Spectral analysis for characterizing program power and performance

Performance and power analysis in modem processors requires managing a large amount of complex information across many time-scales. For example, thermal control issues are a power sub-problem with relevant time constants of millions of cycles or more, while the so-called dl/dt problem is also a power sub-problem but occurs because of current variability on a much finer granularity: tens to hundreds of cycles. Likewise, for performance issues, program phase analysis for selecting simulation regions requires looking for periodicity on the order of millions of cycles or more, while some aspects of cache performance optimization require understanding repetitive patterns on much finer granularities. Fourier analysis allows one to transform a waveform into a sum of component (usually sinusoidal) waveforms in the frequency domain; in this way, the waveform's fundamental frequencies (periodicities of repetition) can be clearly identified. This paper shows how one can use Fourier analysis to produce frequency spectra for some of the time waveforms seen in processor execution. By working in the frequency domain, one can easily identify key application tendencies. For example, we demonstrate how to use spectral analysis to characterize the power behavior of real programs. As we show, this is useful for understanding both the temperature profile of a program and its voltage stability These are particularly relevant issues for architects since thermal concerns and the dl/dt problem have significant influence on processor design. Frequency analysis can also be used to examine program performance. In particular it can predict the degree of latency tolerance in a program. It can also identify periodic occurrences of important microarchitectural events like cache misses. Overall, the paper demonstrates the value of using frequency analysis in different research efforts related to characterizing and optimizing application performance and power.