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doc: add diagnosis memory leak guide #4590

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doc: add diagnosis memory leak guide
tony-go May 5, 2022
d34dedc
chore: fix linter
tony-go May 5, 2022
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Merge branch 'main' into mem-leak-guide
tony-go May 5, 2022
aba890b
Merge branch 'main' into mem-leak-guide
May 8, 2022
396170d
Merge branch 'main' into mem-leak-guide
tony-go May 10, 2022
785b42c
fix: use md ordered list properly
tony-go May 10, 2022
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Merge branch 'main' into mem-leak-guide
tony-go May 18, 2022
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Merge branch 'main' into mem-leak-guide
tony-go May 30, 2022
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chore: fix linter
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tony-go committed May 5, 2022
commit d34dedca9a22e3f0b6b84fec8610a9a37e7d101a
55 changes: 30 additions & 25 deletions locale/en/docs/guides/diagnosis-memory-leak.md
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Expand Up @@ -13,13 +13,13 @@ For the proposal of this guide, we will use a simple web server empowered with [
1 - Create a new project
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```bash
$ npm init -y
npm init -y
```

2 - Install dependencies

```bash
$ npm i fastify
npm i fastify
```

3 - Create a simple server (server.js)
Expand All @@ -34,7 +34,12 @@ server.get('/write', () => {
const date = new Date().toString();

// don't do this at home
entries.add({ date, arch: os.arch(), platform: os.platform(), cpus: os.cpus() });
entries.add({
date,
arch: os.arch(),
platform: os.platform(),
cpus: os.cpus()
});

return true;
});
Expand All @@ -58,7 +63,7 @@ server.listen(9999, (err, address) => {
4 - Run the application server

```bash
$ node --trace-gc server.js
node --trace-gc server.js
```

It should output something like:
Expand Down Expand Up @@ -131,8 +136,8 @@ The `--trace-gc` flag outputs all garbage collection events in the console. Firs
</table>

We'll only focus on two events here:
- Scavenge
- Mark-sweep
* Scavenge
* Mark-sweep

The heap is divided into "spaces." Amongst these, we have a space called the "new" space and another one called the "old" space.

Expand Down Expand Up @@ -183,19 +188,19 @@ This algorithm is composed of two phases:

Now we can come back to the output of the `--trace-gc` flag and see how we could interpret the console's output.

- First, install (`autocannon`)[https://www.npmjs.com/package/autocannon]:
* First, install (`autocannon`)[https://www.npmjs.com/package/autocannon]:
```bash
$ npm i -g autocannon
npm i -g autocannon
```

- Then, restart the server:
* Then, restart the server:
```bash
$ node --trace-gc server.js
node --trace-gc server.js
```

- Open a new terminal and run the following command:
* Open a new terminal and run the following command:
```bash
$ autocannon http://localhost:9999/write
autocannon http://localhost:9999/write
```

Now, if you come back quickly to the previous terminal window: you'll see that there are a lot of `Mark-sweep` events in the console. We also see that the amount of memory collected after the event is insignificant.
Expand All @@ -213,7 +218,7 @@ An excellent way to inspect memory is to make heap dumps and compare them. So le
import v8 from 'v8';

// then add:
server.get('/heap-dump/:id', (req) => {
server.get('/heap-dump/:id', (req) => {
const id = req.params.id;
return v8.writeHeapSnapshot(`heap-dump-${id}.heapsnapshot`);
});
Expand All @@ -224,27 +229,27 @@ We exposed an endpoint that will perform a heap dump for us. We'll use it to gen
1. It's time to rerun our server (without any flag this time):

```bash
$ node server.js
node server.js
```

2. Open a new terminal, and generate a first dump:

```bash
$ curl http://localhost:9999/heap-dump/start
curl http://localhost:9999/heap-dump/start
```

You should see a `heap-dump-first.heapsnapshot` file in your current directory.

3. Now run the test command:

```bash
$ autocannon http://localhost:9999/write
autocannon http://localhost:9999/write
```

4. Once it is finished, generate a second dump:

```bash
$ curl http://localhost:9999/heap-dump/end
curl http://localhost:9999/heap-dump/end
```

You should see a `heap-dump-end.heapsnapshot` file in your current directory.
Expand All @@ -271,11 +276,11 @@ You should see a `heap-dump-end.heapsnapshot` file in your current directory.
<img style="text-align:center" src="../../../../static/guides/diagnosis-memory-leak/setup-cdt-3.png" alt="setup-cdt-3" width="400" />
</div>

* Click on the "end" snapshot and compare them:
- Click on "Objects allocated between heap-dump-start and heap-dump-end - It will help us see what's happened between our states (start and end).
- Sort object by "Retained size" (size freed if the object and dependents ones are deleted)
- Then you can expand the first line recursively until you find the most significant object
- Once you find it, click on it, and you'll be able to find the name in the description below
* Click on the "end" snapshot and compare them:
* Click on "Objects allocated between heap-dump-start and heap-dump-end - It will help us see what's happened between our states (start and end).
* Sort object by "Retained size" (size freed if the object and dependents ones are deleted)
* Then you can expand the first line recursively until you find the most significant object
* Once you find it, click on it, and you'll be able to find the name in the description below

<div align="center">
<img style="text-align:center" src="../../../../static/guides/diagnosis-memory-leak/setup-cdt-4.gif" alt="setup-cdt-4" />
Expand All @@ -295,6 +300,6 @@ What do you think about fixing this leak, making a new snapshot, and observing t
Hope you appreciate playing with diagnostic tools. In case, did you know that Node.Js documentation contains a list of tools for diagnostics purposes? Feel free to [check it](https://github.com/nodejs/node/blob/master/doc/contributing/diagnostic-tooling-support-tiers.md) out.

## References:
- https://v8.dev/blog/trash-talk
- https://github.com/thlorenz/v8-perf/blob/master/gc.md
- https://developer.chrome.com/docs/devtools/memory-problems/memory-101/
* https://v8.dev/blog/trash-talk
* https://github.com/thlorenz/v8-perf/blob/master/gc.md
* https://developer.chrome.com/docs/devtools/memory-problems/memory-101/