[Development] Question about QCoreApplicationData::*_libpaths

[Jędrzej Nowacki]

On Tuesday 19 of January 2016 13:51:56 Marc Mutz wrote:
> On Tuesday 19 January 2016 11:15:43 Milian Wolff wrote:
> > On Dienstag, 19. Januar 2016 11:51:42 CET Marc Mutz wrote:
> > > I missed one:
> > > 
> > > On Monday 18 January 2016 23:43:37 Marc Mutz wrote:
> > > >     #include <QVector>
> > > >     #include <string>
> > > >     
> > > >     int main() {
> > > >     
> > > >         QVector<QByteArray> l;
> > > >         int oldC = l.capacity();
> > > >         for (int i = 0; i < 10000000; ++i) {
> > > >         
> > > >             char buf[std::numeric_limits<int>::digits + 1];
> > > >             sprintf(buf, "%d", i);
> > > >             l.push_back(buf);
> > > >             int newC = l.capacity();
> > > >             if (newC != oldC)
> > > >             
> > > >                 qDebug("%d", newC);
> > > >             
> > > >             oldC = newC;
> > > >         
> > > >         }
> > > >     
> > > >     }
> > > >     
> > > >     $ time ./test
> > > >     <snip>
> > > >     real    0m1.769s
> > > >     user    0m1.572s
> > > >     sys     0m0.192s
> > > 
> > > Same with std::vector<QByteArray>:
> > > 
> > > real    0m1.776s
> > > user    0m1.616s
> > > sys     0m0.156s
> > > 
> > > > best of three runs, core i7-2720QM, GCC 5.3
> > > 
> > > Ditto.
> > > 
> > > So... is realloc actually the optimisation everyone (incl. me) expected
> > > it to be?
> > 
> > Did you run it through a profiler? Where is the time actually spent?
> 
> No. It's not the IO, though. Removing the qDebug() and capacity tracking, it
> performs the same, within error margins.

Hi, 

  I can not reproduce the numbers on gcc version 5.3.1 20151219 (Debian 
5.3.1-4). But there is a bug in the benchmark, std::vector and QVector have 
different grow model, at least I do not see the same count of qDebug messages.
In addition I think the benchmark may be affected by heap allocation executed 
on each l.push_back.

 The feature is also used in QVariant which tries to avoid allocations. That 
was confirmed as important optimization.

Cheers,
 Jędrek