On Tuesday 14 of July 2009, Thiago Macieira wrote:
> Second, I think that the most memory used by X and by kwin is due to pixmap
> caching. Then remember Qt applications do double-buffering too, which leads
> to triple buffering of everything. There was even a bug in Oxygen that
> caused KWin to enable double-buffering of its own, which would lead to
> quadruple-buffering, but I think that has been fixed for a year.

 That wasn't really a bug. Blending doesn't work without the backing pixmap. 
What happened was rather that Oxygen people worked around the blending 
somehow manually, and this should be rather handled properly. Could it be 
e.g. done in Qt that disabling the backing pixmap using Qt::WA_PaintOnScreen 
would actually get rid of the backing pixmap and not just stop using it? Then 
KWin could enable the backing pixmap only for each paint event and then 
discard the memory again.

 Alternatively, it might make sense to make it possible to avoid the backing 
pixmap altogether in Qt, as it's just additional backing pixmap when 
compositing is enabled, but I guess that would be more difficult.

-- 
Lubos Lunak
KDE developer
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