NOSTalgy: Near-Optimum Run-Time STT-MRAM Quality-Energy Knob Management for Approximate Computing Applications

Abstract

The stochastic switching feature of Spin-Transfer Torque Magnetic RAM (STT-MRAM) provides an attractive knob to trade quality for energy consumption in approximate computing applications. Indeed, the quality of STT-MRAM functionalities (mainly write operation) is increased by consuming more energy to achieve a more stable write. On the other hand, in approximate computing applications, we do not need 100 percent quality for all of the data. Accordingly, in recent years, several approaches have been proposed to find a balance between output threshold quality and energy consumption in approximate computing applications employing STT-MRAM on-chip memories. While approximate computing application output qualities are highly affected by the fluctuations of the environmental-conditions or input variations, none of the previously proposed approaches have considered the effects of these fluctuations on the output quality. In this article, we propose NOSTalgy, a closed-loop cross-layer approach to dynamically trade off the quality of STT-MRAM based cache memories for energy saving in approximate computing applications. NOSTalgy utilizes a feedback managed fine-grained cache-line-level actuation knobs with different levels of quality for individual write accesses. These knobs are adjusted with the support of the operating system and programmer at run-time. Our experimental results using a set of benchmarks show that NOSTalgy satisfies the output quality thresholds while delivering up to 52 percent energy savings with negligible performance and area overheads.