Introduction to TinyOS

Faster, Smaller, Numerous

• Moore’s Law– “Stuff” (transistors, etc)

doubling every 1-2 years

• Bell’s Law– New computing class every

10 years

year

log

(p

eo

ple

pe

r c

om

pu

ter)

Streaming Data to/from the

Physical World

Source: HotChips 2004

Mote Evolution

Source: HotChips 2004

Design Principles

• Key to Low Duty Cycle Operation:– Sleep – majority of the time– Wakeup – quickly start processing– Active – minimize work & return to sleep

Source: HotChips 2004

Sleep

• Majority of time, node is asleep– >99%

• Minimize sleep current through– Isolating and shutting down individual circuits– Using low power hardware

• Need RAM retention

• Run auxiliary hardware components from low speed oscillators (typically 32kHz)– Perform ADC conversions, DMA transfers, and bus

operations while microcontroller core is stopped• There’s wake-up overhead too

Source: HotChips 2004

Reminder

• watt = voltage * amphere– Unit of power

• joule = watt * second– Unit of energy

Active• Microcontroller

– Fast processing, low active power

– Avoid external oscillators

• Radio– High data rate, low power

tradeoffs– Narrowband radios

• Low power, lower data rate, simple channel encoding, faster startup

– Wideband radios• More robust to noise, higher

power, high data rates

• External Flash (stable storage)– Data logging, network code

reprogramming, aggregation– High power consumption– Long writes

• Radio vs. Flash– 250kbps radio sending 1 byte

• Energy : 1.5J• Duration : 32s

– Atmel flash writing 1 byte• Energy : 3J• Duration : 78s

Source: HotChips 2004

• Mica2 (AVR)– 0.2 ms wakeup

– 30 W sleep

– 33 mW active

– 19 kbps

Power Calculation ComparisonDesign for low power

• MicaZ (AVR)– 0.2 ms wakeup

– 30 W sleep

– 33 mW active

– 250 kbps

• Telos (TI MSP)– 0.006 ms wakeup

– 2 W sleep

– 3 mW active

– 250 kbps

Report data once every 3 minutes with two AA batteries (<1% duty cycle)

328 days 945 days453 days

Software challenges

• Power efficiency• Small memory footprint• Application specific• Modularity• Concurrency-intensive operations

– Multiple, high-rate data flows (radio, sensor, actuator)

• Real-time– Real-time query and feedback control of physical world– Little memory for buffering: Data must be processed on the fly– TinyOS: No buffering in radio hw: deadline miss data loss– TinyOS provides NO real-time guarantees!– EDF (Earliest Deadline First) scheduling supported in TinyOS 2.x

• http://www.tinyos.net/tinyos-2.x/doc/html/tep106.html

“General-Purpose” OS?

MANY functionalities & programming APIs• Protection between “untrusted” applications

and kernel– Overhead for crossing kernel/user boundary & interrupt

handling

• Multi-threaded architecture– Large number of threads large memory– Context switch overhead

• I/O model– Blocking I/O: waste memory on blocked threads– Polling: waste CPU cycles and power

• Need a new OS architecture!

TinyOS Overview• Application = scheduler + graph of components

– Compiled into one executable

• Event-driven architecture• Single shared stack• No kernel/user space differentiation

CommunicationActuating Sensing Communication

Application (User Components)

Main (includes Scheduler)

Hardware Abstractions

Traditional OS Architectures

Problems

• Large memory & storage requirement

• Unnecessary and overkill functionality

• Address space isolation, complex I/O subsystem, UI

• Relatively high system overhead, e.g, context switch

• Require complex and power consuming hardware support

VM I/O Scheduler

Application 1 Application 2

Monolithic kernel

HW

NFS I/O

Scheduler

Application 1

Micro-kernel

HW

IPC VM

Desirable OS architecture

• Extremely small footprint

• Extremely low system overhead

• Extremely low power consumption

NO Kernel Direct hardware manipulationNO Process management Only one process on the flyNO Virtual memory Single linear physical address space NO Dynamic memory allocation Assigned at compile timeNO Software signal or exception Function call

Goal: to strip down memory size and system overhead.

TinyOS Architecture Overview

I/O COMM . …….

Scheduler TinyOS

Application Component

Application Component

Application Component

TinyOS Architecture Overview

• Characteristics of TinyOS– No UI, power constrained– Application specific HW and SW– Multiple flows, concurrent bursts– Extremely passive vigilance (power saving )– Tightly coupled with the application

Mote Hardware

Actuating Sensing

Simplified

TinyOS

ArchitectureActive Message

Communication

Application (User Components)

Main (includes Scheduler)

TinyOS component model

• Component has:– Frame: storage– Tasks: computation– Interface

• Command: request to lower-level components

• Event: deal with hardware events

• Frame: static storage model - compile time memory allocation (efficiency)

• Command and events are function calls (efficiency)

Messaging Component

Internal StateInternal Tasks

Commands Events

An Example Application

RFM

Radio byte (MAC)

Radio Packet

i2c

Tempphoto

Messaging Layer

clocksbit

byte

packet

Routing Layer

sensing applicationapplication

HW

SW

ADC

messaging

routing

TinyOS Two-level Scheduling

• Tasks do computations– Non-preemptive FIFO scheduling– Bounded number of pending tasks

• Events handle concurrent data flows– Interrupts trigger lowest level events

– Events preempt tasks, tasks do not– Events can signal events, call commands, or post tasks

Hardware

Interrupts

eve

nts

commands

FIFO

TasksPOST

Preempt

Time

commands

Two-Level Scheduling• Event and Task

– Tasks cannot preempt other tasks or events– Single shared stack

• Used by both interrupts and function calls

– Simple FIFO scheduler – Events can preempt a task– Events can preempt each other– When idle, scheduler shuts down the node

except for clock

Event implementation Event is independent of FIFO scheduler. • Lowest level events are supported directly by

Hardware interrupt • Software events propagate from lower level to

upper level through function call

How to handle multiple data flows?

