doc: Add a README for benchmark tests

The README.md documents how to run node core benchmark tests and
how to write new tests.

benchmark/README.md

@@ -0,0 +1,115 @@
+# Node.js core benchmark tests
+
+This folder contains benchmark tests to measure the performance for certain
+Node.js APIs.
+
+## How to run tests
+
+There are two ways to run benchmark tests:
+
+1. Run all tests of a given type, for example, buffers
+
+```sh
+node benchmark/common.js buffers
+```
+
+The above command will find all scripts under `buffers` directory and require
+each of them as a module. When a test script is required, it creates an instance
+of `Benchmark` (a class defined in common.js). In the next tick, the `Benchmark`
+constructor iterates through the configuration object property values and run
+the test function with each of the combined arguments in spawned processes. For
+example, buffers/buffer-read.js has the following configuration:
+
+```js
+var bench = common.createBenchmark(main, {
+    noAssert: [false, true],
+    buffer: ['fast', 'slow'],
+    type: ['UInt8', 'UInt16LE', 'UInt16BE',
+        'UInt32LE', 'UInt32BE',
+        'Int8', 'Int16LE', 'Int16BE',
+        'Int32LE', 'Int32BE',
+        'FloatLE', 'FloatBE',
+        'DoubleLE', 'DoubleBE'],
+        millions: [1]
+});
+```
+The runner takes one item from each of the property array value to build a list
+of arguments to run the main function. The main function will receive the conf
+object as follows:
+
+- first run:
+```js
+    {   noAssert: false,
+        buffer: 'fast',
+        type: 'UInt8',
+        millions: 1
+    }
+```
+- second run:
+```js
+    {
+        noAssert: false,
+        buffer: 'fast',
+        type: 'UInt16LE',
+        millions: 1
+    }
+```
+...
+
+In this case, the main function will run 2*2*14*1 = 56 times. The console output
+looks like the following:
+
+```
+buffers//buffer-read.js
+buffers/buffer-read.js noAssert=false buffer=fast type=UInt8 millions=1: 271.83
+buffers/buffer-read.js noAssert=false buffer=fast type=UInt16LE millions=1: 239.43
+buffers/buffer-read.js noAssert=false buffer=fast type=UInt16BE millions=1: 244.57
+...
+```
+
+2. Run an individual test, for example, buffer-slice.js
+
+```sh
+node benchmark/buffers/buffer-read.js
+```
+The output:
+```
+buffers/buffer-read.js noAssert=false buffer=fast type=UInt8 millions=1: 246.79
+buffers/buffer-read.js noAssert=false buffer=fast type=UInt16LE millions=1: 240.11
+buffers/buffer-read.js noAssert=false buffer=fast type=UInt16BE millions=1: 245.91
+...
+```
+
+## How to write a benchmark test
+
+The benchmark tests are grouped by types. Each type corresponds to a subdirectory,
+such as `arrays`, `buffers`, or `fs`.
+
+Let's add a benchmark test for Buffer.slice function. We first create a file
+buffers/buffer-slice.js.
+
+### The code snippet
+
+```js
+var common = require('../common.js'); // Load the test runner
+
+var SlowBuffer = require('buffer').SlowBuffer;
+
+// Create a benchmark test for function `main` and the configuration variants
+var bench = common.createBenchmark(main, {
+  type: ['fast', 'slow'], // Two types of buffer
+  n: [512] // Number of times (each unit is 1024) to call the slice API
+});
+
+function main(conf) {
+  // Read the parameters from the configuration
+  var n = +conf.n;
+  var b = conf.type === 'fast' ? buf : slowBuf;
+  bench.start(); // Start benchmarking
+  for (var i = 0; i < n * 1024; i++) {
+    // Add your test here
+    b.slice(10, 256);
+  }
+  bench.end(n); // End benchmarking
+}
+```