Optimize KConfigIniBackend::parseConfig by reducing allocations.
Review Request #118587 - Created June 6, 2014 and submitted
|dfaure, mdawson, ossi|
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.
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.
Why? Now it could clash with an application class BufferFragment, on a system without "hidden visibility". Or it might confuse gdb, even with hidden-visibility? not sure how that works.
This isn't always the case though. kmail does that, yes, but e.g. kdeglobals doesn't. Nor most desktop files, nor konquerorrc (except that some video player added per-file groups into mine!). What do we lose if the config file has no repeated keys? Memory (the hash) and CPU (inserting and looking up into the hash), right? I have a hard time being sure that the tradeoff is always in our favour.
Review request changed
Make KConfigIniBackend::BufferFragment a public class. It's still internal, as KConfigIniBackend is a private class. This makes the code simpler (no friends required) and also allows us to keep the lookup free-function static.
Revision 3 (+44 -7)
note that you could make it even more efficient with a more drastic approach: have the entry map hold buffer fragments to start with. the entries would also have variants (or at least byte arrays) as a cache for the already-converted (and overwritten) values. that would mean keeping the raw config file in memory, but that shouldn't be significant compared to everything else. i have no idea whether keeping a string table of the converted values on top of that would still add value. it could if the many identical values are actually all used (identical keys should not matter, as they are never converted - unless you enumerate them).
Review request changed