Where and what is "OpenGL memory" used by VBOs, etc

Question

I am learning how to use VBOs and, as the book says,

"...you can free up CPU memory by moving vertex data to the OpenGL memory on the GPU."

Well, just exactly what can a GPU handle in this regard? Is it acceptable to assume that the "OpenGL memory" can store the vertex data for millions of polygons? What about the GPU in a mobile device?

While developers are used to having a frame of reference for memory restrictions on a CPU, learning OpenGL is partly challenging because I don't know much about GPUs and what to expect from their hardware. So when I read a vague statement like the above, it makes me nervous.

Answer 1

OpenGL has an abstract device and memory model. And technically in the world of OpenGL there is not CPU and GPU memory, but client and server memory. OpenGL buffer objects live on the server side. Server, that simply means everything the OpenGL driver abstracts away. And the OpenGL driver is perfectly allowed to swap out data from the GPU to the CPU if the GPU memory, which acts like a cache, is not sufficient. Hence what your book states:

"...you can free up CPU memory by moving vertex data to the OpenGL memory on the GPU."

Is not entirely correct, as the data in a OpenGL buffer object may very well reside in CPU memory.

Answer 2

There are minimal requirements in spec, but in general, the amount of GPU memory is quite broadly available information which you certainly noticed when buying your PC (overhyped by sellers). However, as @datenwolf said, you can't really know where the data actually is; all that matters is that you can destroy your temporary buffers.

You should take capabilities of the targetted hardware into account regardless of the technology used.