X-bit labs

First in DirectX 10.1: ATI Radeon HD 3850/3870 (page 17)

Category: Video

by Alexey Stepin , Yaroslav Lyssenko, Anton Shilov

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

Vertex Shader Performance

ATI Radeon HD GPUs have always been strong at geometry, and the RV670 is no exception, becoming the new leader in the field. The reason of this victory is simple: the GPU can use all of its 320 ALUs for processing geometry whereas Nvidia’s chips just do not have such resources. Well, tests of this type have little to do with real applications in which shader processors are mostly busy with computations of quite a different kind.

This test is rather simple except that the total amount of vertexes in it is over 4 million. There is only one light source. The outcome is predictable: the ATI Radeon HD family enjoys a confident victory, not differing much among themselves.

The Complex Vertex Shader test is far more interesting: the performance of both Radeon HD 3800 models is declining as the display resolution grows up. We can’t explain it by a lack of memory bandwidth because the Nvidia GeForce 8800 GT doesn’t behave that way but has a lower memory bandwidth than the Radeon HD 3870. The speed of the TMUs is not an answer, either, or the Radeon HD 2900 XT would slow down, too. So, this must be the result of some deeper changes in the RV670 architecture we don’t know anything about.