Review Request 118587: Optimize KConfigIniBackend::parseConfig by reducing allocations.

[Milian Wolff]

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https://git.reviewboard.kde.org/r/118587/
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Review request for kdelibs.


Repository: kdelibs


Description
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Optimize KConfigIniBackend::parseConfig by reducing allocations.

Yet another awesome application of the Qt implicit sharing trick.
Since config files often contain only few different keys and even
value strings, we can share them. This reduces memory consumption
and also speeds up parsing, as we do not have to allocate the
duplicated strings, but can simply reuse the previous values.

The most extreme case for this of my knowledge, is KatePart:
katesyntaxhighlightingrc has more then 20k lines which triggered
about 30k allocations on startup. With this patch applied, this
value goes down dramatically. I added a simple static counter for
the cache hit/miss ratio, which resulted in 5442 cache misses compared
to 43624 cache hits across all KConfig files parsed by kwrite on startup.


Diffs
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  kdecore/config/bufferfragment_p.h 5a753ad 
  kdecore/config/kconfigini.cpp 8ec43c5 
  kdecore/config/kconfigini_p.h d7aa43e 

Diff: https://git.reviewboard.kde.org/r/118587/diff/


Testing
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Unit tests all pass. My malloc tracer shows that the allocations are all gone.

My malloc tracer showed before:

17421 allocations at:
0x7fee73692b97 QByteArray::QByteArray(char const*, int) /usr/lib/libQtCore.so.4
0x7fee73bb7cee ? /usr/lib/libkdecore.so.5
0x7fee73bb7fc4 ? /usr/lib/libkdecore.so.5
0x7fee73ba1320 ? /usr/lib/libkdecore.so.5
0x7fee73ba1731 KConfig::KConfig(QString const&, QFlags<KConfig::OpenFlag>, char const*) /usr/lib/libkdecore.so.5
0x7fee64830c06 KateHlManager::KateHlManager() in /ssd/milian/projects/kde4/kate/part/syntax/katesyntaxmanager.cpp:76 /ssd/milian/projects/compiled/kde4/lib/libkatepartinterfaces.so.4

12699 allocations at:
0x7fee73692b97 QByteArray::QByteArray(char const*, int) /usr/lib/libQtCore.so.4
0x7fee73bb7cd7 ? /usr/lib/libkdecore.so.5
0x7fee73bb7fc4 ? /usr/lib/libkdecore.so.5
0x7fee73ba1320 ? /usr/lib/libkdecore.so.5
0x7fee73ba1731 KConfig::KConfig(QString const&, QFlags<KConfig::OpenFlag>, char const*) /usr/lib/libkdecore.so.5
0x7fee64830c06 KateHlManager::KateHlManager() in /ssd/milian/projects/kde4/kate/part/syntax/katesyntaxmanager.cpp:76 /ssd/milian/projects/compiled/kde4/lib/libkatepartinterfaces.so.4

These where the allocation hotspots number #1 and #3 respectively. With the patch applied, the two locations are not under the top 10 anymore.


Thanks,

Milian Wolff

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