[cairo] Redraw speed issues

[Charles Tuckey]

Hi David,

Thank you for your answers to our last set of questions. We are 
continuing to explore using the OpenGL backend for our product.

I ran two instantions of the cairo-demo program on a RedHat 9 machine; 
one program using the glx backend and the other using xrender. I 
expected the redraw time for the glx demo to be at least as fast as the 
xrender demo. However, this was not the case. The glx demo redraw was 
substantially slower than the xrender demo redraw (by substantially,  I 
mean about 5-10 times slower). I observed substantially the same 
performance with the spline and knockout demos as well.

Do you have any idea why this might be? Suggestions for further study 
would be appreciated as well.

Here is some relevant information:

     OS distro: Linux RedHat 9

     XFree86 version: 4.3.0
     xdpyinfo reports GLX as an extension

     OpenGL vendor string: NVIDIA Corporation
     OpenGL renderer string: GeForce2 MX/AGP/SSE/3DNOW!
     OpenGL version string: 1.4.1 NVIDIA 53.36

     Using cairo-0.1.19-gl-r6  and  libglc-0.1.0-r3

I don't if this information is of use but here is the experiment I did 
and the results. I ran cairo-demo and then occluded the demo window with 
another window. I uncovered the demo window, waited for it to redraw and 
  then repeated the process. I gathered CPU usage data as follows:
            Demo program     X Server
xrender       10%              81%
glx           16%              74%

An anomaly that I observed was that the glx demo CPU usage climbed to 
about 75% after I quit covering and uncovering the window. It stayed 
there for about 10 seconds before dropping. This didn't happen with the 
xrender demo.


Thanks for your time David.


charlie