Dynamic MIPS rate stabilization in out-of-order processors

Today's microprocessor cores reach high performance levels not only by their high clock rate but also by the concurrent execution of a large number of instructions. Because of the relationship between power and frequency, it becomes attractive to run an OoO (Out-of-Order) core at a frequency lower than its nominal frequency in the context of embedded or real-time systems. Unfortunately, whereas OoO pipelines have high average throughput, their highly variable and hard-to-predict execution rate makes them unsuitable for real-time systems with hard or even soft deadlines. In this paper, we demonstrate that the execution time of an OoO processor can be stable and predictable by controlling its MIPS (Mega Instructions Per Second) rate via a PID (Proportional, Integral, and Differential gain) feedback controller and DVFS (Dynamic Voltage and Frequency Scaling). The stabilized processor uses much less power per committed instruction, because of the reduced average frequency. The EPI (Energy Per Instruction) is also cut by an average of 28% across our benchmark programs. Since a stable MIPS rate is maintained consistently with lower power/energy per instruction, OoO processors stabilized by a feedback controller can realistically be deployed in real-time systems. To demonstrate this capability we select a subset of the MiBench benchmarks that displays the widest execution rate variations and stabilize their MIPS rate in the context of a 1GHz Pentium III-like microarchitecture.