[Webkit-unassigned] [Bug 90375] Parallel image decoders

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=90375





--- Comment #77 from Nick Carter <nick at chromium.org>  2012-08-09 14:55:50 PST ---
I'm a little bit worried about the rendering behavior of the parallel image decoding as proposed here. I haven't interacted with a compiled binary containing these changes, but here is what I expect it to behave like. luxtella or skyul, can you respond to the following analysis?

 - The first attempt to draw a BitmapImage essentially sets up a race between the parallel decode, and the draw operation's attempt to read the frame.
 - Given the timings involved, the draw operation will likely proceed before the decode operation really starts in earnest.
 - Usually, we'll expect the draw operations to complete next frame.
 - As a result, we'll visually see a flash of a transparent region while waiting for the image data to become available. That flash is arguably justifiable (?) since images are expected to load progressively anyway.
 - If an <img>'s data is available immediately (e.g. the compressed image data is in cache because it was prefetched via XHR), the race parallel decode will typically add a frame of latency to the presentation of the image.
 - If the MemoryCache is under extreme pressure and thrashing, what used to be a performance problem (slower paint operations because allocation/decoding happens each draw operation) will devolve to a kind of nondeterministic behavior (some timing-dependent subset of decoded images -- possibly all images -- are never available at draw time, because the parallel decode work is evicted before it's consumed in the next frame).

Incurring flashing or an extra frame of latency to the visibility of every new <img> element, this does not seem desirable if serial decoding would only take a small fraction of the frame time.

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