What's the actual use of the Atomics object in

问题:

The ECMAScript specification defines the Atomics object in the section 24.4.

Among all the global objects this is the more obscure for me since I didn't know about its existence until I didn't read its specification, and also Google hasn't many references to it (or maybe the name is too much generic and everything gets submerged?).

According its official definition

The Atomics object provides functions that operate indivisibly (atomically) on shared memory array cells as well as functions that let agents wait for and dispatch primitive events

So it has the shape of an object with a number of methods to handle low-level memory and regulate the access to it. And also its public interface makes me suppose it. But what's the actual use of such object for the end-user? Why is it public? Are there some examples where it can be useful?

Thank you

回答1:

Atomics are for synchronising WebWorkers that share memory. They cause memory access into a SharedArrayBuffer to be done in a thread safe way. Shared memory makes multithreading much more useful because:

• It's not necessary to copy data to pass it to threads
• Threads can communicate without using the event loop
• Threads can communicate much faster

Example:

var arr = new SharedArrayBuffer(1024);

// send a reference to the memory to any number of webworkers
workers.forEach(worker => worker.postMessage(arr));

// Normally, simultaneous access to the memory from multiple threads 
// (where at least one access is a write)
// is not safe, but the Atomics methods are thread-safe.
// This adds 2 to element 0 of arr.
Atomics.add(arr, 0, 2)

SharedArrayBuffer was enabled previously on major browsers, but after the Spectre incident it was disabled because shared memory allows implementation of nanosecond-precision timers, which allow exploitation of spectre.

In order to make it safe, browsers need to run pages a separate process for each domain. Chrome started doing this in version 67 and shared memory was re-enabled in version 68.

回答2:

If you have some complex computation you may need WebWorkers, in order to your main script may continue his work while heavy things are done in parallel.

Problem that Atomics solve is how WebVorkers can comunicate between each other (easy, fast and relaible). You can read about ArrayBuffer, SharedArrayBuffer, Atomics and how you can use them for your benifits here.

You shouldn't bother about it if:

• You are creating something simple (e.g. shop, forum etc)

You may need it if:

• You want to create something complex, memory consuming (e.g figma or google drive)
• You need that if you are looking to work with WebAssembly or webgl and you want to optimize performance
• Also you may need it if you want to create some complex Node.js module
• Or if you are creating complex app via Electron like Skype or Discord

Thank you Pavlo Mur and Simon Paris for yours answers!

回答3:

In addition to what Arseniy-II and Simon Paris stated, Atomics are also handy when you embed JavaScript engine into some host application (to enable scripting in it). Then one can directly access shared memory from different concurent threads simultaneously, both from JS and from C/C++ or whatever language your host application is written, without involving JavaScript API for access on C/C++/OtherLanguage side.

回答4:

Atomic operation is "everything or nothing" group of smaller operations.

Let's have a look at

let i=0;

i++

i++ is actually evaluated with 3 steps

1. read current i value
2. increment i by 1
3. return the old value

What happens if you have 2 threads doing the same operation? they can both read the same value 1 and increment it at the exact same time.

But this and Javascript, isn't it's single threaded?

Yes! JavaScript indeed single threads but browsers / node allows today usage of several JavaScript runtimes in parallel (Worker Threads, Web Workers).

Chrome and Node (v8 based) creates Isolate for each thread, which they all run in their own context.

And the only way the can share memory is via ArrayBuffer / SharedArrayBuffer

What will be the output of the next program?

Run with node > =10

node --experimental_worker example.js

const { isMainThread, Worker, workerData } = require('worker_threads');

if (isMainThread) {
  const shm = new SharedArrayBuffer(Int32Array.BYTES_PER_ELEMENT);
  process.on('exit', () => {
    const res = new Int32Array(shm);
    console.log(res[0]); // expected 5 * 500,000 = 2,500,000
  });
  Array(5).fill(null).map(() => new Worker(__filename, { workerData: shm }));
} else {
  const arr = new Int32Array(workerData);
  for (let i = 0; i < 500000; i++) {
    arr[i]++;
  }
}

The output might be 2,500,000 but we don't know that and in most of the cases it won't be 2.5M, actually, the chance that you'll get the same output twice is pretty low, and as programmers we surely don't like code that we have no idea how it's going to end.

