benchmark: cleanup child_process IPC benchmark

Squashed from:
- child_process: fix IPC bench to obey send() ret val
- child_process: fix IPC benchmark message has two more bytes
- child_process: use setImmediate for IPC bench

PR-URL: https://github.com/nodejs/node/pull/10557
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

benchmark/child_process/child-process-read-ipc.js

@@ -1,14 +1,20 @@
 'use strict';
 if (process.argv[2] === 'child') {
   const len = +process.argv[3];
-  const msg = `"${'.'.repeat(len)}"`;
-  while (true) {
-    process.send(msg);
-  }
+  const msg = '.'.repeat(len);
+  const send = () => {
+    while (process.send(msg));
+    // Wait: backlog of unsent messages exceeds threshold
+    setImmediate(send);
+  };
+  send();
 } else {
   const common = require('../common.js');
   const bench = common.createBenchmark(main, {
-    len: [64, 256, 1024, 4096, 32768],
+    len: [
+      64, 256, 1024, 4096, 16384, 65536,
+      65536 << 4, 65536 << 8
+    ],
     dur: [5]
   });
   const spawn = require('child_process').spawn;
@@ -18,7 +24,7 @@ if (process.argv[2] === 'child') {
     const dur = +conf.dur;
     const len = +conf.len;
 
-    const options = { 'stdio': ['ignore', 'ignore', 'ignore', 'ipc'] };
+    const options = { 'stdio': ['ignore', 1, 2, 'ipc'] };
     const child = spawn(process.argv[0],
       [process.argv[1], 'child', len], options);
 
@@ -29,8 +35,7 @@ if (process.argv[2] === 'child') {
 
     setTimeout(function() {
       child.kill();
-      const gbits = (bytes * 8) / (1024 * 1024 * 1024);
-      bench.end(gbits);
+      bench.end(bytes);
     }, dur * 1000);
   }
 }