Guide: How To Inspect Memory Usage in Node.js

What you will learn

• How to get information about the memory of a Node.js process
• How Node.js organizes the memory

Requirements

To follow along you need a basic understanding of Javascript and a Node.js installation.

How does Node.js organizes the memory?

A blog post won't be enough to explain all the machinery: you should just be concerned about the heap for now so let's start by breaking down the basics.

The heap is a memory segment used for storing objects, strings and closures. This is where all the magic happens.

The heap is part of something bigger though: a running Node.js process store all its memory inside a Resident Set. You can think of it as of a big box which contains some more boxes.

The Resident Set contains also the actual Javascript code (inside the Code segment) and the Stack, where all the variables live. Hopefully the following illustration will clarify the concept:

Sounds complicated? We need to focus only on the heap for now, so don't worry if you can't grasp all the terminology right now!

Querying the Node.js heap

We know what the Resident Set is and we also know that the Heap is what we're looking for: now we need to find out how much heap is used by a given Node.js process. But how do we access the heap?

process is a global Node.js object which contains information about the current Node.js process, and it provides exactly what we were searching for: the memoryUsage() method.

memoryUsage returns an object with various information: rss, heapTotal, heapUsed, external:

• rss stands for Resident Set Size, it is the total memory allocated for the process execution
• heapTotal is the total size of the allocated heap
• heapUsed is the actual memory used during the execution of our process

. external is, according to the documentation, the memory used by "C++ objects bound to JavaScript objects managed by V8"

Armed with the knowledge we are ready to see some examples.

Testing reverse() memory usage for a small array

In our first example we will see how much memory is used by a very basic method: reverse(). reverse is an array method: it takes an array and reverses its content.

Create a new file named reverse_node.js:

const arr = [1, 2, 3, 4, 5, 6, 9, 7, 8, 9, 10];
arr.reverse();
const used = process.memoryUsage().heapUsed / 1024 / 1024;
console.log(`The script uses approximately ${Math.round(used * 100) / 100} MB`);

Now if you run this script you should see the following output:

This script uses approximately 3.6 MB of memory

In the example above I picked the property heapUsed from process.memoryUsage() but if you run process.memoryUsage() by itself, you will have access to the other properties as listed in the previous section. For instance by running:

const arr = [1, 2, 3, 4, 5, 6, 9, 7, 8, 9, 10];
arr.reverse();
const used = process.memoryUsage();
for (let key in used) {
  console.log(`${key} ${Math.round(used[key] / 1024 / 1024 * 100) / 100} MB`);
}

You’ll see the following output:

rss 25.63 MB
heapTotal 5.49 MB
heapUsed 3.6 MB
external 0.01 MB

Now, this is not the most accurate way for measuring the memory consumption in Node.js but at least you'll get the job done.

There is more to take care about when doing instrumenting, in particular you should account for the Node.js garbage collector. Besides that, with memoryUsage you have at least an indication about how much memory a given script/process takes.

Let's continue with another example!

Testing reverse() memory usage for a big array

How about testing a very big array now?

Create a new file inside your playground directory named reverse_node_bigArray.js:

let arr = Array(1e6).fill("some string");
arr.reverse();
const used = process.memoryUsage().heapUsed / 1024 / 1024;
console.log(`The script uses approximately ${used} MB`);

Array(1e6) creates an empty array with a length property of 1 million. Then the fill() method fills every position of the array with a string. At this point we have an array with 1 million of elements into it.

By running the new script you should see the following output:

This script used approximately 11.23 MB

Things will start to get more interesting if we increase the size of our array. In the script change the following line from:

let arr = Array(1e6).fill("some string");

to

let arr = Array(1e8).fill("some string");

Array(1e8) creates an empty array with a length property of 100 millions. Again, the fill() method fills every position of the array with a string.

At this point we have an array with 100 million of elements into it, and by running the script again you will see how the memory usage increases:

This script uses approximately 1278.83 MB

Now go check your code and see if you can optimize some function!

Where to go from here

We’ve seen how to measure the memory usage of a Node.js process. Keep in mind, for the most common needs the memoryUsage() method will suffice but if you were to investigate a memory leak in an Node.js application you need more.

To learn more about this topic I suggest:

• a tour of v8 garbage collection
• Visualizing memory management in V8 Engine

Thanks for reading and stay tuned!