A Performance-Correctness Explicitly-Decoupled Architecture: Technical Report

Optimizing the common case has been an adage in decades of processor design practices. However, as the system complexity and optimization techniques’ sophistication have increased substantially, maintaining correctness under all situations, however unlikely, is contributing to the necessity of extra conservatism in all layers of the system design. The mounting process, voltage, and temperature variation concerns further add to the conservatism in setting operating parameters. Excessive conservatism in turn hurt performance and efficiency in the common case. However, much of the system’s complexity comes from advanced performance features and may not compromise the whole system’s functionality and correctness even if some components are imperfect and introduce occasional errors. We propose to separate performance goals from the correctness goal using an explicitly-decoupled architecture. In this paper, we discuss one such incarnation where an independent core serves as an optimistic performance enhancement engine that helps accelerate the correctness-guaranteeing core by passing high-quality predictions and performing accurate prefetching. The lack of concern for correctness in the optimistic core allows us to optimize its execution in a more effective fashion than possible in optimizing a monolithic core with correctness requirements. We show that such a decoupled design allows significant optimization benefits and is much less sensitive to conservatism applied in the correctness domain.