Profiling using --prof inside worker_threads does not work

Generating flamegraphs with worker_thread produces isolate files that do not contain useful information.

Based on https://github.com/mattheworiordan/nodejs-encoding-benchmarks

Running

npm install

You need 0x to generate flamegraphs:

npm i -g 0x

Run using experimental workers:

node --prof --experimental-worker index.js

This will produce two isolate files, one for the main thread and one for the worker thread.

You have to move it to some subfolder, because 0x does some weird clean up after processing a file and will remove the other file. Also 0x accepts files in format isolate-0xNNNNNNNNN-PPPP-v8.log (where P is pid).

Values between isolate-0x and -v8.log will be different every time you run it.

mv isolate-0x105803c00-v8.log /tmp/isolate-0x102803600-0000-v8.log
cd /tmp
0x --visualize-only .
cp flamegraph.html ~/some-folder/thread-1.html
cd ~/some-folder/
mv isolate-0x104800000-v8.log /tmp/isolate-0x102803600-0000-v8.log
cd /tmp
0x --visualize-only .
cp flamegraph.html ~/some-folder/thread-2.html

Run using single threaded version:

node --prof --experimental-worker benchmark.js
mv isolate-0x104800000-v8.log isolate-0x104800000-1111-v8.log
cd 1231.0x
0x --visualize-only .

You should see something similar to this for single threaded version:

single-threaded-node-js-flamegraph

And this for worker_threads:

multi-threaded-node-js-flamegraph-thread-1

multi-threaded-node-js-flamegraph-thread-2

Side note:

What makes me think that this is a bug inside node not 0x is that isolate files and processed files (generated by running node --prof-process) contain pretty much the same info that is on the flame graphs. I used flamegraphs because they are easier for me to read.