Strange speed differences on composite

[Owen Taylor]

On Thu, 2006-02-09 at 18:55 +0100, Tomasz Torcz wrote:
>   Hi,
> 
>   recently I've noticed something suspicious going on with compisite
> speed. 
>   First subject was Xephyr, CVS version dated 11th August 2005,
> build using jhbuild. I turned it on as a 1024x768 window with GNOME
> session inside. The I've run "xcompmgr -c" inside. Shadows appeared.
> Now, moving windows in this Xephyr window is pretty fast and CPU isn't
> used much (10% max), so it's pretty usable.
>   Second test was on normal X server. I'm running Xorg 6.9.0 from
> Slackware -current, my graphics card is Matrox G550 with mga driver
> and Semprom 3500+ CPU. EXA is not available on mga.
> When I run "xcompmgr -c" everything becomes very slow. Window movement
> is painful with cpu stuck at 100%. Switching to other virtual desktop
> takes 2-3 seconds to redraw windows.
> 
>   The question: how it comes that software-only Xephyr is _faster_ at
> drawing shadows than fully hw accelerated X server with mga driver?

If you ever have to read data from the frame buffer, that's *slow* - 
you lose far more than you gain from any hardware acceleration you
might be getting.

With xcompmgr inside Xepyhr, you are just working in system memory
than pushing the final result to the frame buffer, with no reading
back from it.

Regards,
					Owen