[Qt-interest] Making a Qt program into a dynamic library

[Thiago Macieira]

Em Quinta-feira 17. Dezembro 2009, às 15.28.50, Allan Sandfeld Jensen 
escreveu:
> On Wednesday 16 December 2009, Oliver.Knoll at comit.ch wrote:
> > Why would this speed up an application? I would expect that this would
> > take even longer (compared to a single huge executable which is loaded at
> > once into memory), since dynamic linkage also takes its time. Or do you
> > mean "Code is only dynamically loaded/linked when actually needed?"
> 
> No, it loaded the entire executable binary as a dynamic libary. C++ linking
> was very expensive in earlier binutils, and before hidden symbols support
>  from gcc and Qt. Being loading from an application that already was linked
>  to most libraries you used, meant only your own symbols needed to be
>  linked instead of all symbols in all the libraries you used. This reduced
>  startup time for KDE applications from 10 seconds to around 1 second.
> 
> Even today this trick is still marginally faster, but it is not a killer
>  like it used to be.

Oliver is right: it's more expensive if you compare to one application's 
loading.

But this is used for many applications, most of which at KDE startup. It 
probably breaks even at 2 or 3 applications (I'm guessing here). And it keeps 
paying off whenever you launch any application that uses the "kdeinit trick".

The same trick allows us to hide the "NVidia cost". Since NVidia's 
libGLcore.so.1 is *not* position-independent code, it costs about 10 MB per 
process linking to QtOpenGL, in terms of non-sharable code segments. That cost 
you pay simply by linking -- you don't have to use OpenGL at all. We hide the 
cost by preloading libplasma (which links to QtWebKit and QtOpenGL) and 
sharing their non-sharable code segments.

Since at least plasma-desktop and kwin are constantly running on a KDE 
workspace, we've saved you already 10 MB of memory. If you use Amarok -- which 
also links to libplasma -- then it's another 10 MB saved.

Maemo also uses the same trick on the N900.

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
  Senior Product Manager - Nokia, Qt Development Frameworks
      PGP/GPG: 0x6EF45358; fingerprint:
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