Review Request: KImageCache optimization

[Aaron J. Seigo]

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kdecore/util/kshareddatacache.cpp
<http://git.reviewboard.kde.org/r/104052/#comment8776>

    this needs a check on destination being not-null. e.g. if (destination && !temp.isNull()) above


- Aaron J. Seigo


On Feb. 23, 2012, 6:21 p.m., Mark Gaiser wrote:
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> http://git.reviewboard.kde.org/r/104052/
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> (Updated Feb. 23, 2012, 6:21 p.m.)
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> 
> Review request for kdelibs, David Faure and Michael Pyne.
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> Description
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> I was running KWin through callgrind to see where possible bottlenecks are. I wasn't expecting much since it improved greatly during the 4.8 dev cycle, however one stood out. The saving of PNG images was taking about 1/5th of the time in KWin that i could see directly. That looked like something i might be able to optimize.
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> What this patch is doing is storing the actual image bits to prevent saving a PNG image to the mmapped cache. That was a hot code path in time (cycles), not even in calls. I've also reduced the amount of memory copies to a bare minimum by adding a rawFind function to KSharedDataCache which fills a QByteArray::fromRawData thus preventing a expensive memory copy. The rawFind is used for looking up an image and fetching it's data without copying it. That is done because QImage seems to make a copy itself internally. I don't have any performance measurements, however, prior to this patch my kwin test was using up ~5.000.000.000 cycles. After this patch it's using up 1.370.000.000. I don't have raw performance numbers to see if the cache itself is actually faster, it certainly has become a lot cheaper to use the cache. Logic wise i would say creating a QImage from the cached data should be way faster now since there is no step involved anymore in decoding the image. Storing is certainly an order of magnitude faster.
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> Special thanks go to David Faure for helping me a great deal with this.
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> Diffs
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>   kdecore/util/kshareddatacache.h 339cecc 
>   kdecore/util/kshareddatacache.cpp 9fe3995 
>   kdeui/tests/CMakeLists.txt 63788f6 
>   kdeui/tests/kimagecachetests.h PRE-CREATION 
>   kdeui/tests/kimagecachetests.cpp PRE-CREATION 
>   kdeui/util/kimagecache.cpp a5bbbe1 
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> Diff: http://git.reviewboard.kde.org/r/104052/diff/
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> Testing
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> I've also written a bunch of test cases (greatly improved by David Faure) to see if i didn't break anything. According to the test (which is also comparing the actual image bits) it's all passing just fine.
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> 
> Thanks,
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> Mark Gaiser
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>

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