launchpad-dev team mailing list archive

[Richard Harding <rick.harding@xxxxxxxxxxxxx>]

On Thu, 20 Sep 2012, Gavin Panella wrote:

> On 19 September 2012 14:25, Deryck Hodge <deryck.hodge@xxxxxxxxxxxxx> wrote:
> > It's not just the assignment or lookup alone; there's also the
> > performance hit for creating that namespace.  Also, that assignment
> > hit has to be paid over and over in each test.  Every JavaScript book
> > will note that nested properties are slow in every browser.  We've
> > just been lazy about this.  Now we have definitive proof that it costs
> > us something -- i.e. Rick had a test that failed due to a timeout
> > error, due to these long namespaces.  So it makes sense to fix it now,
> > or at least not make it worse going forward.
> 
> I guess it's an interpreter problem, though Curtis says there's
> massive improvements in WebKit since Lucid. I assume these problems
> are being observed on post-Lucid machines?
> 
> It seems unbelievable that four/five lookups should be so expensive,
> especially compared to, say, manipulating the DOM, but I guess
> JavaScript has the potential for prototype lookups too along the way.
> 
> Do you know of any decent profilers that could incontrovertibly
> confirm the root cause? I know you guys have shown that reducing
> namespace lookups helps, but I'm still amazed that this is a problem,
> and I don't know how else to silence my brain's WTF lobe.

Gavin, I've not had a chance to dig into it a ton, but I did toss together
a quick perf test to check browsers and their speed of dealing with nested
namespaces.

Check out this following url in various browsers.

http://jsperf.com/js-object-lookup

Below are screenshots of tests with Chrome dev version, FF nightly, and an
IE test in a Virtualbox setup.

Chrome: http://ubuntuone.com/4m2YYq3itkCaorK2DP4g2F
Firefox: http://ubuntuone.com/1QlZSJOxWWCiHBDJrETdHu
IE: http://ubuntuone.com/3ZEJ2yoTBapMPk01E6C3Wo


You can see how FF and IE show a very scaled change as the namespace
deepens. I can imagine a very old webkit showing this pattern and I'm sure
as the namespace widens as well as deepens the lookup hit would be larger.
I still don't think that in the limited JS code in use here it should have
caused these timeouts, but that'll take more work to narrow down which
lines were really killing me or the test runner.

-- 

Rick Harding

Launchpad Developer
https://launchpad.net/~rharding
@mitechie