[webkit-dev] RenderArena: Teaching an old dog new tricks

[Ryosuke Niwa]

On Thu, Nov 15, 2012 at 12:22 AM, Chris Evans <cevans at chromium.org> wrote:

> On Wed, Nov 14, 2012 at 11:32 PM, Maciej Stachowiak <mjs at apple.com> wrote:
>
>> rather than DOM vs. not DOM. Also having a per-document arena for DOM
>> nodes (as is done for render objects via RenderArena) seems irrelevant to
>> the security goal and likely to cause bad memory fragmentation.
>>
>
> My read on the Arena is that it's fragmentation resistant (i.e. it will
> not repurpose a larger free chunk to satisfy a smaller allocation.)
> However, memory usage at any given time is defined by peak usage since it
> cannot release pages back to the system without ruining its security
> guarantee. Interestingly, it can't be super bad: we already bite this
> bullet for RenderArena as used by RenderObjects. The RenderArena lifetime
> is the same as the document / DOM and I was surprised to recently be told
> that we don't throw away the RenderArena on a full layout.
>

Not really.

Render tree is really small. It's in the order of a few megabytes on most
websites. On the other hand, DOM tree and CSS objects can consume as much
as tens, if not hundreds, of megabytes because there are many DOM nodes
that are not displayed on the screen.

Also, a large proportion of render objects tend to be allocated and
deallocated at the same time while DOM nodes tend to be created and deleted
at different times on many script heavy page.

These two characteristics of render tree makes it particularly attractive
to make use of memory management strategies like the one used in
RenderArena. I'm not convinced that using the same strategy for DOM nodes
is a good idea.

- R. Niwa
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