[webkit-dev] Limiting slow unload handlers (Re: Back/forward cache for pages with unload handlers)

Maciej Stachowiak mjs at apple.com
Wed Sep 16 22:57:35 PDT 2009

On Sep 16, 2009, at 10:33 PM, Darin Fisher wrote:

> On Wed, Sep 16, 2009 at 9:59 PM, Maciej Stachowiak <mjs at apple.com>  
> wrote:
> On Sep 16, 2009, at 4:49 PM, Darin Fisher wrote:
>> Counting work instead of time is much more robust.  The getTime  
>> call counts is a measure of work, albeit approximate.
> The way JavaScriptCore execution time limit works is that the clock  
> doesn't start ticking until JS execution begins. So it's unlikely  
> that a full timeout cycle will occur while the process is swapped  
> out or paused, since the clock won't start running until the process  
> is actually executing JS. And the actual timeout check is only done  
> occasionally (every N loop back edges or function calls, for some  
> value of N). So even if there's a context switch in the middle of JS  
> execution, it's unlikely that JS processing will be terminated  
> immediately upon return. So maybe a different solution is  
> appropriate for JavaScriptCore than V8.
> Consider what happens if during JS execution garbage collection  
> runs.  That could cause portions of the VM to be swapped into RAM,  
> which could cause significant wall clock delay.  Do you discount  
> time spent in GC?

We don't exclude time spent in GC - slow is slow. But in practice we  
haven't seen the scenario you describe come up under similar  

> To help us decide whether (and how) to tackle this for non-V8 ports  
> of WebKit, can the Chrome team share the data they have on the  
> following:
> (1) Frequency of pages doing a busy loop in an unload handler. I've  
> heard it's common but no specific data.
> (2) A few examples of URLs to pages that do this, so we can study  
> what they are doing and why.
> (3) Frequency of a date-based loop being used to implement the busy  
> loop.
> (4) Average additional delay imposed by unload busy loops.
> (5) Proportion of sites that use busy looping in unload solely for  
> link tracking and not for any other purpose.
> You can find links to example sites in the Chromium bug report:
> http://code.google.com/p/chromium/issues/detail?id=7823
> The bug contains some distilled data.

I found a couple of URLs (which addresses #2) but I couldn't easily  
find the other data I asked about. Will I find it if I carefully read  
all 80 comments on that bug, or should I assume it's not available?

> By the way, the issue is not with trouble sites but with trouble ad  
> networks and/or producers.  I believe the web sites are just victims  
> here.
> The reason I'm interested in (1)-(4) is to determine if doing  
> nothing is really worse than doing something hackish, as suggested  
> by Adam.
> The reason I'm interested in (5) is to determine if <a ping> is an  
> adequate replacement. I think if we break existing techniques, we  
> need to give authors a replacement. unload fires when the user  
> leaves the page in any way whatsoever, including closing the window  
> or typing in the location field. So sites could use I/O in unload  
> plus a busy loop to track the amount of time the user spent on the  
> page, or to save state. If sites are doing that, then <a ping> won't  
> be an adequate replacement, so we'll have to do something like  
> Adam's suggestion to guarantee completion of I/O that is initiated  
> in the unload handler. The reason I think it's possible sites care  
> about more than just link tracking is that if that's all they care  
> about, they could just use redirect links, and get a better user  
> experience today than busy looping in unload. If sites are not using  
> redirects for link tracking today, why would they use <a ping> in  
> the future?
> The reason why I don't think they are using it for critical data is  
> because they have a timeout.  If they were trying to persist  
> critical data then they would just use a synchronous XHR.  In this  
> case, they are trying to increase the probability of successfully  
> sending a ping by giving themselves a few 100 ms.

I'm not saying it's necessarily critical data, just that I suspect  
they may want to detect when the user leaves the page for a reason  
other than a link, and therefore may not be satisfied with <a ping>.  
If they only care about link tracking, why don't they just convert  
links to redirects?


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