Performance change from X in Fedora Core 4 to Fedora Core 5

[Felix Bellaby]

On Thu, 2006-07-13 at 07:52 +0900, Carsten Haitzler wrote:
> > I must confess that it was a guess on my part. If nvidia have confided
> > otherwise then bully for them. I wonder how they do it, and whether it
> > is as fast as drawing to a stable drawable? 
> 
> it is give and take. you incur some extra expense for a temporary pixmap
> allocation and de-allocation in return for not holding on to that video ram
> permanently and thus eventually running out of video ram and needing to swap
> things in and out of video ram and possibly force your os to start swapping
> to/from disk. chances that you need all of that tmp pixmap space (the worst
> case scenario is a whole window in size per window for all windows) all at the
> same time for redrawing is very unlikely. so it is again a trade-off. i would
> much sooner save the video ram as it is so incredibly easy to fill it up and
> run out this way and then watch performance drop through the floor, that you
> want to save the ram, if you can. the allocation expense, in my experience with
> a range of x drivers and cards over a decade or so of writing x code that loves
> to do this kind of stuff, is very minimal to the point of "don't worry about
> it".

I based my gloomy forecast on my recent experience of trying to allocate
and deallocate GLXPixmaps on nVidia hardware. Even when the new
GLXPixmap references exactly the same Pixmap (which remains X allocated)
this process takes longer than copying the Pixmap. I can only presume
that this is a synchronisation problem, but I do not really know.

Felix