Review Request: KImageCache optimization

Mark Gaiser markg85 at
Thu Feb 23 19:23:05 GMT 2012

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(Updated Feb. 23, 2012, 7:23 p.m.)

Review request for kdelibs, David Faure and Michael Pyne.


Someone wants benchmarks ;)

Description (updated)

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.

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 ~ 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.

Benchmark numbers. insert(write) and find(read)

-- Before patch --
READ : 0.019 msecs per iteration (total: 79, iterations: 4096)
WRITE: 0.010 msecs per iteration (total: 88, iterations: 8192)

-- After patch --
READ : 0.019 msecs per iteration (total: 79, iterations: 4096)
WRITE: 0.0026 msecs per iteration (total: 87, iterations: 32768)

Reading is equal in speed, writing is ~5x faster after the patch.

Special thanks go to David Faure for helping me a great deal with this.


  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 



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.


Mark Gaiser

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