Does AGP Really Improve Performance?

10% Reduction In CPU Performance

The other side of the performance equation is the CPU. What happens to the CPU when AGP texturing is activated? When AGP texturing is turned on, the graphics card takes control of the main memory bus in order to access texture data. When this happens, the CPU is locked out of main memory. If the CPU is not very busy, this may not be a problem. But if the CPU is engaged in a computationally challenging task (such as a game) it is highly possible that the CPU may stall, waiting for its turn to access main memory.

Right now there is no good way to physically test this scenario. It must be modeled. For this and other reasons, I have built a rather complex software model of the entire PC architecture. Using this model, I am able to estimate the CPU performance impact of main memory arbitration conflicts resulting from AGP texturing.

Intel has publicly released figures that show that a 300 MHz P2 requires about 100 MB/s of external bandwidth while running 3D Winbench. Third party testing has demonstrated that games can create a CPU bandwidth demand of 50 to 120 MB/s from main memory. In the face of this load, AGP texturing could also place a concurrent main memory bandwidth demand of about 50 megaBytes per second (or more).

Using this data, my system performance model shows that main memory conflicts between the CPU and the graphics controller will result in a CPU performance reduction of more than 10%. This assumes the use of a Pentium II with the back side cache intact. The cacheless Covington would be brought to its knees under these circumstances.

The assumption that AGP is a little faster now, but will be a lot faster when it is "really turned on" is completely false. In fact the opposite is true. In most cases, system which actually use AGP for textures will be potentially 40% slower than systems which use local graphics memory for textures instead.

Is this enough motivation to make sure you get a graphics card with enough memory to do the job?