Demystifying Memory Leaks in JavaScript

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Today it is possible to write complex applications with only a handful of developers in a short time frame relying upon Frameworks and tooling. But what happens when the application crashes with out of memory errors? How is it possible to guarantee fast response times? These problems are still considered difficult to solve.

Let's dive into the Node.js internals and learn about profilers and the garbage collector. Understanding how the system works in depth let's you write code that is safer, faster and less error prone.

Let's make sure you always provide the best experience for everyone. Yourself and your customer. Let us find that memory leak and fix it.

Learnings

  •         Participants understand the Node.js memory handling and their shortcomings.
  •         Participants know when to profile their application to identify memory leaks and slow code.
  •         Participants are able to find and address most memory leaks.

Ruben Bridgewater
Ruben Bridgewater
33 min
24 Jun, 2021

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Video Summary and Transcription

The Talk discusses demystifying memory leaks in JavaScript, covering topics such as memory allocation, typical memory leaks and issues, handling file descriptors and event listeners, tools and techniques for identifying memory leaks, fixing memory leaks and restarting applications, and Ruben's personal experience with memory leaks.

1. Demystifying Memory Leaks in JavaScript

Short description:

Hello, everyone. My name is Ruden Britschwurter. I'm a NodeJS TC member. I work as a principal software architect at MyConvolt, and I'm happy to be here today at the Node conference. Today I'm going to talk about demystifying memory leaks. So memory leaks are often considered something difficult and hard to solve. But is that actually so? And how can we ease the process? Memory leak is when a computer program incorrectly manages random access memory. In a way that memory, which is no longer needed, is not released. And the blue line is clearly a memory leak because over time you allocate just more and more memory without actually freeing it again. And this is bad. So how do we handle memory in JavaScript in particular? Because we do not have to worry about it, right? This is all done transparently and this is perfect. And memory is just freed. Why should there be a memory leak in the first place?

Hello, everyone. My name is Ruden Britschwurter. I'm a NodeJS TC member. I work as a principal software architect at MyConvolt, and I'm happy to be here today at the Node conference.

And today I'm going to talk about demystifying memory leaks. So memory leaks are often considered something difficult and hard to solve. But is that actually so? And how can we ease the process?

To get to that, I would first like to answer the question what a memory leak actually is. And I'm consulting Wikipedia to answer that question. So memory leak is when a computer program incorrectly manages random access memory. In a way that memory, which is no longer needed, is not released. As such, we are going to stack up more and more memory over time. And the program, in its worst case, might just crash because there is no more memory to allocate. And that's really the worst-case scenario. Or maybe you're in a cloud environment and you're going to have to pay much more money because you have an auto-scaling active and more and more memory is allocated there.

So here we have a graph which clearly shows you how memory leak would look like in comparison to the memory usage and the time the program runs. And the yellow line is a perfect program pretty much. It starts up and you allocate some memory and then there are some ups and downs and this is perfect for each program pretty much. Sometimes spikes might be higher or going a little bit down again, but on average it's the line, the flat line. And the blue line is clearly a memory leak because over time you allocate just more and more memory without actually freeing it again. And this is bad.

So how do we handle memory in JavaScript in particular? Because we do not have to worry about it, right? This is all done transparently and this is perfect. And memory is just freed. Why should there be a memory leak in the first place? So memory is divided into a stack memory and a heap memory. The heap memory is a dynamically allocated memory and the stack is done by the operating system in a typical case. Each thread has some stack memory. It is the last in-first-out algorithm, also called lethal. It's very simple, it's super fast and the memory that is allocated on the stack is automatically reclaimed as soon as the function exits. When we compare that with the heap memory, there is a lot of things going on because the stack memory normally only contains pointers to the function that is currently running. And if you push onto the stack, the currently running pointer, then we get to the heap, which is the dynamically allocated memory. And here we have a nice overview from V8 that is done for the Times of India.

2. Memory Allocation and Garbage Collection

Short description:

There is a lot of things going on. JSObjects are allocated, we have JavaScript source code, optimized code, we have regular expression code, strings, and much, much more. The heap memory, in V8 in particular, is divided into three areas. We have the young generation, intermediate generation, and old generation. The garbage collector is responsible for automatically freeing the memory we allocate, but it can sometimes go haywire.

There is a lot of things going on. JSObjects are allocated, we have JavaScript source code, optimized code, we have regular expression code, strings, and much, much more. So, this is the main memory for our program in this case.

And the heap memory, in V8 in particular, is divided into three areas. Again, we have the young generation. So, as soon as we allocate a new variable, let's say we say let the pool is the string test, then you are going to allocate the memory test and it's going to be put into the young generation. This memory is relatively small. Mostly JavaScript will have intermediate variables that you use to compute the next value immediately. And you do not need the variable as soon as you compute your next value. This is all done synchronously. So, we want to dispose all of that memory that we do not need as soon as possible. So, this young generation will only survive one so-called scavenge run. This is the first run where our program to create a memory is going to try to get rid of those not-anymore-used variables. And if any variable survives that run, it is then pushed into the intermediate generation. And if it survives the second run, then it's pushed into the old generation. This is the bigger part of the application. It's normally used for long and used variables. So, things that you reuse in each application that might hold pointers to a lot of things. And we use a different algorithm to free the memory in this case. And the algorithm used there is called MarkSweep. We start off from our root object. The root object in a browser would be the window object, and in Node.js, it's global. While in modern JavaScript, it would be in both, just global disk. And we use a somewhat like, similar to a recursive algorithm where we start off with the root object and we just connect each dot. We try to connect each node that is somewhat connected to their root object. All other nodes, all other variables, or allocations are going to be freed. And this should be mostly ideal to free all the memory that is not used.

So, I already spoke about that we as a developer do not have to worry about the memory that we allocate because it's going to be automatically freed. What is done in the background is a so-called garbage collector is run and that garbage collector has a lot to do and at some point it might actually even go haywire and not, does not work as we anticipated it would be. So we have to look into that a little bit closer.

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