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Theoretical Tests

Each time we meet a new graphics processor architecture, we run a full round of synthetic tests to expose all its properties.

Synthetic Tests: Fill Rate

Fillrate Tester opens our cycle of synthetic tests by measuring the scene fill-rate speed and the speed of executing pixel shaders.

Let’s measure the texturing speed of the new graphics core, in the simplest case. The graphics processor’s standard operation mode: color writes and Z writes are enabled.

That’s predictable: the GeForce 6800 Ultra can process 16 pixels at once and thus shows the highest results in this test. Without textures, the NV40 is close to the theoretical maximum. When there’s one texture, the efficiency of the core drops down, but smoothly improves as there appear more textures, i.e. when there are more clock cycles necessary to process groups of 16 pixels. The reduction of efficiency at mapping one texture (i.e. at the maximum pixel output speed) is probably due to the insufficient speed of writing pixels and Z values into the frame buffer, that is, due to the low memory bus bandwidth. A simple calculation tells that when we use 32-bit textures, a 32-bit frame buffer and Z-buffer and map one texture, the GeForce 6800 Ultra has to write into the graphics memory (into caches, to be precise, and then into the memory), 128 bytes of data per clock cycle or about 52 gigabytes per second, while the bandwidth of its memory bus is “only” 32GB/s. When there are more textures to be mapped, the processor performs memory writes less frequently – the memory bus bandwidth doesn’t play the limiting role then and the NV40 performs close to its theoretical maximum.

Z writes disabled, we see no surprises: the NV40 is more than two times faster than its competitors. And again, the stretch between no-textures and one-texture points is steeper with the NV40 than with the GeForce FX 5950 and the RADEON XT. But with two textures, the GeForce 6800 Ultra again shows a higher efficiency than the graphics processor of the last generation.

Disabling color writes we switch the NV40 into a mode where the graphics processor calculates 32 Z values per clock cycle, that is, the fill-rate doubles. However, the results don’t comply with the theory – the NV40’s fill-rate should be about 12,800 MPixels/second, while the test produces results around 20,000 MPixels/second. By some reason, the GeForce 6800 Ultra shows higher results, than the theoretical maximum!

 
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