Re: [Libvisual-devel] GL library linkage

[Burkhard P. <pl...@ip...>]

> We can run 3 X servers independently. He-he. On different machines!
> I'll try that later.

If you have them on different machines, the OpenGL calls will be
tunneled through the X-Protocol to different Servers. X-Protocol
calls are driver independent.
Why does the client machine need separate Gl Libs then?

> Not actually. It depends...

If you want to do it right, you must.

If you pass a context pointer with each OpenGL call,
you implicitely allow people intermix calls with different contexts
(I know that it's nosense, but you never know what programmers do,
when nobody watches them) And then you must use glxMakeCurrent
in each call, which adds some cycles to the overhead calculation
below.

If you forbid to intermix such calls, you don't need the context
pointers and leave things as they are in plain GLX/OpenGL.

> Hm, let one function call overhead will be 100 instructions
> (VERY high overhead). 

I think 100 is realistic. I'm no assembler/CPU guru, but calling
functions via dlsym pointers seems a bit slower than direct calls.

> It will be (less than, due to architecture)
> 15000*100=1.500.000 CPU clocks. On a 1GHz it's 1.5ms.
> Is that much?

Ok, I made some errors before:

I have 5120 Triangles, 9 OpenGL calls per triangle
(3 * (glNormal + glTexCoord + glVertex)) makes 46080 calls per frame.
The mesh is much too complicated to make a triangle strip.

Overhead becomes then 4.608 ms for 1GHz CPU speed.
Assuming the lemuria framerate of ca. 30 fps, it's 13.8 %.

And the 46080 calls are not the only ones which are necessary to
draw the picture.
This time could be used to make better looking animations than
enabling features which are useless for 99% of all users.

If the usage of multiple OpenGL libs cannot be avoided:
Would it make sense to have them in separate processes? The LD_PRELOAD
environment variable can easily force a certain libGL.so to be used,
and OpenGL can be used as we learned it.

-- 
_____________________________
Dr.-Ing. Burkhard Plaum
Institut fuer Plasmaforschung
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