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~ Understanding Memory Behavior on NodeJS The memory concept in 
 JavaScript engines @raphamundi

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Raphael Amorim This guy seems like a nice person, but he doesn’t. Seriously. He likes topics related to JavaScript, Python, Clojure, WebGL, Algorithms and sometimes force some git push. 
 Working most part of his time in useless globo.com • js foundation/jQuery member • Mozillian • Metido a besta • @raphamundi This guy seems like a he doesn’t. Seriously. related to JavaScript, WebGL, Algorithms and some git push. 
 Working most part of h open source projects. some git push. 
 Working most part of his open source projects.

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talentos.globo.com

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[Question] Would you board a plane programmed by JavaScript?

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[Question] Would you board a plane programmed by JavaScript? 54% Yes 32% No 14% I don’t know

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Motivations (?)

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Developers (not all) don’t considers (or just doesn’t know) how tricky can be memory management in Nodejs.

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INTRO

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Computer memory is any physical device capable of storing information temporarily or permanently.

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Computer memory is any physical device capable of storing information temporarily or permanently.

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the ‘ is 
 from 
 other ‼

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Memory is Limited

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NODEJS

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Initially created for Google Chrome, but it can also be used as a standalone.

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83%* of all INTERPRETERS AREN’T IN JAVASCRIPT *invented number

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NodeJS is a C++ program controlled via V8 *https://github.com/v8/v8/wiki/Getting%20Started%20with%20Embedding

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Dynamic memory allocation

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Dynamic memory allocation Weakly typed

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Dynamic memory allocation Weakly typed "V8's stop-the-world"

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Dynamic memory allocation Weakly typed "V8's stop-the-world" "generational"

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V8: garbage collection

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Every program that consumes memory requires a mechanism for reserving and freeing space.

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In C and C++, this is accomplished by malloc() and free() Every program that consumes memory requires a mechanism for reserving and freeing space.

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char * buffer; buffer = (char*) malloc (42); // Do something with buffer free (buffer);

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char * buffer; buffer = (char*) malloc (42); // Do something with buffer free (buffer); What this means?

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In this case (C++) the programmer is responsible for freeing heap memory that is no longer required.

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Programmers make mistakes.

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/* Allocate 30 bytes to house a string */ char* str = new char [30]; /* Clear those 30 bytes and make str point nowhere */ delete [] str;

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/* Allocate 30 bytes to house a string */ char* str = new char [30]; /* Give str another memory address with the first one gone forever */ str = new char [60]; delete [] str;

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Fail to remove data that can not be referenced.

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When a portion of memory allocated for certain operation is not released when it's no longer needed.

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Fail to remove data that can not be referenced. When a portion of memory allocated for certain operation is not released when it is no longer needed. It’s a 
 Memory Leak

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Garbage collection is double-edged sword

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positive side: languages that use it become more simple.

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negative side: no control of memory.

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ECMAScript specification doesn't expose any interface to garbage collector.

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Performance of garbage collected languages is not strictly better or worse than languages without managed memory!

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V8: memory scheme

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Resident Set ~ V8 uses a scheme similar to the Java Virtual Machine and divides the memory into segments ~

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Resident Set Code Segment Stack Heap Used Heap

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~Code Segment~ The actual code being executed

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[|||||] Stack Variable

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char *buff[500]

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Contains all value types with pointers referencing objects on the heap.

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[|||||] Heap Variable

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char *buff = (char *)malloc(500)

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A memory segment dedicated to storing reference types like objects, strings and closures.

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process.memoryUsage() *https://nodejs.org/api/process.html#process_process_memoryusage

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console.log(process.memoryUsage()); { rss: 4935680, heapTotal: 1826816, heapUsed: 650472, external: 49879 }

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console.log(process.memoryUsage()); { rss: 4935680, heapTotal: 1826816, heapUsed: 650472, external: 49879 } rss refer to resident set size.

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console.log(process.memoryUsage()); { rss: 4935680, heapTotal: 1826816, heapUsed: 650472, external: 49879 } heapTotal and heapUsed refer to V8's memory usage.

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console.log(process.memoryUsage()); { rss: 4935680, heapTotal: 1826816, heapUsed: 650472, external: 49879 } external refers to the memory usage of C++ objects bound to JavaScript objects managed by V8.

