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2. Diagnostics (Basics, Debugging, Performance) 5%

Performance profiling with perf and 0x

perf

  • apt-get install linux-tools-common linux-tools-generic linux-tools-$(uname -r)
  • node --perf_basic_proof_only_functions server.js
  • perf record -F 99 -p <pid> -g -- sleep 30 - take 99 stack trace snapshots every second during 30 seconds
  • perf script > stacks.out

0x

  • npm install 0x -g
  • 0x -c gen stacks.out > flamegraph.html

Coredump generation on Uncaught exception with llnode inspecting

  1. In order to generate coredump
ulimit -c unlimited # to allow coredump generation
node --abort-on-uncaught-exception server.js # it will generate coredump on uncaught exception

Coredump will be created either in /cores folder or in current folder.

  1. Install llnode
apt search lldb # find available versions
# in my case I installed 6.0 version
sudo apt install lldb-6.0 python-lldb-6.0 liblldb-6.0-dev
sudo ln -s /usr/bin/lldb-6.0 /usr/bin/lldb
npm i -g llnode
  1. Inspect coredump
llnode -c <path to coredump file>

for example

llnode -c core # open coredump file from current folder

then it will open llnode terminal ([q] + [Enter] to exit)

v8 help #show all commands

More information about how to work with llnode here -> https://developer.ibm.com/node/2016/08/15/exploring-node-js-core-dumps-using-the-llnode-plugin-for-lldb/

Memory leaks inspection with heapdump

App

npm install --save heapdump

require('heapdump')

Production server

kill -USR2 <pid>

Google Chrome DevTools

profiles -> Load (2 dumps) -> Comparison

Common causes of memory leaks:

  1. Frequent garbage collections Scavenge GC in New Object Space which loads CPU:

    • Expensive operations of clone and merge of objects
    • Frequent objects creation

  2. Old Object Space

    • Closures
    • Timers

  3. Large Object Space

    • JSON stringify and parse
    • strings concatenation

Debugging via chrome devtools

node --inspect server.js

or if you want to break on the first line of the application code

node --inspect-brk server.js

But I'm personally a big proponent of https://github.com/GoogleChromeLabs/ndb