Bug#648222: Significant 2D performance regression with ColorTiling

[Iustin Pop]

On Thu, Nov 10, 2011 at 06:15:43PM +0100, Michel Dänzer wrote:
> On Don, 2011-11-10 at 18:01 +0100, Michal Suchanek wrote: 
> > On 10 November 2011 17:46, Alex Deucher <alexdeucher at gmail.com> wrote:
> > > On Wed, Nov 9, 2011 at 12:36 PM, Iustin Pop <iustin at debian.org> wrote:
> > >>
> > >> 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.
> > >
> > > 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.
> > 
> > FWIW I have color tiling enabled and have no speed issues in urxvt -
> > TrueType fonts, AA enabled, etc.
> > 
> > Unlike xterm urxvt (rxvt-unicode) uses some special font-rendering
> > libraries, however.
> > 
> > If I understand it correctly xterm would use the in-server bitmap font
> > rendering which the X server can accelerate as much as it wants.
> 
> Core bitmap fonts are completely unaccelerated so far with EXA. xterm
> can also use Xft for font rendering via the -fa option though, which is
> well accelerated.

Hi all, thanks for the replies.

I've tested further and this seems to be a problem specific to xterm
(and possibly other software? not sure how to test e.g. firefox's UI
speed):

Test file: ~20K lines. I've chosen the font sizes so as to have the same
number of lines in full-screen. The timing is simply "time cat file".

ColorTiling disabled:

xterm -fn fixed:      ~3.3s
xterm -fa Mono -fs 8: ~8.6s
rxvt-unicode -fn fixed: ~0.07s
rxvt-unicode -fn xft:Mono:pixelsize=11: ~0.05s

ColorTiling enabled:

xterm -fn fixed:      ~21s
xterm -fa Mono -fs 8: ~7.8s
rxvt-unicode -fn fixed: ~0.6s usually, sometimes ~0.1s??
rxvt-unicode -fn xft:Mono:pixelsize=11: ~0.05s

So it seems to me that:

a) whatever xterm does, it is very sub-optimal
b) while xterm's -fa mode is indeed sped up by ColorTiling, it's still
   many times slower than with ColorTiling disabled and using core
   fonts
c) also rxvt-unicode with core fonts is slowed down by color tiling,
   sometimes very much so (10x)

Based on this I think this is mostly an xterm bug (-fa should/could be
much faster), but I wonder if it also applies to other purely software
2D rendering.

thanks,
iustin
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