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Terms: generationals

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How Memory Allocation works in Nodejs

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// allocation for Number var num = 42; // allocation for String var str = “my string”;

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// allocation for Number var num = 42; // allocation for String var str = “my string”; Rvalue Lvalue

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// each element is a not named reference var arr = [1, null, "ABC"]

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// each element is a not named reference var arr = [1, null, "ABC"] *In-Object slack tracking

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In-Object slack tracking

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// each element is a not named reference var arr = [1, null, "ABC"] *In-Object slack tracking *overflow array

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// callable - executable object function fun(a) { return a + 10 }

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// functions with expressions do object allocation myElement.addEventListener(‘click’, () => {
 myElement.style.opacity = 0
 }, false)

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Cyclic reference

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// cycle allocation var a = {} var b = {} a.c = b b.d = a

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Hightlight: Garbage Collector Sample

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> var myObject = {
 person: { name: "raphael", age: 21, }
 }

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var myObject = {
 person: { name: "raphael", age: 21, }
 } > var anotherObject = myObject

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var myObject = {
 person: { name: "raphael", age: 21, }
 } var anotherObject = myObject > myObject = 1

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> var yetAnotherObject = anotherObject.person person: { name: "raphael", age: 21, }
 } var anotherObject = myObject myObject = 1

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> anotherObject = “some-random-string” var yetAnotherObject = anotherObject.person }
 } var anotherObject = myObject myObject = 1

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> yetAnotherObject = null anotherObject = “some-random-string” var yetAnotherObject = anotherObject.person var anotherObject = myObject myObject = 1

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Hightlight: Building A Leak

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easy, right?

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function LeakingClass() { // do-nothing }

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function LeakingClass() { // do-nothing } var leaks = []

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function LeakingClass() { // do-nothing } var leaks = [] setInterval(function() { for (var i = 0; i < 100; i++) { leaks.push(new LeakingClass) } console.error('Leaks: %d', leaks.length) }, 1000)

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it’s so fun, let’s create another!

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const developer = { name: “amorim”, company: “globocom” } doWork(() => { const data = developer.name // doing a bunch of work })

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one more!

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const http = require(‘http') const server = http.createServer((req, res) => { for (var i=0; i<1000; i++) { server.on('request', function LeakFunc() {}) } res.end(‘Hello, The Conf!’) }).listen(8585, ‘127.0.0.1') server.setMaxListeners(0) console.log(`Server running at 127.0.0.1:8585/. Process PID: ${process.pid}`)

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Hightlight: Debugging A Leak

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let’s get back to the leak examples

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function LeakingClass() { // do-nothing } var leaks = [] setInterval(function() { for (var i = 0; i < 100; i++) { leaks.push(new LeakingClass) } console.error('Leaks: %d', leaks.length) }, 1000) leak.js

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node --inspect leak.js

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

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Hint2: It's important to use named constructors, otherwise the heap snapshots will not produce useful outputs for you.

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const http = require(‘http') const server = http.createServer((req, res) => { for (var i=0; i<1000; i++) { server.on('request', function LeakFunc() {}) } res.end(‘Hello, The Conf!’) }).listen(8585, ‘127.0.0.1') server.setMaxListeners(0) console.log(`Server running at 127.0.0.1:8585/. Process PID: ${process.pid}`) server-leak.js

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npm i memwatch-next or npm i node-memwatch

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› ab -n 1000 -c 100 http://127.0.0.1:8585/

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There’s a lot of tools to help you! - node-memwatch - node-heapdump - node-inspector - nodetime - v8-profiler - memwatch-next - ...

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const developer = { name: “amorim”, company: “globocom” } doWork(() => { const data = developer.name // doing a bunch of work }) closure-leak.js

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const developer = { name: “amorim”, company: “globocom” } doWork(() => { const data = developer.name // doing a bunch of work developer = null })

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Hightlight: Wrap up

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"Even valid code can cause memory leaks."

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theThing = null

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References: - https://blog.codeship.com/understanding-garbage- collection-in-node-js - https://speakerdeck.com/pantuza/memory-leak-em- javascript - https://medium.com/@_lrlna/garbage-collection-in-v8-an- illustrated-guide-d24a952ee3b8 - http://jayconrod.com/posts/55/a-tour-of-v8-garbage- collection - https://github.com/v8/v8/wiki

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References: - https://stackoverflow.com/questions/19550253/memory- leaks-and-closures-in-javascript-when-and-why - https://www.nearform.com/blog/self-detect-memory-leak- node/ - https://github.com/felixge/node-memory-leak-tutorial - https://stackoverflow.com/questions/6261201/how-to- find-memory-leak-in-a-c-code-project

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Thank You @raphamundi