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Re: [agenda] Web Performance WG Teleconference #81 Agenda 2012-09-12

From: Boris Zbarsky <bzbarsky@MIT.EDU>
Date: Thu, 13 Sep 2012 12:47:21 +0100
Message-ID: <5051C7C9.5080107@mit.edu>
To: public-web-perf@w3.org
On 9/13/12 12:14 PM, Paul Bakaus wrote:
> Precisely the fact that similar structures don't "copy" memory, but some
> others do, is why we'd like to find out. Let's say I create 100 similar
> formed objects (objects on a map), and only the x/y coordinates change 
> I'd like to know how much more of a memory footprint additional of these
> objects consume to better gauge how large my total "pool" of objects
> should be. Right now, the only thing we can do is create a JS test and
> then watch the memory usage of the tab/browser rise.

OK.  But the point is that creating a single object and measuring its 
memory usage won't tell you what you want here, right?  You actually 
have to create 100 objects and ask how much memory those are using as a 
set.  Or something?

> I understand your argument and it is one I've been hearing from many
> browser developers. We have the specific issue that we can only have a
> given amount of textures loaded at the same time  so imagine a game that
> has multiple levels and assets that to some extend change per level. We
> need to be able to reliably understand how much memory the assets consume

My point is that this presupposes definitions for "memory" and "consume" 
that are not obvious to me.

> Even though it doesn't guarantee anything, on most mobile platforms today,
> there's a single app usage pattern, meaning you use one app at a time, so
> it is reasonably safe to assume that memory consumption won't dramatically
> increase by another app thread on a device while browsing your site.

But it can increase for other browser tabs, no?  Also for the browser's 
own internal bookeeping.

>> What does this mean in a world where parts of GC happen async on
>> separate threads?
>
> I wasn't aware of the fact that GC can happen async nowadays and have
> never seen it in action.

I said "parts of GC".  It happens all the time in Spidermonkey; for 
example finalization has been on a background thread for most gcthings 
since Firefox 6.  Other parts of GC happen on the main thread.

It sounds like what you're really looking for is "pause time on main 
thread due to gc" not "gc duration"?  This also matters because some GC 
implementations are incremental, with GC yielding control after running 
for a bit but before finishing the GC.  Again, Spidermonkey certainly 
does this.

> Same answer :) It is is happening on the UI thread, we need to know.

Yes, but what do you need to know, exactly?  Total main-thread GC time, 
individual main-thread pause times, something else?

>> You would have to do a good bit of convincing on this one, I think.  My
>> first reaction being somewhere between "heck, no" and "no way".  ;)
>> This is a huge footgun, and one incredibly likely to get misused all
>> over the place.
>
> Very true. This one can only be implemented if all of the above is
> implemented - meaning, when the user has total awareness of what code or
> asset uses memory (and when), when he is in theoretical full control, this
> option makes sense.

I'm not convinced it does, because in my experience people have a 
tendency to think they know what's going on when they actually don't... 
  I mean in terms of the "total awareness" you describe, not in terms of 
competence.

In particular, there are allocation sources that are effectively tied 
into GC that don't seem like they would be reported on if "all of the 
above" is implemented.  Worse yet, these are likely to be 
browser-specific, and it would be easy for pages to not know they need 
to check for them at all, even if they were exposed.

> 1) I know the total number of assets my game requires
> 2) I do all the asset/code pooling myself
> 3) I'm reasonably confident to be able to keep the heap at a certain level
> on my own
> 4) I want no-compromise, maximum execution performance

I think that #3 is a huge assumption that is likely to be false in 
practice...

> This is a scenario where I'd like to try disabling the GC. You know this
> better than I though  is this technically simple?

Disabling GC?  It's obviously pretty easy technically to add a "don't 
GC" flag.  Making it not just explode in people's face is the hard part.

How do you see the "no gc" thing working when multiple pages share a GC 
heap?  Would you be just trying to turn off page-local GC or also global GC?

-Boris
Received on Thursday, 13 September 2012 11:47:49 GMT

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