> For the complete documentation index, see [llms.txt](https://sliu583.gitbook.io/blog/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://sliu583.gitbook.io/blog/specific-work/seminar-and-talk/berkeley-system-seminar/immortal-threads-multithreaded-event-driven-intermittent-computing-on-ultra-low-power-microcontroll.md). # Immortal Threads: Multithreaded Event-driven Intermittent Computing on Ultra-Low-Power Microcontroll ### Presentation #### Low-power computing * LoWPANs * Volcano monitoring, structural monitoring, smart home and IoT * Types of wireless network * High cost, high power <-- -->Low cost, low power <-- --> ultra-low cost, ultra-low power * E.g. wifi, bluetooth, LoWPAN (embedded mesh network) * Low-power embedded devices * Hamilton sensor platform * 10 dollars excluding the battery (manufacture) * 32 KiB RAM, 250 kb/s IEEE 802.15.3 radio * All SRAM, no DRAM * RAM limited: power consumption (reason) * This is an example of embedded programming * No OS-supported features into it * Two questions * How should we program such devices? * How can we compose them into larger embedded networked sensor system? * Early thoughts (late 1990s and early 2000s) * Requirements that shape the design of network sensor system * Low power and small size: constrain the processing, storage, and interconnect capability * Highly concurrent: primary mode of operation ... is to flow information from place to place * Little physical parallelism: number of independent controllers, the capabilities of the controllers * More burden put on the CPU * Diverse designs * Application-specific * Unusually robust * No human in the loop * Concurrency model * Threads v.s events debate * Threads are much easier to program, but tricky to sync * Events allow for faster task switching and scale to more concurrent tasks * Writing event-driven code is cumbersome * I2C: bus * State machines * Generate state machines underneath the hood (some of the existing techs) * Making event-driven programming easier * TinyOS * Stack-based cooperative multithreading * Contiki * Preemptive multithreading as a library * Portothreads example * Minimizing the thread states, but complex code in run * pt -> lc (2 bytes) * Stackless: no stack, lose the variables #### Paper * Energy harvesting battery-less devices * Future sensing devices are tiny, sustainable and run forever * Capacitor on the device to store energy * A typical battery-less sensor architecture * Q: FRAM (map to physical address space and it's non-volatile) * Program failures: intermittent execution * Existing techniques * Checkpoint: back up entire state * Costly * Consistent snapshot * Event-driven tasks * Globals: Non-v ram (FRAM) * Task: atomic; non-preemptive and stackless * Checkpoint easily * Stackful concurrency * Programming explicitness * Problem statement * Programming model that * No cognitive load and lightweight as task-based model * Stackful concurrency (preemption + multithreading) * Minimal wasted progress * Pseudo-stackful preemptive multithreading * Immortal threads * Think only event-driven aspects * Identify the events * Threads as event handler * Manage state management and transitions * Compiler frontend * Almost free checkpoints: save only the program counter rather than all registers and memory (just 2 bytes) * Counter of the switch statements of where you were * Stack is always on Non-V memory * Just-in-time privatization * Creates private copies of variables dynamically to keep non-V memory consistent Questions * Immortal threads * break up the execution into idempotent reads and writes * checkpoint after each such memory operation * Why is this faster than prior SOTA * Is intermittent computing really necessary * IPSN 2019: capacity over capacitance for reliable energy * Concurrency is less important now * Depends on the idea: lose power frequently * Add a second power-backup * Reasonable: power budget --> decrease the cost of checkpointing * No dynamic power measurements to do the prediction * Does it support loop and if statement? * Dynamic loop * Statically check each branch in loop body