[Qt-interest] performance issue

[João Abecasis]

João Abecasis wrote:
> Btw, did you try opening the device in Unbuffered mode as well? From
> inspecting the code, it seems that Qt may be doing double-buffering
> otherwise (once by Qt itself and again by using standard streams) :-/ --
> I should fix this...

I looked some more into this issue and the double-buffering is indeed an 
issue that can explain the decrease in readLine performance. 
Double-buffering started in Qt 4.3.0. I just committed a fix for it in 
the 4.6 branch.

Contrary to my earlier suggestion, switching to Unbuffered mode won't 
really help, quite the contrary: readLine would default to 
byte-at-a-time reads.

A more proper workaround would be:

     QFile fileHandle("file.txt");

     if (fileHandle.open(QIODevice::ReadOnly | QIODevice::Unbuffered) {
         QFile file;

         // shouldn't fail
         file.open(fileHandle.handle(), QIODevice::ReadOnly);
     }

Cheers,


João