NoMali: Simulating a realistic graphics driver stack using a stub GPU

Abstract

Since the advent of the smartphone, all high-end mobile devices have required graphics acceleration in the form of a GPU. Today, even low-power devices such as smartwatches use GPUs for rendering and composition. However, the computer architecture community has largely ignored these developments when evaluating new architecture proposals. A common approach when evaluating CPU designs for the mobile space has been to use software rendering instead of a GPU model. However, due to the ubiquity of GPUs in mobile devices, they are used in both 3D applications and 2D applications. For example, when running a 2D application such as the web browser in Android with a software renderer instead of a GPU, the CPU ends up executing twice as many instructions. Both the CPU characteristics and the memory system characteristics differ significantly between the browser and the software renderer. The software renderer typically executes tight loops of vector instructions, while the browser predominantly consists of integer instructions and complex control flow with hard-to-predict branches. Including software rendering results in unrepresentative benchmark performance. In this paper, we use gem5 to quantify the effects of software rendering on a set of common mobile workloads. We also introduce the NoMali stub GPU model that can be used as a drop-in replacement for a real Mali GPU model. This model behaves like a normal GPU, but does not render anything. Using this stub GPU, we demonstrate how most of the problems associated with software rendering can be avoided, while at the same time simulating a representative graphics stack.