Debugging memory leaks in node.js

This is a quick tutorial for debugging memory leaks in node.js.

Step 1: Install the debugging tools

First of all, you need to install the v8-profiler module. Without that required from within your app, you will not be able to take heap snapshots in the debugger.

mkdir node_modules
npm install v8-profiler

Once you have that installed, you need to install the node inspector utility by doing:

npm install -g node-inspector

Step 2: Create some leaking code

In order to debug a memory leak, we first need to create one. This is as easy as adding a bunch of objects to an array every 1s inside a file called leak.js (also included in this repo).

require('v8-profiler');

// It is important to use named constructors (like the one below), otherwise
// the heap snapshots will not produce useful outputs for you.
function LeakingClass() {
}

var leaks = [];
setInterval(function() {
  for (var i = 0; i < 100; i++) {
    leaks.push(new LeakingClass);
  }

  console.error('Leaks: %d', leaks.length);
}, 1000);

Step 3: Debug the memory leak

With all the above in place, you can now debug your memory leak by running the leak.js file in debug mode:

node --debug leak.js

Note: You can also send the SIGUSR1 signal to an existing node process to enable the debug mode as you need it.

Now that your app is running (and leaking memory), start the node inspector in a new shell/tab to debug the memory leak:

node-inspector

In order to use the inspector, use Google Chrome to open up the url of the debugger which should be: http://0.0.0.0:8080/debug?port=5858.

You should see a typical Chrome Developer Tools window:

• Select "Profiles" from the tabs on top
• Hit the "Enable Profiling" button
• Now click the eye icon on the bottom left to take a heap snapshot
• Wait a few seconds
• Click the eye icon again to take another heapsnapshot
• Click the "Count +/-" column on top to sort by it.

You should now see something like this:

In our case this lets you see that there have been 2200 new LeakingClass objects created since the last snapshot, and that there are 29000 of them total in the heap at this point.

Note: In order to find out which objects prevent your LeakingClass instances from being garbage collected, you can use this v8-profiler fork to explore the graph of retainers (screenshot).

Update: You can also try out Nodetime's memory profiler. It uses V8's heap profiler to take snapshots and show the heap as seen from the retainers, e.g. variables and properties. Installing Nodetime is just two simple steps: 1) npm install nodetime and 2) require('nodetime').profile() (before other require statements). It will then print out a link for accessing the profiler's web console at nodetime.com. More on memory profiling with Nodetime in the blog post Detecting Memory Leaks in Node.js Applications (with screenshot).

That's it, you have learned the basics of tracking down memory leaks in node.js.