I've encountered a CPU issue which is somewhat of a puzzler for me.

Here's a description of what I'm doing: While investigating why an 
application written for Node.js was consuming what seemed to us as 
unreasonable amounts of CPU I ended up discovering that most of the CPU 
usage is actually attributed to framework we're using - Express.
I've validated this by comparing two "Hello World" style applications, both 
in single process mode and when using clustering.
I've implemented this super simple application using vanilla Node and then 
again using express:

Express version:
var express = require('express');
var app = express();

let userCount = 0;
app.get('/', function (req, res) {
  const hello = {msg: "Hello world", userCount: userCount};

app.listen(3000, function () {
  console.log('Example app listening on port 3000!');

Plain Node version
var http = require('http');

let userCount = 0;
var server = http.createServer(function (request, response) {
    response.writeHead(200, {"Content-Type": "application/json"});
    const hello = {msg: "Hello world", userCount: userCount};

console.log("Server running at";);

I've then used wrk2 from a load generator machine (c4.large - 2 CPUs) to 
send a constant load (2k reqs/sec) to the machine (another c4.large) 
running each of these 4 setups.
I was expecting to see higher CPU with express (after all it does more) and 
I was expecting the clustered versions to use somewhat more CPU than the 
single process versions.
What I actually saw was that indeed the express version uses more CPU than 
the plain Node version, but while the extra total CPU used when putting the 
plain Node version in a cluster was a bit higher, the same test for the 
Express version yielding MUCH more CPU.

Here are the grand totals in all cases this includes both CPUs:

   - single process - plain Node version - total 12.95%
   - 2 processes using Node's cluster module - plain Node version - total 
   - single process - Express version - total 19.05%
   - 2 processes using Node's cluster module - Express version - total 

So, the results for the plain Node version are consistent with what I was 
expecting, but the results for the Express version are not.
As an initial step, I'd like to understand why does putting the same code 
inside a cluster perform so much worse when using express.
So I'm wondering regarding two things:

   1. Are my assumption even correct? Perhaps with increased processing it 
   IS actually expected to see such deterioration when moving from single to 
   2. If my assumption is correct, which tools (if any...) can I use to 
   diagnose this so I'll know what to look for when I dive into the code. My 
   initial intuition was increased context switches, but looking at vmstat 
   output I could not find proof for that.

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