Bug 90630 – Drawing to accelerated 2D canvas causes compositor to recompute layer tree

• Component: New Bugs
• Status: RESOLVED FIXED

[Justin Novosad]

Drawing to accelerated 2D canvas causes compositor to recompute layer tree

[Justin Novosad]

Created attachment 150979 [details]
Patch

[Justin Novosad]

A key piece of code that is relevant to this patch is method RenderLayer::contentChanged (not modified by this patch). This is the method that was triggering the rebuild of the compositor tree when a 2D canvas is drawn to.

[Justin Novosad]

kbr expressed concern that we must make sure that accelerated canvas layers will still trigger the compositor in all situations with this change.  I have verified that this is the case: RenderLayer::addChild will trigger the compositor (via RenderLayer::dirtyZOrderLists) when a new canvas is added.

[Simon Fraser (smfr)]

What is the code path that causes the full layer rebuild? Is it via RenderLayer::contentChanged()?

[Justin Novosad]

(In reply to comment #4)
> What is the code path that causes the full layer rebuild? Is it via RenderLayer::contentChanged()?

Yes.

All the methods in CanvasRenderingContext2D that draw something to the canvas end up calling CanvasRenderingContext2D::didDraw, which calls RenderBoxModelObject::contentChanged(), which calls RenderLAyer::contentChanged()

[Simon Fraser (smfr)]

But that should only cause a layer rebuild if updateLayerCompositingState() returns true. Does it?

[Justin Novosad]

(In reply to comment #6)
> But that should only cause a layer rebuild if updateLayerCompositingState() returns true. Does it?

updateLayerCompositingState is returning true because RenderLayerBacking::updateGraphicsLayerConfiguration() is returning true because of this bit of code:

else if (isAcceleratedCanvas(renderer)) {
    HTMLCanvasElement* canvas = static_cast<HTMLCanvasElement*>(renderer->node());
    if (CanvasRenderingContext* context = canvas->renderingContext())
        m_graphicsLayer->setContentsToCanvas(context->platformLayer());
    layerConfigChanged = true;
}

BTW, an important benefit of this patch is that will bypass the call to
updateLayerCompositingState which was a hot spot that I identified in a profiling experiment. When rendering a whole bunch of small primitives in a tight loop, all the calls to updateLayerCompositingState (can be thousands per frame) are often a much heavier burden than the compositor tree rebuild. In some experiments, I've seen up to 20% of CPU time being spent there.

[Simon Fraser (smfr)]

(In reply to comment #7)
> (In reply to comment #6)
> > But that should only cause a layer rebuild if updateLayerCompositingState() returns true. Does it?
> 
> updateLayerCompositingState is returning true because RenderLayerBacking::updateGraphicsLayerConfiguration() is returning true because of this bit of code:
> 
> else if (isAcceleratedCanvas(renderer)) {
>     HTMLCanvasElement* canvas = static_cast<HTMLCanvasElement*>(renderer->node());
>     if (CanvasRenderingContext* context = canvas->renderingContext())
>         m_graphicsLayer->setContentsToCanvas(context->platformLayer());
>     layerConfigChanged = true;
> }

OK, for some reason Mac doesn't hit this code. I think your original patch is OK.

> BTW, an important benefit of this patch is that will bypass the call to
> updateLayerCompositingState which was a hot spot that I identified in a profiling experiment. When rendering a whole bunch of small primitives in a tight loop, all the calls to updateLayerCompositingState (can be thousands per frame) are often a much heavier burden than the compositor tree rebuild. In some experiments, I've seen up to 20% of CPU time being spent there.

Please file a new bug on that.

[Justin Novosad]

> 
> Please file a new bug on that.

Done.
https://bugs.webkit.org/show_bug.cgi?id=90695

[Simon Fraser (smfr)]

Comment on attachment 150979 [details]
Patch

What is the code path that causes the full layer rebuild? Is it via RenderLayer::contentChanged()?

[Simon Fraser (smfr)]

> What is the code path that causes the full layer rebuild? Is it via RenderLayer::contentChanged()?
Ignore that comment.

[WebKit Review Bot]

Comment on attachment 150979 [details]
Patch

Clearing flags on attachment: 150979

Committed r121987: <http://trac.webkit.org/changeset/121987>

[WebKit Review Bot]

All reviewed patches have been landed.  Closing bug.