Multiple clock domains in recent NVIDIA graphics hardware
Starting with the NV40, NVIDIA have started clocking parts of its GPUs at different rates. There's a reference clock of sorts, then three clock domains that NVIDIA software will identify as Graphics, Shader and Raster. The Graphics clock is rate the fragment rasteriser runs at. That part of the chip takes geometry output from the vertex hardware and turns it into pixel fragments for the fragment shaders to work on. The Shader clock is self explanatory, the Raster clock the frequency of the ROPs.The ROP hardware is responsible for pixel output, buffer combining, some surface filtering and blending, colour and depth compression and sampling (for anti-aliasing), amongst other things. In the NV4x series of chips, it appears that all the clock domains run in lock-step with each other. So you clock the chip at 400MHz using a tool like Coolbits or RivaTuner and all the clock domains run at that clock.
With G70 it's common for the Graphics clock to run out of sync with the others and indeed that's the case with GeForce 7800 GTX. With GeForce 7800 GT, it seems that all three clocks run at the same rate. Here's a shot of the XFX GT in 3D clock mode, from an instrumented ForceWare graphics driver. Click for the bigger version.
Notice all three in lock-step at exactly 450MHz. Clocking parts of the chip at different rates allows for power savings and possible higher performance. Imagine a scenario where you're limited only by the rate the triangle rasteriser can spit out data for the fragment hardware. You only have to clock that part of the chip up to remove the limitation. That functionality can be tied in to the ForceWare driver's profile system, for a per application setting of clocks like that.
Analysis of a game or application using freely available developer tools might give you metrics that show limitations in one part of the graphics pipeline, that can be fed back to NVIDIA and included in later driver revisions as an offset and clock range. Whether that happens currently isn't known.
