X-bit labs

First in DirectX 10.1: ATI Radeon HD 3850/3870 (page 16)

Category: Video

by Alexey Stepin , Yaroslav Lyssenko, Anton Shilov

[ 01/25/2008 | 10:31 AM ]

Pixel Shader Performance

The ATI Radeon HD 3870 and ATI Radeon HD 2900 XT behave identically in this test. The RV670-based solution enjoys but a small advantage which is proportional to the difference in the clock rates of the two cards. The Radeon HD 3850 is slower than the two mentioned cards but faster than the Nvidia GeForce 8800 GT except in the PS 2.0 Per Pixel test.

The results of the latter card are interesting, too, as they do not exactly copy the results of the GeForce 8800 GTX when it comes to simple shaders. We know that the G80 and G92 have identical shader processors, so the difference must be due to texture lookups because it is the TMU architecture that differentiates Nvidia’s new high-performance GPU the most from the older one.

The ATI Radeon HD 3870 behaves like a copy of the Radeon HD 2900 XT, taking the difference in their clock rates into account, but there are serious discrepancies as well: the new card is much slower than its predecessor when executing dynamic branching shaders. Such shaders make use of complex mathematics with loops and conditional jumps but have few texture lookups. Anyway, we can only explain such a big difference by a lower performance of the memory subsystem of the newer card. Perhaps the explanation lies somewhere deeper but we cannot find it basing on the available information about the architecture of the new GPU.

The pixel shader performance test from 3DMark06 shows nothing extraordinary. The ATI Radeon HD 3870 is ahead of the Radeon HD 2900 XT just as expected. Interestingly, the Radeon HD 3850 is almost as fast as the R600-based cards despite its lower GPU frequency. Since this test mostly measures the speed of texture lookups and the efficiency of the memory controller and caches, we can suppose that the RV670’s memory controller has indeed been seriously redesigned and the size of the caches has been increased.