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copied!<blockquote> <p>1. How would I run several web workers? I tried a for-loop looking like that:</p> </blockquote> <p>There's no problem with creating more than one worker, even if you don't keep track of them in an array. See below.</p> <blockquote> <p>2. How would I control that all have finished their work? (I need to reassembly the array and work with it later)</p> </blockquote> <p>They can post a message back to you when they're done, with the results. Example below.</p> <blockquote> <p>3. How many web workers really bring an improvement?</p> </blockquote> <p>How long is a piece of string? :-) The answer will depend entirely on the target machine on which this is running. A lot of people these days have four or more cores on their machines. Of course, the machine is doing a lot of other things as well. You'll have to tune for your target environment.</p> <blockquote> <p>4. Is there any advanced tutorial, besides the MDN-entry?</p> </blockquote> <p>There isn't a lot "advanced" about web workers. :-) I found <a href="http://www.html5rocks.com/en/tutorials/workers/basics/" rel="noreferrer">this article</a> was sufficient.</p> <p>Here's an example running five workers and watching for them to be done:</p> <p>Main window:</p> <pre><code>(function() { var n, worker, running; display("Starting workers..."); running = 0; for (n = 0; n < 5; ++n) { workers = new Worker("worker.js"); workers.onmessage = workerDone; workers.postMessage({id: n, count: 10000}); ++running; } function workerDone(e) { --running; display("Worker " + e.data.id + " is done, result: " + e.data.sum); if (running === 0) { // <== There is no race condition here, see below display("All workers complete"); } } function display(msg) { var p = document.createElement('p'); p.innerHTML = String(msg); document.body.appendChild(p); } })(); </code></pre> <p><code>worker.js</code>:</p> <pre><code>this.onmessage = function(e) { var sum, n; sum = 0; for (n = 0; n < e.data.count; ++n) { sum += n; } this.postMessage({id: e.data.id, sum: sum}); }; </code></pre> <p>About the race condition that doesn't exist: If you think in terms of true pre-emptive threading, then you might think: I could create a worker, increment <code>running</code> to <code>1</code>, and then before I create the next worker I could get the message from the first one that it's done, decrement <code>running</code> to <code>0</code>, and mistakenly think all the workers were done.</p> <p>That can't happen in the environment web workers work in. Although the environment is welcome to start the worker as soon as it likes, and a worker could well finish before the code starting the workers finished, all that would do is <em>queue</em> a call to the <code>workerDone</code> function for the main JavaScript thread. There is no pre-empting. And so we know that all workers have been started before the first call to <code>workerDone</code> is actually executed. Thus, when <code>running</code> is <code>0</code>, we know they're all finished.</p> <p>Final note: In the above, I'm using <code>onmessage = ...</code> to hook up event handlers. Naturally that means I can only have one event handler on the object I'm doing that with. If you need to have multiple handlers for the <code>message</code> event, use <code>addEventListener</code>. All browsers that support web workers support <code>addEventListener</code> on them (youdon't have to worry about the IE <code>attachEvent</code> thing).</p>

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