jakub-g / nodejs_profiling

What is this

It's a tiny script that allows to log all invocations of node executable when you run a script which runs nested node processes (i.e. when you're calling JS scripts or globally installed modules like npm, eslint, prettier etc. through spawn or exec).

I used it to debug slowness of a precommit hook in our project. It turns out it indirectly invoked node 18 times (!), due to:

• suboptimal code in our codebase and overreliance on npm scripts in package.json as a way of composing tasks
• suboptimal config of pre-commit package
• suboptimal code inside pre-commit package
• suboptimal handling of npm shell calls on Windows by npm

See https://gist.github.com/jakub-g/46c2ef2b9677eaba86837a2160752e42 and npm/cli#3088.

Logging vs writing to files

Naive thing would be to console.log each invocation to stdout, but this does not show all invocations: in case the parent process swallows the stdout/stderr, not all processes will show up.

Hence apart from logging, it also writes to tmp files (one file per node process created).

You can display contents of all written files with cat /path/to/nodejs_profiling/tmp* | tee log.log. (When running things multiple times, remember to clean up those tmp files before each run).

Usage

# NODE_OPTIONS: see https://nodejs.org/api/cli.html#cli_node_options_options
# START_TIME: we create unix timestamp before starting node, to show timestamps relative to it as we execute commands one by one
START_TIME=$(date +%s%N | cut -b1-13) NODE_OPTIONS='--require C:\git\nodejs_profiling\debug_rich.js' node /path/to/somefile.js

Example output

[ 1451] C:\NODE_DIR\node.exe C:\git\myproject\node_modules\pre-commit\index.js
[ 2038] C:\NODE_DIR\node.exe C:\NODE_DIR\node_modules\npm\bin\npm-cli.js prefix -g
[ 2724] C:\NODE_DIR\node.exe C:\NODE_DIR\node_modules\npm\bin\npm-cli.js run precommit-hook --silent