[Webkit-unassigned] [Bug 68353] [Qt] Apply ParalellJobs for ImageBuffer::platformTransformColorSpace method

Wed Oct 5 01:30:43 PDT 2011

https://bugs.webkit.org/show_bug.cgi?id=68353


--- Comment #19 from Andras Piroska <pandras at inf.u-szeged.hu>  2011-10-05 01:30:43 PST ---
> Could you explain this to me?
> Why is this better than deriving the optimal thread count from the number of scanlines total?
If the image is 5x120 the algorithm will separate the picture (at 4 cores) to four 5x30 image, but for so small images the overhead much bigger then the gain.
Our experience showed the overhead disappears below approx. 256x256 pixel per thread.
You should also consider there 120x5 image which is also not a good candidate for parallelization so we should check the if the image has the minimum area(width * height) and a minimum scaleline as well.
And must be sure that the count of threads not be more than the height of the image.

> I like that you're sharing the code between the threaded and non-threaded paths.
> Could we refactor it slightly to look less thread-specific?
> For example having a transformColorSpace(yStart, yEnd, width, bits, bytesPerLine, lookupTable) method instead of passing a ThreadParameters object.
> Then the non-threading code doesn't need to know about ThreadParameters.
Not, the worker function must be get only one parameter. That is a limitation of Parallel_Jobs API.

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