Systems Wireless EmBedded Macroprogramming Eric Brewer (with help from David Gay, Rob von Behren, and Phil Levis)

Systems Wireless EmBedded 6/17/2002WEBS Retreat2 Outline Introduction NesC Midterm Plans Macroprogramming

Systems Wireless EmBedded 6/17/2002WEBS Retreat3 Language Goals Short term (NesC) –Make it much easier to program single motes –Gain experience with the issues (please tell us!) –Build intermediate language for future work Long term (macroprogramming) –Write high-level programs for groups of motes –Deal with failure and uncertainty, varying numbers of motes –Abstract issues of time, location, neighbors –Provide implicit communication and data sharing –Enable low power and bandwidth efficiency

Systems Wireless EmBedded 6/17/2002WEBS Retreat4 NesC Midterm Goals Version 1 ready now –Cleans up wiring and code –Smaller, faster, faster to compile code Potential areas for the next version: –Simplify split-phase calls »Blocking calls at the higher level? »At least simplify creation of the command/event pair »Manage state across split-phase calls –“pass” values (like message buffer) »Pass by reference, but object leaves your scope »Implies passed thing is NOT shared »Simplifies capabilities, events, pipelines

Systems Wireless EmBedded 6/17/2002WEBS Retreat5 NesC Midterm (cont) Definition of messages –Define a message type and then create surrounding code Dynamic Linking? –Enable larger programs, multi-mode motes –Load modules from EEPROM? from network? Support for proxies? Sleep modes/timing? Explicit support for state machines? –Probably for interfaces/protocols

Systems Wireless EmBedded 6/17/2002WEBS Retreat6 NesC Midterm (cont) Big goal: race-free, deadlock-free programs Key idea: 1.We can know all of the global resources statically 2.We can find atomic sections for every resource use 3.We can track which resources are in use 4.We can avoid running things that conflict (run things that release resources) We believe that we can detect and enforce every critical section. Requires a scheduler (and no multiprocessing) Claim: would have prevented Therac-25, Mars Rover

Systems Wireless EmBedded 6/17/2002WEBS Retreat7 Macroprogramming Roughly: programming language plus runtime system Basics: –One program for many motes –Shared view of the world, some shared state –Higher level programming language Should provide for sure: –Logical location (2D for now), with error radius »Explicitly know mobile or fixed –Time synchronization, with error bound »Global sleep scheduling? –Time series as a first class type »A sequence of values over time (with error bounds!) »Operations on the sequence (e.g. window, max, last)

Systems Wireless EmBedded 6/17/2002WEBS Retreat8 Macro: Tree Support Claim: spanning tree is a key concept Runtime provides a logical spanning tree –One per partition, split/merge automatically –Changes over time, but operations can still be ordered –Motes need not know whole tree, just local area/direction Enables tree operations: –Barrier (local to partition) –Broadcast –Gather with aggregation (statistical) –Scatter (can affect on way down) –Gather and broadcast with aggregation

Systems Wireless EmBedded 6/17/2002WEBS Retreat9 Macro: Tree Aggregation Key idea: combining function (cf) v’ = cf(v, a) // new value is a function of old plus an argument Cf is associative and commutative cf(cf(v, a), b) == cf(cf(v, b), a) Given two args, a and b c = G(a, b) Require: cf(cf(v, a), b) = cf(v, c) – this allows us to combine args without knowing v –… and thus combine args up the tree Can also compute result after fixing a partition –Log only the aggregate arg, not all of the updates Examples: addition, min/max, latest

Systems Wireless EmBedded 6/17/2002WEBS Retreat10 Macro: using CF trees At language level: –Enables reliable aggregation efficiently –Can sometimes compute G from cf given by user –Can also allow explicit definition of G (and cf) Easy to do: –Shared state that maintains latest value, min or max –“horizontal” arrays with global operations! –Readers/Writers shared state with cf for writes »App just makes local update; we handle the communication and consistency Harder: –Automatic conflict resolution after partition ends? –Automatic generation of cf given an expression? –Multi-arg combiner functions, including error terms

Systems Wireless EmBedded 6/17/2002WEBS Retreat11 Macro: 2D Spaces Alternative framework to logical spanning tree Based on 2D virtual grid –Motes have locations (not on grid points) –Operations for areas, gradients, spatial neighbors Easier for fixed placements Two phases: discovery and use –Discovery does all-way localization (using some known nodes), and determines neighbor structures –Creates a quad tree instead of logical spanning tree –Use is both tree operations and spatial operations Claim: shared infrastructure is the key to NEST

Systems Wireless EmBedded 6/17/2002WEBS Retreat12 Other issues Multi-mode programs? –Construct different programs for different kinds of motes automatically How to exploit high resource motes? –Power, bandwidth, connectivity? –Rough approach: bimodal »Lite motes worry about power/bandwidth »Heavy motes do not »Two programs generated (or one with two parts) Weakly consistent sharing (e.g. bayou)? Generate software for proxies, PCs? Support 3D spaces? 2½ D

Systems Wireless EmBedded 6/17/2002WEBS Retreat13 Conclusions Key is really the shared runtime system –Logical spanning tree –Logical 2D grid –Shared state Can’t make new routing for each app… Error propagation/merging must be central Looking for feedback! –What are the problems we need to solve?