On 12/14/07, James Richard Tyrer <[EMAIL PROTECTED]> wrote: > I guess that I could just let you and Nicolas Boulay fight it out since > he says that fixed use hardware is a bad idea. But, I would have to > take the orthodox engineering position that it is an optimization > problem that I don't pretend to know the answer to. Fixed function > hardware can only perform a fixed function which suggests that you might > need more of of it -- even to the point of having 4-word vector
OGA1 isn't entirely unconfigurable. It's just that the pixel and color transforms are arranged in a fixed sequence. But each stage has numerous settings that affect what it does. And the sequence is far from arbitrary. > processors limits what can be done with the hardware. OTOH, to the > extent that hardware is general purpose, you can do more things with it > which suggests that you would need less of it. To me, having several > 4-word processors is a reasonable compromise. I don't see how the clock > rate and pipeline length (which are usually antithetical) are relevant > here except that faster clocks and shorter pipes are always nice. Your analogy with CPU pipelines isn't quite on point here. CPUs have dependencies from later stages to earlier ones. A graphics pipeline doesn't. The main thing to recognize about a long pipeline without dependencies is the number of pixels in flight at one time. If there are 100 stages, then 100 pixels can be in flight at one time, and each one takes effectively one cycle to process (streaming). If those same 100 "stages" had to be processed sequentially by a more traditional processor, then each pixel would take, effectively, 100 clock cycles. So to get the same throughput, you need a clock rate 100 times faster. I'm sure you realize that there is some exaggeration there because ATI and nVidia chips have some degree of pipelining. But it's not an exaggeration if you were to try to do this with a general-purpose CPU. > As far as area is concerned, it appears to me that most of the hardware > area is going to be consumed by the hardware multiplier arrays. So, > other minor differences would be minor. This also means that you should > avoid making fixed use of a multiplier array if that meant that it would > stand idle part of the time. Do some reading about data-flow processors. A fixed-function GPU is like a data-flow processor, where the flow has been fixed in advance. And it's fixed that way because it's a good way to handle 3D rendering. -- Timothy Normand Miller http://www.cse.ohio-state.edu/~millerti Open Graphics Project _______________________________________________ Open-graphics mailing list [email protected] http://lists.duskglow.com/mailman/listinfo/open-graphics List service provided by Duskglow Consulting, LLC (www.duskglow.com)
