JavaScript Performance Stack


Something that’s frequently befuddled is the differentiation between where JavaScript is executing and where performance hits are taking place. The difficulty is related to the fact that many aspects of a browser engine are reliant upon many others causing their performance issues to be constantly intertwined. To attempt to explain this particular inter-relationship I’ve created a simplified diagram:

To break it down, there’s a couple key areas:

  • JavaScript – This represents the core JavaScript engine. This contains only the most basic primitives (functions, objects, array, regular expression, etc.) for performing operations. Unto itself it isn’t terribly useful. Speed improvements here have the ability to affect all the various object models.
  • Object Models – Collectively these are the objects introduced into the JavaScript runtime which give the user something to work with. These objects are generally implemented in C++ and are imported into the JavaScript environment (for example XPCOM is frequently used by Mozilla to achieve this). There are numerous security checks in place to prevent malicious script from accessing these objects in unintended ways (which produces an unfortunate performance hit). Speed improvements generally come in the way of improving the connecting layer or from removing the connecting layer altogether.
    • XMLHttpRequest and Timers – These are implemented in C++ and introduced into the JavaScript engine at runtime. The performance of these elements only indirectly affect rendering performance.
    • Browser – This represents objects like ‘window’, ‘window.location’, and the like. Improvements here also indirectly affect rendering performance.
    • DOM and CSS – These are the object representations of the site’s HTML and CSS. When creating a web application everything will have to pass through these representations. Improving the performance of the DOM will affect how quickly rendering changes can propagate.
  • Parsing – This is the process of reading, analyzing, and converting HTML, CSS, XML, etc. into their native object models. Improvements in speed can affect the load time of a page (with the initial creation of the page’s contents).
  • Rendering – The final painting of the page (or any subsequent updates). This is the final bottleneck for the performance of interactive applications.

What’s interesting about all of this is that a lot of attention is being paid to the performance of a single layer within the browser: JavaScript. The reality is that the situation is much more complicated. For starters, improving the performance of JavaScript has the potential to drastically improve the overall performance of a site. However there still remain bottlenecks at the DOM, CSS, and rendering layers. Having a slow DOM representation will do little to show off the improved JavaScript performance. For this reason when optimization is done it’s frequently handled throughout the entire browser stack.

Now what’s also interesting is that the analysis of JavaScript performance can, also, be affected by any of these layers. Here are some interesting issues that arise:

  • JavaScript performance outside of a browser (in a shell) is drastically faster than inside of it. The overhead of the object models and their associated security checks is enough to make a noticeable difference.
  • An improperly coded JavaScript performance test could be affected by a change to the rendering engine. If the test were analyzing the total runtime of a script a degree of accidental rendering overhead could be introduced as well. Care needs to be taken to factor this out.

So while the improvement of JavaScript performance is certainly a critical step for browser vendors to take (as much of the rest of the browser depends upon it) it is only the beginning. Improving the speed of the full browser stack is inevitable.

Posted: February 28th, 2008


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