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Introduction to TinyOS
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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
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Mote Evolution
Source: HotChips 2004
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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
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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
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Reminder
• watt = voltage * amphere– Unit of power
• joule = watt * second– Unit of energy
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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
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• 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
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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
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“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!
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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
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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
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Desirable OS architecture
• Extremely small footprint
• Extremely low system overhead
• Extremely low power consumption
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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
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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)
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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
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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
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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
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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
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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
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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
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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
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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
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Analysis and Evaluation
• Space, Power and Time
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Code and Data Size Breakdown
Scheduler: 178 Bytes codeTotals: 3450 Bytes code
226 Bytes data
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Time Breakdown
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Power Breakdown…
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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
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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.
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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
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Interface
• Bidirectional interfaces support split-phase execution
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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
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Module• Surge: Get a sensor reading and send a
message every second
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Configuration
• Wire TimerM and HWClock components together
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Example: SurgeC configuration
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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
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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
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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
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Effect of inlining
• Impact of inlining on footprint and performance
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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
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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
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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
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• 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++)
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• 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
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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”