[whatwg] Canvas 2D memory management

Ashley Gullen ashley at scirra.com
Sat Jan 12 11:52:45 PST 2013

I'd be curious to learn more about how IE handles it.  Does it use
least-recently-used or some other heuristics?  I think letting the browser
guess could result in sub-optimal load times.  Consider a game with two
Level 1: uses texture set A and B
Level 2: uses texture set A and C
When switching from level 1 to level 2, we want to release texture set B
then load texture set C, leaving set A in memory.  If the browser guesses
about releasing textures, it may release textures from set A while loading
set C, when it would be more appropriate to release textures from set B.
 Since level 2 still needs set A, it will load them again.  This means
either the game still janks during level 2, or when loading textures from
set A are needlessly released and loaded again, increasing the load time.

I hadn't heard of ImageBitmap before, and it looks like it could solve the
problem, assuming the existence of an ImageBitmap implies the texture is
already loaded in memory (which it looks like the intent is).  If set A's
ImageBitmap objects are still referenced, and set B are unreferenced and
collected when loading set C if the device runs out of memory, that appears
to be optimal behavior.  What's the implementation status of ImageBitmap?
 Do any browsers support it yet?

Ashley Gullen

On 11 January 2013 22:56, Rik Cabanier <cabanier at gmail.com> wrote:

> On Fri, Jan 11, 2013 at 2:36 PM, Robert O'Callahan <robert at ocallahan.org>wrote:
>> On Sat, Jan 12, 2013 at 6:41 AM, Rik Cabanier <cabanier at gmail.com> wrote:
>>> Some of you concerns with memory management could be addressed with
>>> WeakMaps.
>>> Basically, you can put all your images in a WeakMap and during the draw
>>> cycle, you pull them out and use them. If they're no longer there, it
>>> meant
>>> that the garbage collector has kicked in to free up memory and you need
>>> to
>>> reload (which will unfortunately cause a jank). I'm unsure how smart the
>>> garbage collector is because you probably want the items in the WeakMaps
>>> to
>>> be deleted last.
>> WeakMaps don't work like that. You can only test whether something is in
>> a WeakMap by looking it up with a key object. If the key object stays alive
>> and the WeakMap stays alive, then the value must also stay alive. On other
>> hand, if the key object or the WeakMap have been collected, obviously you
>> can't do the lookup. So you can't observe whether GC has happened using a
>> WeakMap. (This is by design.)
>> Sorry about that!
> yes, you are correct that you can't use it that way.
> Someone told me that WeakMaps work the same way as Flash's dictionary with
> weak keys, but that is not the case.
> FWIW, flash game developers use dictionaries with 'weak keys' as I
> described earlier. If the memory runs low, the garbage collector will
> delete objects from that cache.

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