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

[Frank Reininghaus]

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Review request for KDE Frameworks and David Faure.


Repository: kio


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 

Diff: https://git.reviewboard.kde.org/r/118452/diff/


Testing
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Unit tests still pass.

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.

I still haven't set up a Qt5/KF5 build in release mode (shame on me!), so I cannot present any benchmark results.


Thanks,

Frank Reininghaus

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