The emergence of energy harvesting devices creates the potential for batteryless sensing and computing devices. Such devices operate only intermittently, as energy is available, presenting a number of challenges for software developers. Programmers face a complex design space requiring reasoning about energy, memory consistency, and forward progress. This paper introduces Alpaca, a low-overhead programming model for intermittent computing on energy-harvesting devices. Alpaca programs are composed of a sequence of userdefined tasks. The Alpaca runtime preserves execution progress at the granularity of a task. The key insight in Alpaca is the privatization of data shared between tasks. Updates of shared values in a task are privatized and only committed to main memory on successful execution of the task, ensuring that data remain consistent despite power failures. Alpaca provides a familiar programming interface and a highly efficient runtime model. We also present an alternate version of Alpaca, Alpaca-undo, that uses undo-logging and rollback instead of privatization and commit. We implemented a prototype of both versions of Alpaca as an extension to C with an LLVM compiler pass. We evaluated Alpaca, and directly compared to three systems from prior work. Alpaca consistently improves performance compared to the previous systems, by up to 23.8x, while also improving memory footprint in many cases, by up to 17.6x. C1:A program must preserve progress despite losing volatile state on power failures. C2: A program must have a consistent view of its state across volatile and non-volatile memory. C3: A program must respect atomicity constraints (e.g., sampling related sensors together).G1: Applications should place as few restrictions on the hardware as possible.G2: Applications should be tunable at design time to use the energy storage capacity efficiently. G3: Applications should minimize runtime overhead and memory footprint. Recent work made progress toward several of these goals, but necessarily compromised on others. This paper develops Alpaca 1 , a programming and execution model that allows software to execute intermittently. Like state-of-the-art systems, Alpaca preserves progress despite power failures (C1) and ensures memory consistency (C2). Alpaca uses a static task model that can adhere to programmer-provided atomicity constraints and energy availability (G2, C3). Memory updates made by an Alpaca task only commits atomically when the task completes. By discarding memory updates on power failure, Alpaca can restart a task with negligible cost, without checkpointing the volatile state as in prior work [Lucia and Ransford 2015;; Van Der Woude and Hicks 2016] (G3). Unlike prior work that requires the entire memory to be non-volatile [Van Der Woude and Hicks 2016], Alpaca can leverage both volatile and non-volatile memory (G1). We present two different versions of Alpaca with different design choices. Alpaca's design differences, relative to state-of-the-art systems, translate into performance gains of 4-5.2x o...

Energy harvesting computers enable general-purpose computing using energy collected from their environment. Energy-autonomy of such devices has great potential, but their intermittent power supply poses a challenge. Intermittent program execution compromises progress and leaves state inconsistent. This work describes Chain: a new model for programming intermittent devices. A Chain program is a set of programmer-defined tasks that compute and exchange data through channels. Chain guarantees forward progress at task granularity. A task is restartable and never sees inconsistent state, because its input and output channels are separated. Our system supports language features for expressing advanced data exchange patterns and for encapsulating reusable functionality. Chain fundamentally differs from state-of-the-art checkpointing approaches and does not incur the associated overhead. We implement Chain as C language extensions and a runtime library. We used Chain to implement four applications: machine learning, encryption, compression, and sensing. In experiments, Chain ensured consistency where prior approaches failed and improved throughput by 2-7x over the leading state-of-the-art system.

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Energy-neutral Internet of Things requires freeing embedded devices from batteries and powering them from ambient energy. Ambient energy is, however, unpredictable and can only power a device intermittently. Therefore, the paradigm of intermittent execution is to save the program state into non-volatile memory frequently to preserve the execution progress. In task-based intermittent programming, the state is saved at task transition. Tasks are fixed at compile time and agnostic to energy conditions. Thus, the state may be saved either more often than necessary or not often enough for the program to progress and terminate. To address these challenges, we propose Coala, an adaptive and efficient task-based execution model. Coala progresses on a multi-task scale when energy permits and preserves the computation progress on a sub-task scale if necessary. Coala's specialized memory virtualization mechanism ensures that power failures do not leave the program state in non-volatile memory inconsistent. Our evaluation on a real energy-harvesting platform not only shows that Coala reduces runtime by up to 54% as compared to a state-of-the-art system, but also it is able to progress where static systems fail. CCS Concepts: • Hardware → Power and energy; • Software and its engineering → Embedded software; Virtual memory;