Question

I am trying to optimize some OpenGL code and I was wondering if someone knows of a table that would give a rough approximation of the relative costs of various OpenGL functions ?
Something like (these numbers are probably completely wrong) :

method                        cost
glDrawElements(100 indices)    1
glBindTexture(512x512)         2
glGenBuffers(1 buffer)         1.2

If that doesn’t exist, would it be possible to build one or are the various hardware/OS too different for that to be even meaningful ?

Answer 1

There certainly is no such list. One of the problems in creating such a list is answering the question, “what kind of cost?”

All rendering functions have a GPU-time cost. That is, the GPU has to do rendering. How much of a cost depends on the shaders in use, the number of vertices provided, and the textures being used.

Even for CPU time cost, the values are not clear. Take glDrawElements. If you changed the vertex attribute bindings before calling it, then it can take more CPU time than if you didn’t. Similarly, if you changed uniform values in a program since you last used it, then rendering with that program may take longer. And so forth.

The main problem with assembling such a list is that it encourages premature optimization. If you have such a list, then users will be encouraged to take steps to avoid using functions that cost more. They may take too many steps along this route. No, it’s better to just avoid the issue entirely and encourage users to actually profile their applications before optimizing them.