process: improve cwd performance

[BridgeAR]

This caches the current working directory and only updates the variable
if process.chdir() is called.

This improves the process.chdir performance by factor 10 and path.resolve by
up to 66%. That again is used a lot in cjs modules.

---

I guess this is currently not possible since native modules could change the working directory as well, not only JS code. Or is that not a blocker / does not apply?
Do we have a way to detect if any modules are loaded or not? If so, we could have a fallback for the moment a native module is loaded.
And since process.chdir can not be called in workers: how does that work in combination with native modules that could change the working directory? Can we cache the working directory in case we use workers?

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

[addaleax]

I think the native addons situation does make this semver-major, at least, although it’s unlikely that many native addons change the current working directory.

I’m not sure what the most expensive part of caling cwd() is, but we could probably at least cache the JS string instance on the native side.

Can we cache the working directory in case we use workers?

With the approach in this PR, you’d have to notify workers when the cwd changes – for low overhead, you could e.g. use SharedArrayBuffer + Atomics to increase a counter for each call to process.chdir(), for example.

[BridgeAR]

@joyeecheung I tried to add a SharedArrayBuffer to add a counter between the main thread and workers but I am not sure how to actually reach /node/lib/internal/bootstrap/node.js from /node/lib/internal/main/worker_thread.js and /node/lib/internal/worker.js from /node/lib/internal/bootstrap/node.js.

I circumvented this issue by only improving the performance for the main thread for now but it would actually be nice to improve workers as well. process.cwd() is commonly used during bootstrap.

[joyeecheung]

@BridgeAR I think what you need is communication between worker.js and main/worker_thread.js? Or you can expose a wrapper in process/execution.js. Why do you need to go through bootstrap.js? A SharedArrayBuffer containing something like cwd is not supposed to be part of the snapshot.

[BridgeAR]

@joyeecheung the cwd and chdir functions needs access to the shared buffer:

// Worker thread
let cachedCwd = realMethods.cwd();
let lastCounter = -1;
function cwd() {
  const currentCounter = Atomics.load(counter, 0);
  if (currentCounter === lastCounter)
    return cachedCwd;
  lastCounter = currentCounter;
  cachedCwd = realMethods.cwd();
  return cachedCwd;
}

// Main thread only.
// This buffer would have to be shared and passed through to each worker.
const counter = new Uint64Array(new SharedArrayBuffer(8));

function chdir(cwd) {  
  Atomics.add(counter, 0, 1);
  realMethods.chdir(cwd);
}

I could theoretically override the two functions in case the first worker is started and to use a shared buffer at that point but AFAIK we do not want to override the functions anymore at that point, right?

[BridgeAR]

Local benchmark:

// Before
console.time(); for (var i = 0; i < 1e7; i++) process.cwd(); console.timeEnd()
// default: 5596.328ms

// After
console.time(); for (var i = 0; i < 1e7; i++) process.cwd(); console.timeEnd()
// default: 477.811ms

[joyeecheung]

@BridgeAR You could delay the creation of process.cwd and process.chdir until pre-execution; or export the SAB in e.g. process/execution.js (and send it in worker.js whenever a new worker is spawned), then in main_thread_only.js and worker_thread_only.js, wrap the two variants of methods differently in wrapProcessMethods.

[joyeecheung]

um, wait, in case of workers, I suppose the SAB has to be sent together with the LOAD_SCRIPT message, at that point you've already need to run multiple methods that need process.cwd() to do their job, so I guess the best you can do is to cache only after that (it should be fine to override process.cwd() during LOAD_SCRIPT).

[BridgeAR]

I just updated the PR to also work with workers.

[BridgeAR]

@nodejs/tsc PTAL. This needs one more review from the TSC.

[BridgeAR]

Landed in 1d022e8 🎉