Re: penalty for double buffering

[Olexiy Avramchenko <olexiy irtech cn ua>]

Steven Jenkins wrote:

> Soeren Sandmann wrote:
>
> > Some possibilities:
> >
> >         - Your video card doesn't have enough memory. This can cause
> >           the pixmaps to end up in system memory instead of video
> >           memory, causing drawing to not be accelerated.
>
> Doesn't seem likely. The test case display is only 800x600, and the 
> video card is a relatively recent GeForce4 MX 440 with 64 MB of RAM. 

You're right, AGP-base video with mtrr-capable CPU is very fast with 
video memory writes. Hardware blitter from SM to VM will not help much 
in this case.

> >           Worse, if the source pixmap ends up in video memory, and the
> >           temporary double-buffer pixmap doesn't, the data will be
> >           read across the bus from the video memory, which is slow.
Reading video memory via PCI bridge is really slow so this can be the 
reason.

> Can this happen for reasons other than insufficient video memory? A 
> configuration problem of some kind?

You mentioned you're using NVidia chip. What driver are you using ? Try 
both original from XFree86 and one from NVidia site. Also try to test 
your app in system with another video card.

   Olexiy