Write-Light Cache for Energy Harvesting Systems

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Energy harvesting system has huge potential to enable battery-less Internet of Things (IoT) services. However, it has been designed without a cache due to the difficulty of crash consistency guarantee, limiting its performance. This paper introduces Write-Light Cache (WL-Cache), a specialized cache architecture with a new write policy for energy harvesting systems. WL-Cache combines benefits of a write-back cache and a write-through cache while avoiding their downsides. Unlike a write-through cache, WL-Cache does not access a non-volatile main memory (NVM) at every store but it holds dirty cache lines in a cache to exploit locality, saving energy and improving performance. Unlike a write-back cache, WL-Cache limits the number of dirty lines in a cache. When power is about to be cut off, WL-Cache flushes the bounded set of dirty lines to NVM in a failure-atomic manner by leveraging a just-in-time (JIT) checkpointing mechanism to achieve crash consistency across power failure. For optimization, WL-Cache interacts with a run-time system that estimates the quality of energy source during each power-on period, and adaptively reconfigures the possible number of dirty cache lines at boot time. Our experiments demonstrate that WL-Cache reduces hardware complexity and provides a significant speedup over the state-of-the-art volatile cache design with non-volatile backup. For two representative power outage traces, WL-Cache achieves 1.35x and 1.44x average speedups, respectively, across 23 benchmarks used in prior work.