Rethinking OS Design

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Rethinking OS Design
Productivity applications Process control
Personal (PDAs),
Embedded
Workload
You are here
Services API
Internal Structure
Metrics
Policies / Mechanisms
Energy efficiency
Hardware Resources
Processor, Memory, Disks (?), Wireless
IR, Keyboard(?), Display(?), Mic
Speaker, Motors Sensors, GPS, Camera, Batteries
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Energy Efficiency Metrics

• Power consumption in watts (mW).
• Battery lifetime in hours (seconds, months).
• Energy consumption in Joules (mJ).
• Energy Delay
• Watt per megabyte

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Physics Basics

• Voltage is amount of energy transferred from a
  power source (e.g., battery) by the charge
  flowing through the circuit.
• Power is the rate of energy transfer
• Current is the rate of flow of charge in circuit

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Relationships

• Power (watts) Voltage (volts) Current (amps)
• Power (watts) Energy (Joules) / Time (sec)
• Energy (Joules) Power (watts) Time (sec)
• Energy (Joules) Voltage (volts) Charge
  (coulombs)
• Current (amps) Voltage (volts) / Resistance
  (ohms)

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Terminology and Symbols

• Concept Symbol Unit Abbrev.
• Current I amperes A
• Charge Q coulombs C
• Voltage V volts V
• Energy E joules J
• Power P watts W
• Resistance R ohms W

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Relationships

• Energy (Joules) Power (watts) Time (sec)
• E P t
• Power (watts) Voltage (volts) Current (amps)
• P V I
• Current (amps) Voltage (volts) / Resistance
  (ohms)
• I V / R

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Battery Terminology

• Primary (non-reusable) and Secondary
  (rechargable)
• Voltages Voc (initial no-load)V (operating
  voltage under load)Vcut (cut-off when cell is
  considered discharged - 80 of Voc)
• Capacity expressed in amp-hourstheoretical -
  based on amount of material in cellnominal -
  based on amp-hours obtained when discharged at
  constant current until Vcut

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Battery Terminology

• Discharge time - elapsed time until a fully
  charged cell reaches Vcut
• C rate - discharge current expressed in amps
  relative to nominal capacity
• example for a lead acid battery with nominal
  capacity of 5Ah, a discharge rate of C/20 means
  250mA of current.
• Specific energy - Watt-hours per kilogram
  delivered at constant discharge
• Energy density of cell - Watt-hours per liter

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Battery Technology
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Discharge Behavior
Discharge behavior of lithium-ion cell withVoc
3V and Vcut 1V
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Battery Stuff

• Diffusion At non-zero current, active material
  at electrode-electrolyte interface are consumed
  and replaced by new stuff moving in
• Polarization as current increases At high
  enough current, diffusion is unable to compensate
  for depletion at electrode and cell voltage drops
• Recovery (due to diffusion) when current decreased

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Ragone plot for different chemistries
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Pulsed Discharge

• Exploiting recovery ability to get more out of a
  battery
• Delivered specific energy can be increased by
  pulsed instead of constant discharge for a fixed
  power level.
• Chiasserini and Rao 99 - model analysis
• Is bursty better for battery lifetimes?
• Can durations of idle and busy states be
  optimized?

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Pulsed Discharge
Bipolarlead acid cell Pulse 3msRest 22ms
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Smart Batteries

• Part of Intel Power Initiative
• Embedded battery controller that can be
  controlled by OS.
• Interface
• Battery reports designed capacity, latest full
  charged capacity, remaining capacity.
• Warning levels can be set. User notifications

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Rethinking OS Design
Productivity applications Process control
Personal (PDAs),
Embedded
Workload
Services API
Internal Structure
Metrics
Policies / Mechanisms
Energy efficiency
You are here
Hardware Resources
Processor, Memory, Disks (?), Wireless
IR, Keyboard(?), Display(?), Mic
Speaker, Motors Sensors, GPS, Camera, Batteries
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System Organization
interrupts
Processor
Cache
Memory Bus
I/O Bridge
I/O Bus
Main Memory
Disk Controller
Graphics Controller
Network Interface
Graphics
Disk
Disk
Network
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Power Budgets
ave 18
interrupts
Processor
Cache
Memory Bus
I/O Bridge
I/O Bus
Main Memory
4-17 ave. 9 backlight 23
Disk Controller
Graphics Controller
Network Interface
Graphics
Disk
Disk
Network
Lorch95
appox 20
4-12 ave. 8
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Typical Notebook Power Budgets
(Color to 21W)
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• Harris 95

B/W
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DC-DC
HDD
Watts
4
video
mem
2
CPU
1993 notebook full power
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What are the Costs?Measured Power Consumption
(PalmPilot Pro - 1997 model)
Hotsync
Backlight
Memory intensive
CPU Event Loop (nilevents)
CPU Idle
Sleeping in cradle
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CPU/Memory

• Tiwari94
• 486DX2 Instr current
• (mA)
• NOP 276
• Load 428
• Store 522
• Register add 314
• cache miss 216

• Memory op current
• (mA)
• no access 5-77
• page hit 123
• page miss 248

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Intel Power Initiative Targets
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Power Budget Targets
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interrupts
Processor
Cache
Memory Bus
I/O Bridge
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I/O Bus
Main Memory
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Disk Controller
Graphics Controller
Network Interface
Graphics
Disk
Disk
Network
2- 3
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Intel targets
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Itsy Measurement Methodology

• Isense Vsense /.02
• Sampling rate 5000 per second

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Itsy Results
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PowerScope Flinn

• Statistical sampling approach
• Program counter/process (PC/PID) correlated
  current readings.
• Off-line analysis to generate profile
• Causality
• Goal is to assign energy costs to specific
  application events / program structure
• Mapped down to procedure level
• System-wide. Includes all processes, including
  kernel

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Experimental SetupData Gathering
Multimetersclock drivessampling at period of
1.6ms
Takescurrentsample -gt
InterruptcausesPC/PID sample to be buffered
-gtTrigger next sample
lt-TriggerProfilingcomputer
User-level daemonwrites to disk when buffer 7/8
full
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System Monitor Kernel Mods

• NetBSD
• recording of PC and PID
• fork(), exec(), exit() instrumented to record
  pathname associated with process
• new system calls to control profiling
• pscope_init(), pscope_start(), pscope_stop(),
  pscope_read() (user-level daemon, to disk)

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Energy Analyzer

• Voltage essentially constant, only current
  recorded.
• Each sample is binned into process bucket and
  procedure within process bucket.
• Energy calculated by summing each bucketE
  Vmeas S It Dt

n
t0
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Case Study

• Video applicationoriginal 12.1MB
• Step 1lossy compressionB 7MB, C 2.8MB
• Step 2 display size reduced from 320x240 to
  160x120Asmall 4.9MB, Csmall 1MB
• Step 3 WaveLAN put into standby mode when not
  used
• Step 4 Disk powered off

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Base case
Every optimization
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How to Reduce Energy Consumption?

• Energy S Poweri x Timei
• To reduce energy used for task
• Reduce power cost of power state ithrough better
  technology.

i e powerstates
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Opportunities for Lower Power through Technology

• Circuits
• Gated Clocks - disable functional units that are
  not in use for particular instruction
• Compile for
• Voltage Scaling
• For given circuit
• E is related to V2 and time, f(clockrate)
• Linear relationship between V and clockrate
• Ability of software to dynamically change?

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Displays

• Active Matrix LCDs
• 90 of backlight gets transmitted through the
  layers of display
• Possible future technologies
• Reflective displays use ambient light 1/50th
  energy of active matrix
• