[webkit-dev] An idea to improve render performance by caching image-data's intrinsic size

[Maciej Stachowiak]

On Apr 10, 2009, at 6:51 AM, Johnny Ding wrote:

> Hi all,
>
> I have an idea about caching image's intrinsic size to improve  
> render performance.
> We know the dynamic size change will cause the page re-layout when  
> rendering HTML pages. It hurts render performance. The size change  
> of images are usual cases for this issue.  In HTML pages,  The <img>  
> tag might not be specified width and height, or only has specified  
> width/height.  In this case, the img's render size will be  
> calculated based on the intrinsic size of image-data.
> Before getting the actual image data, WebKit will use default  
> intrinsic value (now is width:0 / height:0) to calculate the size of  
> renderImage object,  aftering receiving the real image data, the  
> intrinsic size of renderImage object will be changed,  the  
> "imageChanged" method will be called and the layout around the  
> renderImage could re-layout.  If the renderImage object is nested in  
> couple of render containers, the area which need to re-layout could  
> be big and even the whole page could need to re-layout.
>
> My idea is after first time requiring a image data, we use a map to  
> cache the intrinsic size of the image data, such as
> http:///www.a.com/1.jpg    200*160  ... (additional properties)
> http:///www.b.com/2.jpg    160*200  ...
> Then next time one renderImage is created, we use its url of image  
> data to look up the previous recorded size info and set it as  
> default intrinsic value. I know the real intrinsic size of some  
> images could be changed, but so far we don't care because the value  
> we cached is only used as default value or you can say it's  
> potential value.  Once we get real size from outside world, the  
> right value will be applied. Actually in most of times, the previous  
> recored intrinsic size is the real sintrinsic size.  Since we use  
> right intrinsic size to calculate the size of renderImage objects  
> from first time,  when real imag datas are coming, we might not need  
> to do lots of re-layout action. It should improve the whole render  
> performace of WebKit.

We've discussed ideas along these lines before. It seems unlikely to  
me that a size cache would be much more effective than the actual data  
cache. And if we have cached data for the image, cached size info does  
not improve things. One way to test this hypothesis: add logging to  
determine how often in a typical browsing session you load an image  
that has been previously loaded, but that is not in the cache. This is  
the only case that will be helped, so it is good to know how common it  
is. Then we can do some estimation to see how big a "size cache" would  
likely be needed to catch these cases.

Regards,
Maciej

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