fs: add promisified readFile benchmark

add a benchmark for fs.promises.readFile

benchmark/fs/readfile-promises.js

@@ -0,0 +1,63 @@
+// Call fs.readFile over and over again really fast.
+// Then see how many times it got called.
+// Yes, this is a silly benchmark.  Most benchmarks are silly.
+'use strict';
+
+const path = require('path');
+const common = require('../common.js');
+const fs = require('fs');
+const assert = require('assert');
+
+const tmpdir = require('../../test/common/tmpdir');
+tmpdir.refresh();
+const filename = path.resolve(tmpdir.path,
+                              `.removeme-benchmark-garbage-${process.pid}`);
+
+const bench = common.createBenchmark(main, {
+  duration: [5],
+  len: [1024, 16 * 1024 * 1024],
+  concurrent: [1, 10]
+});
+
+function main({ len, duration, concurrent }) {
+  try { fs.unlinkSync(filename); } catch { }
+  let data = Buffer.alloc(len, 'x');
+  fs.writeFileSync(filename, data);
+  data = null;
+
+  let writes = 0;
+  let benchEnded = false;
+  bench.start();
+  setTimeout(() => {
+    benchEnded = true;
+    bench.end(writes);
+    try { fs.unlinkSync(filename); } catch { }
+    process.exit(0);
+  }, duration * 1000);
+
+  function read() {
+    fs.promises.readFile(filename)
+      .then((res) => afterRead(undefined, res))
+      .catch((err) => afterRead(err));
+  }
+
+  function afterRead(er, data) {
+    if (er) {
+      if (er.code === 'ENOENT') {
+        // Only OK if unlinked by the timer from main.
+        assert.ok(benchEnded);
+        return;
+      }
+      throw er;
+    }
+
+    if (data.length !== len)
+      throw new Error('wrong number of bytes returned');
+
+    writes++;
+    if (!benchEnded)
+      read();
+  }
+
+  while (concurrent--) read();
+}