benchmark: add vfs module-graph cold-load benchmark

Measures loading a large CJS or ESM module graph from a mounted VFS,
relying on the unmount cache purge so every iteration is a cold load.

Author: mcollina

benchmark/vfs/module-graph.js

@@ -0,0 +1,62 @@
+'use strict';
+const path = require('path');
+const { pathToFileURL } = require('url');
+const common = require('../common.js');
+
+const bench = common.createBenchmark(main, {
+  type: ['cjs', 'esm'],
+  files: [1e2, 1e3],
+  n: [10],
+}, { flags: ['--experimental-vfs', '--no-warnings'] });
+
+// Builds a module graph of `files` packages, each with its own package.json,
+// an index requiring a package-local file and a shared root module, and an
+// entry point that pulls in every package.
+function buildGraph(layer, files, type) {
+  const entryRequires = [];
+  if (type === 'esm') {
+    layer.writeFileSync('/package.json', '{"type":"module"}');
+  }
+  layer.writeFileSync('/shared.js',
+                      type === 'cjs' ? 'module.exports = 0;' : 'export default 0;');
+  for (let i = 0; i < files; i++) {
+    layer.mkdirSync(`/${i}`, { recursive: true });
+    if (type === 'cjs') {
+      layer.writeFileSync(`/${i}/package.json`, '{"main":"index.js"}');
+      layer.writeFileSync(`/${i}/lib.js`, 'module.exports = 1;');
+      layer.writeFileSync(
+        `/${i}/index.js`,
+        'require("./lib.js"); require("../shared.js"); module.exports = __filename;');
+      entryRequires.push(`require('./${i}/');`);
+    } else {
+      layer.writeFileSync(`/${i}/package.json`, '{"type":"module"}');
+      layer.writeFileSync(`/${i}/lib.js`, 'export default 1;');
+      layer.writeFileSync(
+        `/${i}/index.js`,
+        'import "./lib.js"; import "../shared.js"; export default import.meta.url;');
+      entryRequires.push(`import './${i}/index.js';`);
+    }
+  }
+  layer.writeFileSync('/entry.js', entryRequires.join('\n'));
+}
+
+async function main({ n, type, files }) {
+  const vfs = require('node:vfs');
+  const layer = vfs.create();
+  buildGraph(layer, files, type);
+
+  bench.start();
+  for (let i = 0; i < n; i++) {
+    const mountPoint = layer.mount();
+    const entry = path.join(mountPoint, 'entry.js');
+    if (type === 'cjs') {
+      require(entry);
+    } else {
+      await import(pathToFileURL(entry).href);
+    }
+    // Unmounting purges the module caches for the mount prefix, so every
+    // iteration is a cold load of the full graph.
+    layer.unmount();
+  }
+  bench.end(n * files);
+}

test/benchmark/test-benchmark-vfs.js

@@ -0,0 +1,7 @@
+'use strict';
+
+require('../common');
+
+const runBenchmark = require('../common/benchmark');
+
+runBenchmark('vfs');