Program Performance

요약

nkang

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ES7에서 사용 가능
Shared Workers survive the termination of a port connection if other port connections are still alive, whereas dedicated Workers are terminated whenever the connection to their initiating program is terminated.
Because a shared Worker can be connected to or from more than one program instance or page on your site, the Worker needs a way to know which program a message comes from. This unique identification is called a "port" -- think network socket ports. So the calling program must use the **port object of the Worker for communication:**

    `w1.port.addEventListener( "message", handleMessages );

    // ..

    w1.port.postMessage( "something cool" );`

If your site or app allows for loading multiple tabs of the same page (a common feature), you may very well want to reduce the resource usage of their system by preventing duplicate dedicated Workers; the most common limited resource in this respect is a socket network connection, as browsers limit the number of simultaneous connections to a single host.
All these are great questions to consider as we explore a feature added to the web platform circa HTML5 called "Web Workers." This is a feature of the browser (aka host environment) and actually has almost nothing to do with the JS language itself. That is, JavaScript does not currently have any features that support threaded execution.

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"Structured Cloning Algorithm" (https://developer.mozilla.org/en-US/docs/Web/Guide/API/DOM/The_structured_clone_algorithm) is used to copy/duplicate the object on the other side. This algorithm is fairly sophisticated and can even handle duplicating objects with circular references. The to-string/from-string performance penalty is not paid, but we still have duplication of memory using this approach. There is support for this in IE10 and above, as well as all the other major browsers.
"Transferable Objects" (http://updates.html5rocks.com/2011/12/Transferable-Objects-Lightning-Fast). What happens is that the object's "ownership" is transferred, but the data itself is not moved. Once you transfer away an object to a Worker, it's empty or inaccessible in the originating location -- that eliminates the hazards of threaded programming over a shared scope. Of course, transfer of ownership can go in both directions.
You may notice a common characteristic of most of those uses, which is that they require a large amount of information to be transferred across the barrier between threads using the event mechanism, perhaps in both directions.

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yujo

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