Scalable Processor Array
There’s no big change in the architecture, which is still based on what Nvidia calls Scalable Processor Array (or Streaming Processor Array, depending on whom you ask). The G80’s SPA was organized like this:
Eight TPCs (Texture Processor Clusters), each equipped with a texture unit and two Streaming Multiprocessors (SM). With the GT200, Nvidia has increased the number of units to 10 TPCs, each still equipped with a texture unit, but now with three multiprocessors.
This change is evidence of the orientation of modern shaders, which put the accent on arithmetic instructions.The texture units of each TPC use the same model as those used for the G84 and G92 – there’s as much address capacity as filtering capacity, unlike the G80, which had twice as much filtering capacity as address capacity. So, in a simple filtering mode with RGBA8 (nearest or bilinear) textures, the texture units of the G84/G92/GT200 have twice the performance of the G80. With more evolved filtering modes or RGBA16 textures, the change makes no difference.
In an improvement that’s more specific to the GT200, Nvidia says they’re now using a more effective scheduler to manage texturing operations, which is supposed to come closer to the peak performance of a G92. Let’s check that using Fillrate Tester:
The move up from 64 to 80 texture units, coupled with the difference in GPU frequency, should give the GTX 280 an advantage of only 11% over the 9800 GTX. Yet we measured 43% with quad texturing, and up to 118% with dual texturing! The improvement in the scheduler alone can’t explain that difference. However, the increase in the number of ROPs (doubled) also plays a part. In any event, it’s clear that the GTX 280 is much closer to the theoretical fill rate values in single or dual texturing (97%) than to the 9800 GTX (between 80 and 91%), meaning that the improvements Nvidia has made there have paid off in practical terms. As we explained previously, the AMD bi-GPU board, which also has a faster clock frequency than Nvidia’s, is only 32% behind the GTX 280 in quad texturing.
Now let’s see what happens with the RightMark3D 2.0 PS 4.0 texturing test, which tests texture lookups.
The result for the first shader (Fur) is surprising: a 14% gain, which isn’t a lot given the optimizations of blending, geometry shaders and fill rate, all again dependent on the shader implementation. On the other hand the 59% gain observed with Steep Parallax Mapping is more spectacular, in line with expectations, and very promising.
