Review Request 113591: Reduce UDSEntry memory usage by sharing the contained QStrings if possible

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This review has been submitted with commit 6d8fac9e342cfbcafb20846e647f25696d67da57 by Frank Reininghaus to branch master.

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On Nov. 3, 2013, 6:57 p.m., Frank Reininghaus wrote:
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> (Updated Nov. 3, 2013, 6:57 p.m.)
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> Review request for kdelibs and David Faure.
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> Repository: kdelibs
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> Description
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> This patch is a subset of https://git.reviewboard.kde.org/r/113355/ (I'll continue working on the other part of that request, which can be dealt with separately, at some later point).
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> It adds a unit test to makes sure that saving UDSEntries to a QDataStream and re-loading them works as expected, and makes use of implicit sharing of QStrings in UDSEntryPrivate::load(QDataStream &s, UDSEntry &a) to reduce the memory usage of this class (which is the major consumer of memory in Dolphin and other applications that list the contents of large directory contents with KIO).
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> It caches the most recently loaded QString for each UDS field in a simple QVector<QString>. This works because sharable strings like, e.g., the user and the group, usually appear at the same position in the QDataStream when retrieving a large number of UDSEntries that have been stored by a kioslave.
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> Note that I had made an earlier attempt to achieve the same thing using a QHash<uint, QString> to look up the cached strings ( http://pastebin.kde.org/p52a24b49 ), but the QVector<QString>-based solution turns out to be faster.
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> Diffs
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>   kio/tests/udsentrytest.h PRE-CREATION 
>   kio/tests/udsentrytest.cpp PRE-CREATION 
>   kio/tests/CMakeLists.txt 5a1f9b5 
>   kio/kio/udsentry.cpp 1e1f503 
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> Diff: http://git.reviewboard.kde.org/r/113591/diff/
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> Testing
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> kdelibs unit tests still pass. The memory usage of both Dolphin and a simple test program that uses KIO::listDir to list the contents of a large directory (see r4 of https://git.reviewboard.kde.org/r/113355/) is reduced by ~128 bytes per item according to my tests.
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> A simple benchmark that simulates how 100,000 UDSEntries stored by kio_file are loaded (see r3 of https://git.reviewboard.kde.org/r/113355/) runs in 234 ms instead of 266 ms on my machine - it seems that growing the heap to provide space for the non-shared QStrings is more expensive than comparing all loaded QStrings with the cached values.
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> Thanks,
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> Frank Reininghaus
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