how to debug node.js causing 100% cpu usage?

Question

I have a node app that uses express and redis. On our development server, after a bit of use node starts to use 100% cpu. The application still responds but top reports node using 100%. The cpu doesn't drop until node is restarted.

I have not nailed it down to any particular route or function that is causing it.

What is the best way to diagnose this problem?

I looked at node-inspector with the v8-profiler and it gave me the same error that is reported here https://github.com/dannycoates/v8-profiler/issues/10

Answer 1

You can profile your app with node-tick.

1. Install node-tick by sudo npm -g install tick
2. Run your app with enabled profile node --prof ./app.js
3. After some time with CPU 100% usage stop your app
4. You can see v8.log in your app directory, now you can read it with node-tick-processor
5. Run node-tick-processor and explain results
6. Load v8.log into chrome://tracing to analyse as tree.

node js cpu 100%

Answer 2

I found the problem by writing a script to record every request and then replay them.

The problem was caused because I had a callback that was not being returned.

myAsncFunc(function(err, data) {      if (err) { callback(err) }      //node kept going after the error was returned to the user.     // make sure you, return callback(err)  }) 

Here was my replay.js code for anyone interested.

var request = require('request'); var async = require('async'); var redis = require('redis');   var host = 'http://myhost.com'; var jobs = true;  var client = redis.createClient();  async.whilst(     function () { return jobs; },     function (callback) {         client.lpop('history', function(err, url) {             console.log(url);             if (!url) {                 jobs = false;                 callback();             }             request.get({url:host+url}, function() {                 callback();             });         })     },     function (err) {         console.log('done')     } ); 

And in you're express app.

app.get('/*', function(req, res, next) {     var url = req.originalUrl;     redis.rpush('history', url);        next(); }); 

It's cool because every history item that is played will be added again to the queue so it continually loops and every time you visit a new page, it will add that one to the queue.