Worker threads

[carera]

Hi,
thanks for the lib, next allows us to profile node12 apps!
One question - I noticed job processed on worker_threads is not shown in the profiles. Do you know a way how to work around this?

[hyj1991]

Can you please include a repro if you can?

[carera]

Sure:

const { Worker, isMainThread, parentPort } = require("worker_threads");
const fs = require("fs");

function work() {
  const fib = n => (n <= 1 ? 1 : fib(n - 1) + fib(n - 2));
  let x;
  // Just do some expensive calculations
  for (let i = 0; i < 1e4; i++) {
    x = fib(1 + (i % 24));
  }
  if (parentPort) parentPort.postMessage(x);
}
const proftitle = "fibonacci";

if (isMainThread) {
  const v8Profiler = require("v8-profiler-next");
  v8Profiler.setSamplingInterval(100);
  v8Profiler.startProfiling(proftitle, true);
  new Worker(__filename).on("message", result => {
    const profiler = v8Profiler.stopProfiling(proftitle);
    profiler.export(function(error, result) {
      fs.writeFileSync("profile.cpuprofile", result);
      profiler.delete();
    });
    console.log(result);
  });
} else {
  work(); // worker thread
}

This is the profile:

(The whole thing takes ~500ms. It looks like only the orchestration on the main thread is captured and then it's idling, until the worker posts message back)

In comparison this is how profile looks when profiling the fibonnaci calculation itself, without using worker_threads:

Flamecharts generated from the cpuprofiles (using http://github.com/brendangregg/FlameGraph):

VS standalone fibonacci function, without worker threads:

[carera]

I thought, well, if that doesn't work, I will just run the profiler inside the worker, right? But when I try to run the profile inside the worker, it exits with error Module did not self-register..

[carera]

Not sure if it is related in any way, but there was once this nodejs/node#24016

[hyj1991]

I think it maybe v8 profiler can't work in work threads because isolate is designed not to be thread-safe. But when the work thread feature gets into Stability 2 (now is Stability 1: Experimental), the kernel may have a solution with it :)

[slonka]

@carera I have a repo that shows what is compatible with worker threads and what is not in terms of profiling. Bear in mind that it is still work in progress: https://github.com/slonka/pprof-nodejs-and-worker-threads/

[carera]

Thanks @hyj1991 and @slonka . Do you by any chance have insight on whether the NodeJS team moves the worker threads into stability 2 and with what version?

[slonka]

@carera it should be in stable pretty soon: nodejs/node#22940 (comment) & https://github.com/nodejs/node/pull/29512/files

[carera]

Hi all, so the workers feature is now flagged stable. What are the consequences?

[hyj1991]

nodejs/node#31569

@carera since we can take heap snapshot from parent thread in this OR, I think the implementation for cpu profiling will come soon.

[Widdershin]

@hyj1991 I'm interested in profiling worker threads, and was wondering if there any updates to the state of affairs.

I had a look through the Node repo and couldn't find any issues/prs to add an equivalent CPU profiling API akin to the heap snapshot one, as it seems they're trying to cover that functionality off with the inspector module.

Do you think it would be possible for us to build worker thread profiling on top of the inspector profiling machinery it already has, or do we need something different added to Node?

I'm willing to help out implementing this but I'd need a bit of guidance.

[slonka]

Disclaimer: I did not work on profiling in node.js since sep 2019.

@Widdershin here is a working example of profiling inside worker threads using inspector module: https://github.com/slonka/nodejs-production-ready-profiling/blob/master/node-inspector.js

[Widdershin]

That's a great reference, thanks @slonka. Just curious, did you run into any issues with the inspector module at any point? My biggest reluctance is that stability banner.

[slonka]

Just curious, did you run into any issues with the inspector module at any point? My biggest reluctance is that stability banner.

I vaguely remember talking to some Node.js devs and them saying that it's the preferred method but I can see that the stability did not change: https://nodejs.org/api/inspector.html#inspector_inspector . I would open up an issue and ask them what's the recommended way to profile inside worker threads with an ability to start/stop on demand.

[Widdershin]

For what it's worth, v8-profiler-node8 seems to reliably profile inside of worker threads as well, with a slightly nicer API and feature set. I'm not sure that there are actually any issues with the node8 version in that regard, so I'll report back after more testing.

[Widdershin]

Ah, I see the problem now. v8-profiler-node8 works for profiling worker threads, but only if it hasn't first been loaded in the main thread.

Seems like we need to be context aware! schroffl/node-lzo#11

I'm going to have a go at making v8-profiler-node8 context aware, hopefully the same can be done for this library.

[Widdershin]

I managed to make v8-profiler-node8 work across threads, should be totally viable to do the same here.

hyj1991/v8-profiler-node8#15

[hyj1991]

npm i v8-profiler-node8@next --save now supports CPU profile in the worker threads.

Refs:

• [WIP] Restructure module for thread safety using context aware initialization v8-profiler-node8#15
• fix: compatibility with old node versions v8-profiler-node8#16
• Add test for worker thread support v8-profiler-node8#17

I will patch it to v8-profiler-next soon.

[oscar-gardiazabal]

this was done?

[hyj1991]

Landed in 8eea608, you can use v8-profiler-next@v1.6.1 to do CPU profiling in worker_threads.

Reference: #37
PR-URL: #39