Studying the Impact of Bit Switching on CPU Energy

Abstract

It has been proposed in previous work that compiler instruction scheduling may reduce energy consumption by reordering instructions to minimize bit switching. Multiple algorithms have been proposed in the literature for performing this form of instruction scheduling. However, the impact of these algorithms on actual energy consumption has not been quantified using real hardware measurements; only simulation results have been reported. In this paper, we study the impact of bit switching on the CPU energy consumption using direct hardware measurements on a modern ARM processor. The measurements are performed using an energy probe provided by ARM. The experimental results show that the switching energy is significant and measurable, thus negating the hypothesis that compiling for performance is equivalent to compiling for energy. Yet, our experimental evaluation of multiple bit-switching-aware algorithms suggests that developing a compiler scheduling algorithm for reducing energy consumption by minimizing bit switching is quite challenging, because bit switching may conflict with execution time. An instruction order that minimizes bit switching but increases execution time may result in an overall increase in CPU energy, because the execution time has a higher impact on CPU energy than bit switching. In conclusion, our experimental results show that although performance is a primary factor that affects energy, it is not the only factor; switching energy is another significant factor.