This is an example for race condition, where n threads race each other and not synced in any way.

Here comes the Atomic operation, that allows us to make arithmetic operations from start to end.

Let's change the program a bit and now run:

const { isMainThread, Worker, workerData } = require('worker_threads');


if (isMainThread) {
    const shm = new SharedArrayBuffer(Int32Array.BYTES_PER_ELEMENT);
    process.on('exit', () => {
        const res = new Int32Array(shm);
        console.log(res[0]); // expected 5 * 500,000 = 2,500,000
    });
    Array(5).fill(null).map(() => new Worker(__filename, { workerData: shm }));
} else {
    const arr = new Int32Array(workerData);
    for (let i = 0; i < 500000; i++) {
        Atomics.add(arr, 0, 1);
    }
}

Now the output will always be 2,500,000

Bonus, Mutex using Atomics

Sometimes, we wish to an operation that only 1 thread can access at the same time, let's have a look at the next class

class Mutex {

    /**
     * 
     * @param {Mutex} mutex 
     * @param {Int32Array} resource 
     * @param {number} onceFlagCell 
     * @param {(done)=>void} cb
     */
    static once(mutex, resource, onceFlagCell, cb) {
        if (Atomics.load(resource, onceFlagCell) === 1) {
            return;
        }
        mutex.lock();
        // maybe someone already flagged it
        if (Atomics.load(resource, onceFlagCell) === 1) {
            mutex.unlock();
            return;
        }
        cb(() => {
            Atomics.store(resource, onceFlagCell, 1);
            mutex.unlock();
        });
    }
    /**
     * 
     * @param {Int32Array} resource 
     * @param {number} cell 
     */
    constructor(resource, cell) {
        this.resource = resource;
        this.cell = cell;
        this.lockAcquired = false;
    }

    /**
     * locks the mutex
     */
    lock() {
        if (this.lockAcquired) {
            console.warn('you already acquired the lock you stupid');
            return;
        }
        const { resource, cell } = this;
        while (true) {
            // lock is already acquired, wait
            if (Atomics.load(resource, cell) > 0) {
                while ('ok' !== Atomics.wait(resource, cell, 0));
            }
            const countOfAcquiresBeforeMe = Atomics.add(resource, cell, 1);
            // someone was faster than me, try again later
            if (countOfAcquiresBeforeMe >= 1) {
                Atomics.sub(resource, cell, 1);
                continue;
            }
            this.lockAcquired = true;
            return;
        }
    }

    /**
     * unlocks the mutex
     */
    unlock() {
        if (!this.lockAcquired) {
            console.warn('you didn\'t acquire the lock you stupid');
            return;
        }
        Atomics.sub(this.resource, this.cell, 1);
        Atomics.notify(this.resource, this.cell, 1);
        this.lockAcquired = false;
    }
}

Now, you need to allocate SharedArrayBuffer and share them between all the threads and see that each time only 1 threads go inside the critical section

Run with node > 10

node --experimental_worker example.js

const { isMainThread, Worker, workerData, threadId } = require('worker_threads');


const { promisify } = require('util');
const doSomethingFakeThatTakesTimeAndShouldBeAtomic = promisify(setTimeout);

if (isMainThread) {
    const shm = new SharedArrayBuffer(Int32Array.BYTES_PER_ELEMENT);
    Array(5).fill(null).map(() => new Worker(__filename, { workerData: shm }));
} else {
    (async () => {
        const arr = new Int32Array(workerData);
        const mutex = new Mutex(arr, 0);
        mutex.lock();
        console.log(`[${threadId}] ${new Date().toISOString()}`);
        await doSomethingFakeThatTakesTimeAndShouldBeAtomic(1000);
        mutex.unlock();
    })();
}