t-kernel – Reliable OS support for WSN

1. t-kernel – Reliable OS support for WSN L. Gu and J. Stankovic, appearing in 4th Proc. of Sensys, Oct. 2006. Best paper award. Presenter: F. E. Bustamante Fabián E. Bustamante, Winter 2007

2. Motivation • Wireless sensor networks (WSNs) with • Resource constrained embedded microcontrollers • Complex application requirements • OS support is very limited; applications (developers) could benefits from • OS protection • Virtual memory • Preemptive scheduling • But microcontrollers do not have hardware support for this • How can we efficiently provide such support w/o hardware help? EECS 443 Advanced Operating SystemsNorthwestern University

3. Context – Resource constrained devices • Very-low-power configurations for • Energy efficiency • Small form factor • Low cost • Minimum assumptions made here (REM) • Reprogrammable • External nonvolatile storage • A certain amount of RAM available (4KB) EECS 443 Advanced Operating SystemsNorthwestern University

4. Context – Complex apps requirements • Three real-world scenarios • VM - VigilNet – large-scale surveillance network • 30 middleware services & 40K SLC • Using overlay in absence of VM is not really an answer • Application specific, inefficient, labor intensive, error-prone • Scheduling - Acoustic sampling app • Timing of microphone sampling inaccurate due to FIFO task scheduling and long-running computation task • Avoiding tasks and use only interrupts doesn’t solve it – now the clock handler’s switch statement is your problem! • OS Control - Extreme scaling • To ensure the OS gets the CPU back • Grenade timer or periodic reboot • Coarse control granularity • Applications must adapt to this • Long time w/o OS control EECS 443 Advanced Operating SystemsNorthwestern University

5. OS (t-kernel) Approach - Naturalization • Load-time code modification • Naturalized program becomes a cooperative program supporting OS protection, VM & preemptive sched. • Naturalizer • In: binary instructions; Out: naturalized instructions (natin) • Page by page • Paging • Storage management • Dispatcher • Controls execution Naturalized program Application EECS 443 Advanced Operating SystemsNorthwestern University

6. Naturalization and control • Modify all branching instructions • Save registers, save destination and go to homeGate (welcomeHome) • welcomeHome – retrieve destination, seeks for a natin page (or create one) and transfer control to it • Transferring control flow to entry point – go to natin page and go through cascading branch chain • Bridging for optimization – basically avoid going to kernel when it’s safe to do so EECS 443 Advanced Operating SystemsNorthwestern University

7. VPC mapping 3 bits • Minimum: 0 Bytes in RAM • To enhance speed, index cache • Hash has 16 possible results 8 bits 6 bits Tag Hash Index VPC Offset VPC Hash EECS 443 Advanced Operating SystemsNorthwestern University

8. Each VPM is hashed to a number of natin pages; need to check all entry points to decide 1. VPC look-aside buffer (fast) 2. Two-associative VPC table 3. Brute-force search on the natin pages (slow) Three-level look up for a VPC EECS 443 Advanced Operating SystemsNorthwestern University

9. Three memory areas • Physical address sensitive memory (PASM) • Virtual/physical addresses are the same • The fastest access • Stack memory • Virtual/physical addresses directly mapped • Fast access with boundary checks • Heap memory • May involve a transition to kernel • The slowest, sometimes involves swapping Example configuration EECS 443 Advanced Operating SystemsNorthwestern University

10. Swapping area organization • Challenge • After 10k writes, a flash page cannot longer be used • If swap-outs evenly distributed to all pages, maximum lifetime • Super page: associative cache, fast swaps • Overflow partition extends longevity – use it when a super page is gone (O(N) seek time, however) • System parameter – Af – associativity of super page (the larger, the faster and short lifetime) 32 Bytes in RAM, 266 days (1,000 swaps/hour), 20% fast swaps EECS 443 Advanced Operating SystemsNorthwestern University

11. Implementation MICA2 EECS 443 Advanced Operating SystemsNorthwestern University

12. Overhead of naturalization • Kernel transaction time: ~20 cycles (2.6microsec on MICA2 @ ~8MHz) • Kernel transaction • Saves/restore registers • Checks the stack pointers • Increments system counters • May need to • Look for destination address • Trigger naturalization of a new page • Re-link naturalized page Relative execution time with iter EECS 443 Advanced Operating SystemsNorthwestern University

13. Overhead from the app’s perspective • PeriodicTask • Wake-up/poll-sensors/communicate • Varying the amount of computation in each task • Keep in mind the CPU idle ration of TinyOS apps EECS 443 Advanced Operating SystemsNorthwestern University

14. Overhead of naturalized VM • Slowest stack access: 16 cycles • Heap access w/o swapping: 15 cycles • Heap access w/ swapping: 149,815 cycles • Swap out time: 20.3ms (near hardware’s limit) Number of pages faults and swap-outs for slidingwin • Balance between swapping speed and longevity • An example: Associativity = 2 • 266 days, 20% fast swaps (assuming 1000swaps/hr) EECS 443 Advanced Operating SystemsNorthwestern University

15. Comparison to VM approach • Comparing with Maté, a VM for TinyOS • A stack based virtual architecture • Comparison with an insertion-sorting program • Initial cost of t-kernel comes from naturalization • After 100 grows slowly since naturalization has a one-time overhead • In contrast, bytecode translation has to be done every time • And sophisticated optimizations for VMs cannot save you here • Of course, you could build Maté/TinyOS on top of t-kernel EECS 443 Advanced Operating SystemsNorthwestern University

16. Conclusions & Future Work • Optimizing compilers • Real-time specification in t-kernel • Characterizing WSN applications • WSN benchmarks • What if power where not an issue? Prof. Alan Epstein - Using computer-chip fabrication techniques to make a gas-turbine engine that fits in the palm of his hand (MIT). EECS 443 Advanced Operating SystemsNorthwestern University