[manover]

Could anybody please explain to me why there is a need for a special treatment of VRAM compared to a regular system RAM in this use case? Assuming, we can perform an allocation in VRAM (probably using OpenCL API), why can't we use tmpfs/ramfs code? Do I understand correctly that PCI maps VRAM to a certain memory region and it is accessible via regular CPU instructions? Is it because CPU caching is different or VRAM is uncacheable? Or is it something else?

[revelation]

VRAM is not mapped to the same memory space as your normal RAM and it is not directly accessible via regular CPU instructions. It's wholly owned by the GPU, and that's who the CPU has to talk to to use it.

This is in fact a (if not the) major limiting factor to expanded use of GPUs for general purpose calculations: you always have to copy input and results between video RAM and normal RAM.

[manover]

Yes, it's owned by the GPU, but you can map it to the regular space. In fact, this is exactly how textures and other data gets loaded to the video card. See http://en.wikipedia.org/wiki/Memory-mapped_I/O

[wtallis]

Your graphics drivers aren't written to be able to share resources with tmpfs. Going through OpenCL ensures that the graphics drivers know about and will respect any VRAM allocations.