Multi-Bit Upsets Vulnerability Analysis of Modern Microprocessors

Miniaturization of integrated circuits brings more devices (thus more functionality) on the same silicon area but also makes them more vulnerable to soft (transient) errors. Assessment and understanding of the magnitude of a microprocessor's vulnerability to soft errors in early stages of the design can steer wise, cost-effective protection decision at the hardware or software level. In recent fabrication technologies, the effect of radiation (neutrons or other particles) is significantly more severe on silicon devices and leads to increased numbers of multi-bit upsets. In this paper, we analyze the effects of multi-bit upsets in modern microprocessors, using microarchitecture level fault injection and a complete system stack. We present details about the effects of multi-bit upsets on 6 major hardware components of an ARM Cortex-A9 CPU modeled on Gem5 microarchitectural simulator, with 15 workloads across 8 fabrication technology nodes. For the purposes of our analysis, we employ and extend the GeFIN (Gem5-based Fault INjector) framework to model and analyze multi-bit faults in the hardware structures of the CPU. The enhanced version of the fault injector models multi-bit faults in adjacent areas of a structure; a very realistic case when modern silicon chips are affected by radiation. Our analysis shows that the architectural vulnerability factor (AVF) significantly increases from 1.5x (+50%) to 3.2x (+220%) between single and triple-bit faults across components. We present the aggregate multi-bit AVF of each hardware structure and each technology node from 250nm to 22nm; our results show significant AVF difference between single bit and aggregate multi-bit measurements, up to 35% as the technology node decreases - this reveals the magnitude of the assessment gap when only single bit errors are considered by any method. We report soft error Failures in Time (FIT) rates for the entire ARM Cortex-A9 CPU across technology nodes and our results show that the contribution of multi-bit upsets in the overall CPU FIT consistently increases across technologies and reaches 21% in 22nm.