• Data are handled by– A sequence of non-blocking event/command (function calls)

through the component graph– Post tasks for computations that are not “emergent”

• Preempting tasks to handle new data

How should network msg be handled?

• Socket/TCP/IP?– Too much memory for buffering and threads

• Data are buffered until application threads read it• Application threads blocked until data is available

– Transmit too many bits (sequence #, ack, re-transmission)

– Tied with multi-threaded architecture

• TinyOS solution: Active Messages

Active Message• Every message contains the name of an event

handler• Sender

– Declare buffer storage in a frame– Name a handler– Request Transmission– Done - completion signal

• Receiver– The corresponding event handler invoked

No blocked or waiting threads on the receiver Behaves like any other events Single buffering

Analysis and Evaluation

• Space, Power and Time

Code and Data Size Breakdown

Scheduler: 178 Bytes codeTotals: 3450 Bytes code

226 Bytes data

Time Breakdown

Power Breakdown…

Summary: TinyOS

• Component-based architecture– An application wires reusable components together in

a customized way

• Tasks and event-driven concurrency– Tasks & events run to completion, but an event can

preempt the execution of a task or an event– Tasks can’t preemt another task or event

• Split-phase operations (for non-blocking operations)– A command will return immediately and an event

signals the completion

A quick look at nesC

• [O2] D. Gay, P. Levis, R. von Behren, M. Welsh, E. Brewer, and D. Culler, The nesC Language: A Holistic Approach to Networked Embedded Systems, ACM Conference on Programming Language Design and Implementation (PLDI), June 2003.

Component• A component provides and uses interfaces

– The interfaces are the only point of access to the component• An interface models some service, e.g., sending a msg• The provider of a command implements the commands,

while the user implements the events

Interface

• Bidirectional interfaces support split-phase execution

Component implementation

• Two types of components: modules & configurations

• Modules provide application code and implements one or more interfaces

• Configurations wire components together– Connect interfaces used by components to

interfaces provided by others

Module• Surge: Get a sensor reading and send a

message every second

Configuration

• Wire TimerM and HWClock components together

Example: SurgeC configuration

Concurrency and Atomicity

• Asynchronous code (AC): reachable from at least one interrupt handler

• Synchronous code (SC): only reachable from tasks– Synchronous code is atomic w.r.t. other synchronous code

• Still potential races between AC and SC– Any update to shared state from AC– Any update to shared state from SC that is also updated from

AC

• Solution: Race-Free Invariant– Any update to a shared variable is by SC or occurs inside an

atomic section

atomic & norace• Atomic

– Disable interrupts– Atomic statements are not

allowed to call commands or events

– If a variable x is accessed by AC, any access of x outside of an atomic section is a compile time error

– An atomic section should be short!

• norace– If a programmer knows a

potential race is not an actual race, declare norace

Disable interrupts

Enable interrupts

Evaluation• Tested for three TinyOS applications

– Surge, Mate, TinyDB• Core TinyOS consists of 172 components

– 108 modules & 64 configurations• Group necessary components, via nesC’s bidirectional interfaces,

for a specific application

Effect of inlining

• Impact of inlining on footprint and performance

Evaluation of TinyOS (and nesC)

• Small memory footprint– Non-preemptable FIFO task scheduling

• Power efficient – Put microcontroller and radio to sleep

• Efficient modularity – Function call (event, command) interface between components

• Concurrency-intensive operations – Event-driven architecture– Efficient interrupts/events handling (function calls, no user/kernel

boundary)

• Real-time – Non-preemptable FIFO task scheduling– No real-time guarantees or overload protection

Is TinyOS a silver bullet?

• Preemptive & deadline-driven scheduling?

• More support for power and energy-efficiency beyond OS?

• End-to-end timing guarantees?

• Reliable synchronization mechanism?

– Most of things, e.g., I/O, are asynchronous

• Comeback of multithreading?

– Most programmers are more used to multithreaded than event-driven programming

– TinyThreads, TOSThreads

Other WSN Operating Systems

• Contiki– Events & Threads: Lightweight threading mechanism, called

protothreads– Flash file system, called Coffee

• Mantis– Preemptive time-sliced multithreading– Synchronous I/O– Concurrency control, e.g., binary & counting semaphores

• Nano-RK– Reservation-based RTOS for WSNs– Fixed priority preemptive multitasking to meet real-time dedadlines– CPU and network bandwidth reservation– Socket-like abstractions and network scheduling and routing

• RTEOS– Dual mode (kernel & user mode)– Application code checking at compile and runtime– Multithreading & support for POSIX 1003.b real-time scheduling

• LiteOS– Traditional Unix-like environment– Built-in hierarchical file system & a wireless Unix-like command

shell– Kernel support for dynamic loading of multithreaded applications– LiteC++ (OO language using a subset of C++)

• For a more comprehensive survey of WSN OS, talk to me to get a copy of: “Providing OS support for wireless sensor networks: challenges and approaches”, IEEE Communication Surveys and Tutorials.

• For more info about TinyOS, visit www.tinyos.net

Next Class

• At the beginning of the next class, submit one-page critique on the S-MAC paper: [M2] “An Energy-Efficient MAC Protocol for Wireless Sensor Networks”