[webkit-dev] setTimeout as browser speed throttle

[Rob Burns]

HI Maciej,

On Oct 3, 2008, at 3:16 PM, Maciej Stachowiak wrote:

>
> On Oct 3, 2008, at 3:10 AM, Rob Burns wrote:
>
>> Hi Darin,
>>
>> On Oct 3, 2008, at 9:37 AM, Darin Fisher wrote:
>>
>>> On Thu, Oct 2, 2008 at 10:36 PM, Maciej Stachowiak <mjs at apple.com>  
>>> wrote:
>>>
>>> On Oct 2, 2008, at 10:09 PM, Darin Fisher wrote:
>>>
>>>> On Thu, Oct 2, 2008 at 9:58 PM, Maciej Stachowiak <mjs at apple.com>  
>>>> wrote:
>>>>
>>>
>>>> (I don't understand your comment about not having to have it on  
>>>> all the time.  Surely if a page is asking for a fast setTimeout  
>>>> repeatedly, it would be on "all the time.")
>>>>
>>>> My understanding is that timeBeinPeriod(1) is currently on all  
>>>> the time in Chrome, even when no short-delay timers are currently  
>>>> pending, thus leading to constant greater power consumption. But  
>>>> there is no need for it to be on when there are not fast timers  
>>>> pending. See WebCore/platform/win/SharedTimerWin.cpp. I think  
>>>> that is a technically better approach than switching based on  
>>>> power management state. Feedback welcome, though, and perhaps you  
>>>> will still come to a different conclusion.
>>>>
>>>> I think that is a good idea too, but it doesn't help when a fast  
>>>> setInterval is active.
>>>
>>> That is true. With the webkit.org version of SharedTimerWin,  
>>> though, you can at least close the problematic tab when you hear  
>>> your fan spin up and stop suffering any power drain. It may be  
>>> that running slower is a better option.
>>>
>>>
>>> Yeah, that's the trade off.  Close the offending tab or let it  
>>> run, but more slowly.
>>
>> Another option, would be to halt timers for all unexposed tabs  
>> (i.e., tabs in windows that are not the frontmost tab). Are there  
>> use-cases or sites that would break with such behavior? The reason  
>> I raise this option is that the loaded page represents important  
>> state to the user, but when it is buried on a window, it is  
>> probably not necessary for it to be actively responding to timer  
>> callbacks. Closing the tab saves battery life, but burying the tab  
>> behind another tab would be preferable for the user (a mobile user  
>> that may not be online when that important state represented by the  
>> page loaded in the tab is sought again).
>
> There are web apps that a user may legitimately want to continue to  
> work in the background. One obvious example is a web-based audio  
> player, although in that case it would be a plugin or <audio>  
> element continuing to do work. Still, if you're going to allow  
> plugins and media elements to continue, you may as well allow  
> timers. Other sites update their title on a timer in a way that is  
> useful for a background tab. For example, GMail updates the unread  
> count, which is quite useful on a background tab label. Given these  
> kinds of examples, I think pausing anything in a background tab  
> would not be a good choice; if that kind of functionality were on  
> offer it should be a more explicit user gesture.

I agree with that. To me burying a tab is an explicit and often  
deliberate user gesture. It would take novice users little time to  
adjust to the pausing of buried tabs (pausing with respect to all  
processing: plugins, animated images, and javascript for example). If  
audio is playing and it pauses because the tab is buried, the user  
will quickly come to understand that the tab needs to be exposed and  
to switch to a new window to create the new tab. The audio still plays  
in a background window, just not in a background tab. Until tab  
management gets up to speed, that would be a prudent approach anyway.

> All in all, I think we shouldn't overreact on the timer issue. Sites  
> that consume egregious amounts of computing resources, whether  
> through timers or otherwise, are the minority, and so long as they  
> suck in all browsers and not just some, users will blame the site  
> and it will be pressured to change. So really (in my opinion),  
> matching or beating the CPU consumption of other browsers is the  
> target, and we shouldn't go crazy with novel ways to restrict timers  
> or anything else. Just avoid looking broken on sites that were only  
> tested in IE or Firefox.

My experience is that the problem is much more of a problem (but I'm  
mobile a lot). Adding a new highres timer is a good step, but if  
that's already available, then providing setTimeout a 15 fold  
difference in clamps between WebKit and IE (the primary test browser)  
will really exacerbate the problem (perhaps as much as a significant  
percentage loss in battery-charge-life).

As for trying to avoid restricting timers that merely leads to  
restricting javascript in total. The only reason I would ever want to  
turn off javascript is to control poorly authored timers (and similar  
poorly authored plugins and plugin authoring). So I lose all  
javascript support simply to avoid timers, where I would prefer to  
lose the timer capabilities and still have the remaining javascript or  
maintain all javascript and lose timers for buried tabs.

If we assume the new highres API will not be abused the way setTimeout  
is abused (and that my require other event APIs as well such as a  
convenience method setting the interval by a specific date-time rather  
than only by interval), then I think there is even more reason to  
enable users to constrain the abuse of setTimeout (and plugins)  
specifically.

Take care,
Rob
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