Using Profiler with AVA

[jcubic]

Is there any way of using a profiler with AVA.

I've tried 0x and clinic. Both return a wall of errors, different depending on what you try.

Example error:

npx 0x -- node ./node_modules/.bin/ava --no-worker-threads tests/test.js
🔥  Profiling
node:net:437
      throw new ErrnoException(err, 'open');
      ^

Error: open EEXIST
    at new Socket (node:net:437:13)
    at Object.<anonymous> (/home/kuba/.config/nvm/versions/node/v24.7.0/lib/node_modules/0x/lib/preload/redir-stdout.js:11:18)
    at Module._compile (node:internal/modules/cjs/loader:1738:14)
    at Object..js (node:internal/modules/cjs/loader:1871:10)
    at Module.load (node:internal/modules/cjs/loader:1470:32)
    at Module._load (node:internal/modules/cjs/loader:1290:12)
    at TracingChannel.traceSync (node:diagnostics_channel:322:14)
    at wrapModuleLoad (node:internal/modules/cjs/loader:238:24)
    at Module.require (node:internal/modules/cjs/loader:1493:12)
    at node:internal/modules/cjs/loader:2024:12 {
  errno: -17,
  code: 'EEXIST',
  syscall: 'open'
}

Node.js v24.7.0
  ✘ tests/test.js exited with a non-zero exit code: 1

this is the code of that file:

'use strict'
const net = require('net')

let isWorker = false
try {
  // Skip redirecting stdout in Worker threads.
  isWorker = !require('worker_threads').isMainThread
} catch (e) {}

if (!isWorker) {
  const socket = new net.Socket({
    fd: 3,
    readable: false,
    writable: true
  })
  Object.defineProperty(process, 'stdout', {
    configurable: true,
    enumerable: true,
    get: () => socket
  })
  Object.defineProperty(process.stdout, 'fd', {
    value: socket._handle.fd
  })
}

[MLuc24]

The file you dug up is the whole story, and the reason it blows up is a collision over file descriptor 3.

AVA copies the parent's node flags into every worker. normalizeNodeArguments ends with

return [...process.execArgv, ...fromConf, ...parsedArgv];

and that array is handed to fork(file, options, apiOptions.nodeArguments), which passes it as execArgv to each worker. So when 0x launches node with its -r .../redir-stdout.js preload, that preload does not stay in the process 0x is profiling — it gets inherited by every test worker AVA starts.

Now put that next to how the workers are created. With --no-worker-threads AVA takes this branch:

worker = childProcess.fork(fileURLToPath(workerPath), options.workerArgv, {
    silent: true,
    serialization: 'advanced',
    ...
});

fork with silent: true gives the child stdio of pipe, pipe, pipe, ipc, so fd 3 in that child is Node's IPC channel — already open, owned by Node. Then the inherited preload runs and does new net.Socket({fd: 3}) to hijack stdout, and you get Error: open EEXIST from exactly the line in your paste.

The guard in that file is the giveaway:

isWorker = !require('worker_threads').isMainThread

It skips worker threads but knows nothing about forked child processes. So --no-worker-threads is the one mode 0x cannot survive, which is presumably why you saw different walls of errors depending on what you tried — you switched AVA into the case the guard does not cover.

Rather than fight that, use the profiler that is already in Node, since it attaches to the process that actually runs your tests instead of the one that merely supervises them:

npx ava --node-arguments="--cpu-prof --cpu-prof-dir=./prof" tests/test.js

That flag travels the same inheritance path, so it lands in the workers where the work happens. It also handles the default threaded mode properly — I checked on Node 24 with a script that spawns one worker, and it writes a separate profile per thread:

CPU.20260821.093314.45744.0.001.cpuprofile
CPU.20260821.093314.45744.1.002.cpuprofile

Drop the one belonging to the test thread into Chrome DevTools or VS Code and you get the same flamegraph 0x would have drawn.

If you would rather keep an external profiler, there is an escape hatch, though a narrower one than it first looks. filterNodeArgumentsForWorkerThreads in your AVA config is applied to that argument list before it reaches the workers, so you can strip 0x's -r preload from them while leaving it on the parent. The catch is the guard around it:

if (workerThreads && 'filterNodeArgumentsForWorkerThreads' in conf) {

so it only takes effect in threaded mode — the --no-worker-threads path you were reaching for is exactly the one it does not cover. And profiling the parent tells you about AVA's scheduling rather than your own code, which is probably not the question you set out to answer.