Architectural time-predictability factor (ATF): a metric to evaluate time predictability of processors

Due to the prohibitive cost of worst-case timing analysis for modern processors, the design of time-predictable processors has become increasingly important for hard real-time and safety-critical systems. However, to the best of our knowledge currently there is no effective and widely accepted metric to quantitatively evaluate time predictability of processors, which greatly impedes the advancement of time-predictable processor design. This paper first introduces the concept of architectural time predictability (ATP), which separates timing uncertainty concerns caused by hardware from software. We then propose a metric called Architectural Time-predictability Factor (ATF) to measure architectural time predictability. Our evaluation on a Very Long Instruction Word (VLIW) processor indicates that ATF is an effective metric to quantitatively evaluate architectural time predictability of a whole processor as well as its architectural and microarchitectural components such as caches, branch prediction, speculative execution, parallel pipelines, and Scratch-Pad Memory (SPM). Thus ATF can be used to quantitatively guide architectural design for enhancing time predictability or making better trade-offs between performance and time predictability.