Review Request 118452: Reduce the memory usage of UDSEntry by using QVector, rather than QHash, for the internal data storage

[David Faure]

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Interestingly, I had a benchmark to compare a number of data structures for UDSEntry (which made me turn the Qt3 QList into the Qt4 QHash). That benchmark is the commented-out "newApiPerformance()" method in jobtest.cpp.

I just extended it to include your suggestion: http://www.davidfaure.fr/2014/udsentry_benchmark.diff

Here are the results I get (in debug mode, because otherwise I'd have to make sure the compiler doesn't optimize away the useless iterations loops :))

Old API: slave code: 5
Old API: app code: 5
QHash+QVariant API: slave code: 9
QHash+QVariant API: app code: 4
QHash+struct API: slave code: 7
QHash+struct API: app code: 3
QMap+struct API: slave code: 8
QMap+struct API: app code: 4
Frank's API: slave code: 13
Frank API: app code: 6

So, unless I'm missing something, this is much slower for the slave (but we could decide to just not use UDSEntry in the slave, as discussed on k-f-d), but it's also slower on the app side....

Can you check if I did something stupid when grabbing your code or writing the testcode for it?
Otherwise the next step is to actually run this with -O2, ensuring the loops still loop.

- David Faure


On June 1, 2014, 1:50 p.m., Frank Reininghaus wrote:
> 
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> This is an automatically generated e-mail. To reply, visit:
> https://git.reviewboard.kde.org/r/118452/
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> 
> (Updated June 1, 2014, 1:50 p.m.)
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> 
> Review request for KDE Frameworks and David Faure.
> 
> 
> Repository: kio
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> Description
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> I am continuing to split up https://git.reviewboard.kde.org/r/113355/ , which attempts to make UDSEntry more efficient memory and CPU-wise, into independent parts. This is the third step after 
> https://git.reviewboard.kde.org/r/113591/ and https://git.reviewboard.kde.org/r/115739/ .
> 
> The present patch modifies the internal data storage of UDSEntry. UDSEntry contains a mapping from unsigned int keys to "Field" values, where Field is a struct that contains a QString and a long long (some keys correspond to numeric values, like size, date, etc, whereas others, like user and group, correspond to a QString).
> 
> Currently, UDSEntry stores all data in a QHash<uint, Field> internally. This ensures that everything can be accessed in O(1) time, but is not very efficient memory-wise because a separate memory allocation is done for each hash node.
> 
> I propose to change this and store both the uint keys and the Field values in a QVector each. This means that accessing a value becomes a O(n) operation, since the entire QVector of keys may need to be scanned in order to find a value, but because the number n of values in a UDSEntry is usually quite small and can currently not exceed a number ~100, this should not matter in practice.
> 
> Some parts of https://git.reviewboard.kde.org/r/113355/ are still missing:
> 
> (a) The QVectors which store the keys (which are usually the same for all items in a directory) are not shared yet. Changing this would reduce the memory usage further, but I decided to keep this change separate in order to keep the current patch small and easy to understand. Moreover, this makes it easier to benchmark other similar approaches (e.g., replace QVector by std::vector, or store keys and values together in a std::vector<std::pair<uint,Field>>).
> 
> (b) No space is reserved in the vectors when key/value pairs are inserted one by one. Implementing this would make UDSEntry faster on the slave side (since repeated re-allocations would not be necessary any more), but this can be done in a later patch. Moreover, it might not be needed any more if UDSEntry is not used directly any more on the slave side, as suggested on the frameworks mailing list by Aaron (good idea BTW!). 
> 
> 
> Diffs
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>   src/core/udsentry.cpp c6ac21a 
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> Diff: https://git.reviewboard.kde.org/r/118452/diff/
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> Testing
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> Unit tests still pass.
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> The memory usage of listjobtest with a directory with 100,000 files is reduced from 71344 K to 35392 K according to KSysGuard. I see similar savings when opening the directory in Dolphin.
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> I still haven't set up a Qt5/KF5 build in release mode (shame on me!), so I cannot present any benchmark results.
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> 
> Thanks,
> 
> Frank Reininghaus
> 
>

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