[Development] optimizing QComposeInputContext / TableGenerator?

[Olivier Goffart]

On Monday 19 January 2015 20:15:22 Milian Wolff wrote:
> Hello all,
> 
> when I run my heaptrack [1] tool on Qt 5 applications, I often stumble upon
> the compose TableGenerator. It initializes many QStrings and also consumes
> ruoghly 400KB of memory. I wonder whether we could optimize this somehow?
> 
> The best approach of course would be to have a OpenDesktop standard that
> allows mmapping the compose table in and using it from there. Probably not
> feasible. Creating our own cache brings the usual issues of having to update
> the cache when the original changes... So what I wonder about is whether
> one could delay the table generation? I usually don't use the compose key,
> so my naive assumption would be that lazy-loading this table would help the
> common case of startup quite a bit already. Or is this required for other
> things that I don't expect?

Looking at the hotspot from void TableGenerator::parseKeySequence:
  
 elem.value = QString::fromLocal8Bit(composeValue).at(0).unicode();

So if I understand correctly, it needs to convert the full line to QString 
just to take the first character.  Surely this can be improved.


The code is here:
http://code.woboq.org/qt5/qtbase/src/plugins/platforminputcontexts/compose/generator/qtablegenerator.cpp.html#_ZN14TableGenerator16parseKeySequenceEPc


But yes, some mmapable on-disk cache would probably be the best.
One would just need to make sure that the cache is invalidated properly when 
it should.

-- 
Olivier 

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