why do gpu's require so much bandwidth?

[her34]

the 8800 gtx has 768MB and bandwidth of 86.4 GB/sec

can someone explain how all the bandwidth is used up?

some numbers would be nice. for example:
a game at 1600x1200 at 32bits, requires ##MB/sec per frame, 8xAA multiplies that by ## per frame, HDR adds ##

86.4GB/sec divided by bandwidth/frame = frames/sec



or something along those lines

 

[Mark R]

Let's say 30 fps @ 1600 x 1200 = 57.6 MPix / s
Approx 3 texels per pixel (overdraw (1 polygon being drawn over the top of another), transparency, multi-texturing/light/shadows, etc.) = 172 MTex / s
Anti-aliasing/super sampling 4x = 700 MTex / s

Using 16 tap anisotropic filtering: each rendered texel requires 16 texels to be retrieved from the original texture = 11 GTex / s
Each Texel is 32 bits (standard dyn range) = 44 GB/s

For 64 bit HDR, that's 88 GB/s - or 128 bit HDR 176 GB/s
If multi-pass rendering is used for special effects (e.g. HDR blooming) then that will further incresae bandwidth requirements.

In practice, GPUs have large amounts of cache memory - and when rendering a polygon, a chunk of the texture will be loaded into the cache. That way repeated texture filtering only needs to retrieve texels from cache, rather than RAM.

I've also not included the bandwidth needed for the framebuffer/temporary render buffers and the Z-buffer.