[Development] QML Demo has a higher memory usage than Wdiget Demo

[Volker Hilsheimer]

QML/Qt Quick and Widgets are based on fundamentally different rendering technologies.

Widget applications have a backing store, which is (simplified) a raster image for each toplevel window; that raster image has as many pixels as required for the size of the window. When a line or text is drawn, pixels are put into that backingstore image, which is then put on screen. Some caching happens in Qt (for glyphs, small pixmaps, etc), but there is no data structure maintained that represents the individual paint instructions.

Qt Quick is based on a scene graph, which the GPU can very efficiently operate on. That scene graph stores vectorized representations of rendering primitives (quads and triangles), as well as textures, shader programmes, etc. This data structures needs to live somewhere in memory.

This is just one aspect. The QML engine’s data structures compared to the purely binary compiled C++ is another aspect.


So, comparing the memory footprint of Quick and Widget applications is comparing apples and pears. Keeping their memory footprint equivalent is, generally speaking and by itself, not a meaningful goal.


Volker




> On 16 Jun 2022, at 14:32, YaNing Lu <dluyaning at gmail.com> wrote:
> 
> >
> You didn't answer the question.
> 
> >
> >
> The difference is that QML is a different technology from widgets.
> 
> >
> >
> If you want more details, please use a memory profiler to size the heap (as 
> 
> >opposed to memory mappings) and find out consumes most memory.
> 
> Thanks Thiago,
> 
> I understand that my program has a high memory footprint, which has a 
> lot to do with my implementation, so I just wrote the demo for comparison.
> QML and widgets use different technologies, can I understand the meaning 
> of no comparison between qml and widgets programs.
> 
> BTW, I re-uploaded my demo in https://bugreports.qt.io/browse/QTBUG-104322.
> In fact, my problem is that there are two:
> 
> Why does qml have higher memory usage than widgets.（At present, I am also 
> analyzing through source code and memory detection tools）
> 
> If optimized based on qml demo, how to make its memory usage consistent
> with widget demo or similar.
> 
> Regards,
> --
> Lu YaNing
> 
> Thiago Macieira <thiago.macieira at intel.com> 于2022年6月15日周三 16:05写道：
> On Wednesday, 15 June 2022 02:51:27 PDT YaNing Lu wrote:
> > We read the USS of the demo for comparison, and the following is the
> > comparison data.
> > 
> > 5.15.2
> > qml-demo : 31.3M
> > widget-demo : 24.7M
> > 
> > 6.2.4
> > qml-demo : 36.0M
> > widget-demo : 22.4M
> 
> What are you measuring here? What tool did you use?
> 
> -- 
> Thiago Macieira - thiago.macieira (AT) intel.com
>   Cloud Software Architect - Intel DCAI Cloud Engineering
> 
> 
> 
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