Bug#648222: Significant 2D performance regression with ColorTiling

[Alex Deucher <alexdeucher@gmail.com>]

On Wed, Nov 9, 2011 at 12:36 PM, Iustin Pop <iustin@debian.org> wrote:
> Package: xserver-xorg-video-radeon
>
> Version: 1:6.14.3-1
> Severity: minor
>
> Hi,
>
> The recent upgrade of xserver-xorg-video-radeon from 1:6.14.2-2 to
> 1:6.14.3-1 enabled ColorTiling for my card, which in turn caused a
> significant performance degradation in 2D (yes, I understand it should
> make 3D faster, but I didn't know it should slow down 2D applications).
>
> I'm using plain 2D environment (openbox, no compositing, anything) and
> plain xterm (bitmap fonts, no AA, etc.). The speed of display text has
> changed significantly enough that I can "see" my mutt refreshing the
> inbox and drawing the lines.
>
> Cat-ing a big (~4K lines of text) file on a full-screen xterm is:
>
> - 6.14.2-2 (low power profile): ~0.8s
> - 6.14.3   (high power)       : ~3.1s
> - 6.14.3   (low power)        : ~5.0s
>
> So it's more than 5x time slower, which makes it "unpleasant".
>
> Simply disabling ColorTiling makes the problem go away, and 6.14.3 is as
> fast as 6.14.2.
>

Tiling will speed up all rendering (2D and 3D).  However, it sounds
like you are using an environment that is mostly software rendering.
As such in order for the CPU to access tiled buffers, the GPU has to
copy them to a linear buffer before CPU can access it properly.

Alex