[whatwg] Proposal for separating script downloads and execution

Boris Zbarsky bzbarsky at MIT.EDU
Wed Feb 9 06:16:23 PST 2011

On 2/9/11 12:27 AM, Kyle Simpson wrote:
> I can't speak definitively as to how the JavaScript engine is
> implemented (and if the code is significantly different between mobile
> and desktop).

In Gecko's case, it's identical (modulo the different JIT backends for 
ARM and x86 and x86-64, of course).

> But I can say that even if their code is substantially the
> same, I could still see it quite plausible that the device itself locks
> up (not the browser) if there's just simply too much going, taxing its
> limited CPU power.

Yes, but that could just happen due to background tabs, workers, etc. 
That's not something a page author can sanely control....

> I can also see it quite plausible that mobile OS's are not as capable of
> taking advantage of multi-threading

That's not the case for the OSes Gecko is targeting on mobile, at least. 
  It's not the case on iOS, last I checked.  I can't speak to WP7 or 
Symbian; I don't have any experience with them.

> Regardless, considering such things is way outside the scope of anything
> that's going to be useful for web developers in the near-term dealing
> with these use-cases.

Yes, but so is the proposal here, no?

> Even if you're right and the fault really lies with the implementor of
> the JavaScript engine (or the OS), that's still a fruitless path for
> this discussion to go down. No matter how good the mobile JavaScript
> engine gets, I promise you sites will find a way to stuff too much
> JavaScript down the pipe at the beginning of page-load in such a way as
> to overload the device. That is a virtual certainty.

Yes, but what makes you think that those very same sites will make good 
use of the functionality we're proposing here?

>> Now you may be right that authors who really want to screw up like
>> that will just do browser-sniffing hacks of various sorts and still
>> screw up. But it's not clear to me that we need to make the barrier to
>> shooting yourself in the foot lower as a result....
> That sounds more like a question of degree (how much we should expose to
> the developer, and how) than the principle (should we expose it).

Yes, of course.  Degree is my only concern here.

> In any case, I don't see much evidence that suggests that allowing an author to
> opt-in to pausing the script processing between load and execute is
> going to lead to authors killing their page's performance. At worst, if
> the browser did defer parsing all the way until instructed to execute,
> the browser simply would have missed out on a potential opportunity to
> use some idle background time, yes, and the user will have to suffer a
> little bit. That's not going to cause the world to come crashing down,
> though.

Neither will the browser eagerly parsing.  ;)

> What's VERY important to note: (perhaps) the most critical part of
> user-experience satisfaction in web page interaction is the *initial*
> page-load experience.

Really?  The pages I hate the most are the ones that make every single 
damn action slow.  I have had no real pageload issues (admittedly, on 
"desktop") in a good long while, but pages that freeze up for a while 
doing sync XHR or computing digits of pi or whatever when you just try 
to use their menus are all over the place....

> So if it's a tradeoff where I can get my page-load
> to go much quicker on a mobile device (and get some useful content in
> front of them quickly) in exchange for some lag later in the lifetime of
> the page, that's a choice I (and many other devs) are likely to want to
> make.

See, as a user that seems like the wrong tradeoff to me if done to the 
degree I see people doing it.

> Regardless of wanting freedom of implementation, no browser/engine
> implementation should fight against/resist the efforts of a web author
> to streamline initial page-load performance.

Perhaps they just have different goals?  For example, completely 
hypothetically, the browser may have a goal of never taking more than 
50ms to respond to a user action.  This is clearly a non-goal for web 
authors, right?  Should a browser be prohibited from pursuing that goal 
even if it makes some particular sites somewhat slower to load initially 
(which is a big "if", btw).

> Presumably, if an author is taking the extraordinary steps to wire up
> advanced functionality like deferred execution (especially negotiating
> that with several scripts), they are doing so intentionally to improve
> performance

My point is that they may or may not be improving performance in the 
process, depending on implementation details.  Unless they tested in the 
exact browser that's running right now, it's hard to tell.

I see this all the time in JS: people time some script in one browser, 
tweak it to be faster, and in the process often make it slower in other 
browsers that have different JIT heuristics or different bottlenecks.

I think you're assuming a uniformity to browser implementations that's 
simply not there.

> and so if they ended up actually doing the reverse, and
> killing their performance to an unacceptable level, they'd see that
> quickly, and back-track.

You're assuming authors will test in multiple browsers.  In my 
experience, bad assumption...

> It'd be silly and unlikely to think they'd go
> to the extra trouble to actually worsen their performance compared to
> before.

You're assuming malice; I'm just assuming insufficient testing (which is 
pretty understandable, by the way; I wouldn't want to invest a few 
thousand dollars in all the various mobile devices needed to run all the 
different mobile browsers to test on my site) and overtuning for the 
things you test.  The latter is pretty likely.

> Really, let's not always assume the worst about web authors.

I'm not.  Really, I'm not.  Above I'm even assuming way better than is 
warranted based on my experience actually interacting with websites and 
attempts by site authors to fix issues.  I'm still led to the conclusion 
that if we let authors overconstrain the UA here that's a bad idea.

> If they do it wrongly and their users suffer, bad on them, not on the rest of
> us.

If they do it wrongly _my_ users suffer.  I'm in the business of 
creating a user-agent here.  Something that represents the user and is 
supposed to provide the user with the best experience I can manage. 
While I agree that authors who really want to screw up can always do it, 
I think we should be careful to make screwing up _hard_, all else being 
equal.  Of course sometimes not all else is equal.

>> In other words, forbid the browser to start parsing the script, right?
>> How would you tell whether a browser implemented this as specified?
> I could tell if the browser let me control execution pretty easily.

I'm talking about parsing here, not execution.

> As for telling if the browser were deferring parsing to a useful degree,
> I'm sure that the only way to actually determine that would be test a
> site with a particularly long script file (length being a rough
> approximation for parsing time), and see just when (between load and
> execution) the big parsing delay (as observed previously) was happening.

Assuming the browser does the parsing on the main thread, yes?  What if 
it doesn't?

>> OK. I definitely agree that better user control over when script
>> _executes_ is a good idea. Sounds like we're on the same page there.
> I think that what should happen is, if a web author chooses to defer the
> execution of a script (easily detectable if it's not ready for execution
> when loading finishes), the browser should get the strong HINT that
> maybe the parsing should be put off until later (and perhaps until
> execution time).

I can live with that.

> Do you see how what I'm asking for is not direct forcible control over
> parsing but some way to strongly encourage the browser to defer both
> parsing and execution until later, when *I* say I want it to?

That wasn't what the initial proposal sounded like, but ok.  If that's 
what you're looking for, that sounds fine.


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