set UV_THREADPOOL_SIZE based on available parallelism if not already set
+  // by the user. The libuv threadpool defaults to 4 threads, which can be
+  // suboptimal on machines with many CPU cores. Use uv_available_parallelism()
+  // as a heuristic, with a minimum of 4 (the previous default) and a maximum
+  // of 1024 (libuv's upper bound).
+  {
+    char buf[64];
+    size_t buf_size = sizeof(buf);
+    int rc = uv_os_getenv("UV_THREADPOOL_SIZE", buf, &buf_size);
+    if (rc == UV_ENOENT) {
+      unsigned int parallelism = uv_available_parallelism();
+      unsigned int threadpool_size = std::min(std::max(4u, parallelism), 1024u);
+      char size_str[16];
+      snprintf(size_str, sizeof(size_str), "%u", threadpool_size);
+      uv_os_setenv("UV_THREADPOOL_SIZE", size_str);
+    }
+  }
+
   if (!(flags & ProcessInitializationFlags::kNoInitializeNodeV8Platform)) {
     uv_thread_setname("node-MainThread");
     per_process::v8_platform.Initialize(

test/parallel/test-uv-threadpool-size-auto.js

@@ -0,0 +1,40 @@
+'use strict';
+const { spawnSyncAndAssert } = require('../common/child_process');
+const assert = require('assert');
+const os = require('os');
+
+const expectedSize = Math.min(Math.max(4, os.availableParallelism()), 1024);
+
+// When UV_THREADPOOL_SIZE is not set, Node.js should auto-size it based on
+// uv_available_parallelism(), with a minimum of 4 and a maximum of 1024.
+{
+  const env = { ...process.env };
+  delete env.UV_THREADPOOL_SIZE;
+
+  spawnSyncAndAssert(
+    process.execPath,
+    ['-e', 'console.log(process.env.UV_THREADPOOL_SIZE)'],
+    { env },
+    {
+      stdout(output) {
+        assert.strictEqual(output.trim(), String(expectedSize));
+      },
+    },
+  );
+}
+
+// When UV_THREADPOOL_SIZE is explicitly set, Node.js should not override it.
+{
+  const env = { ...process.env, UV_THREADPOOL_SIZE: '8' };
+
+  spawnSyncAndAssert(
+    process.execPath,
+    ['-e', 'console.log(process.env.UV_THREADPOOL_SIZE)'],
+    { env },
+    {
+      stdout(output) {
+        assert.strictEqual(output.trim(), '8');
+      },
+    },
+  );
+}