[webkit-dev] Web Components development will continue in a branch in near future

Ryosuke Niwa rniwa at webkit.org
Tue Feb 18 13:41:54 PST 2014

On Tue, Feb 18, 2014 at 7:44 AM, Adam Barth <abarth at webkit.org> wrote:

> On Sat, Feb 15, 2014 at 4:07 PM, Ryosuke Niwa <rniwa at webkit.org> wrote:
>> Now that we've removed all of the existing shadow DOM implementations
>> from trunk in http://trac.webkit.org/changeset/164131, I'm intending to
>> work on new web components implementations in a branch based on the
>> feedback Apple has given on public-webapps and www-style in near future,
>> probably starting with custom elements.
>> I'd like to implement them in a branch to not inadvertently introduce
>> performance regressions.  The old implementation on trunk resulted in 5%
>> overall slowdown in the page load time but we didn't quantify that until we
>> removed the feature entirely last year.  That's probably because the
>> feature was added over years as a pile of small changesets each of which
>> introduced immeasurably small performance regressions.  Development in a
>> branch allows a faithful performance comparison between the two.
> The approach we were successful with in Blink was to restructure how we
> construct the render tree.  At the time Blink branched from WebKit, the
> algorithm we both used to construct the render tree was N^2.  The inner
> loop of the N^2 algorithm contained more complex DOM traversals due to
> shadow DOM.  When you profile the code, it looks like shadow DOM is
> expensive, but the bigger win is just to remove the N^2 algorithm, which
> we've done in Blink.  After removing the N^2 algorithm, shadow DOM related
> code falls off the profile completely.

Makes sense.  I think one big problem with the old implementation was that
it started off with a completely different API and feature set and the code
had to "evolve" as the spec developed further over years and accumulated
design kinks.  I'm sure we can avoid introducing O(n^2) algorithms if we
started from scratch looking at the latest design.

- R. Niwa
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