Then....

Meet the Garbage Collector

Before collection

After collection

memory allocation is cheap

collection is expensive

Definitions

resident set size

is the portion of memory occupied by a process that is held in the RAM, this contains the code itself, the stack and the heap

the stack

contains primitive types and references to objects

the heap

stores reference types, like objects, strings or closures

shallow size of an object

the size of memory that is held by the object itself

retained size of an object

the size of the memory that is freed up once the object is deleted along with its' dependent objects

GC in Node.js (v8)

new space

old space

and

New space

• new objects gets allocated here
• small in size (8mb tops)
• objects living here are called Young Generation
• collected using Scavenge collection

Old space

• objects that survived the Young Generation are moved here
• objects living here are called Old Generation
• collected using Mark-Sweep collection
• collection in the Old Space will only commence once it is getting exhausted (default size: 1.4Gb)

Scavenge Compact Runs

Mark Sweep Runs

Tools to find leaks

npm install heapdump

var heapdump = require('heapdump');

heapdump.writeSnapshot(function(err, filename) {  
  console.log('dump written to', filename);
});

$ kill -USR2 <pid>

or

const heapdump = require('heapdump')
const app = require('koa')()
const router = require('koa-router')()

const log = [];

router.get('/', function *(next) {
  log.push(this.headers)
  this.body = {
    status: 'finding a leak'
  }
})

app
  .use(router.routes())
  .use(router.allowedMethods())

app.listen(process.env.PORT || 3000)

Low-level tools

mdb

extensible utility for:

• low-level debugging and editing of the live operating system
•  operating system crash dumps
• user processes
• user process core dumps

gcore

generate a core dump of a running program with process ID pid.

# create core dump of the node process
gcore `pgrep node`

# search for the all the JS Objects on the heap
> ::findjsobjects

# analyze them using
object_id::jsprint 

# find the retainer of the object
object_id::findjsobjects -r  

Configuring Node.js memory management

for the brave ones

node --v8-options

# expose gc extension
--expose_gc

# print one trace line following each garbage collection
--trace_gc

# Enables optimizations which favor memory size over execution speed.
--optimize_for_size

# max size of the old space (in Mbytes)
--max_old_space_size

# grow the new space based on the percentage of survivors
--experimental_new_space_growth_heuristic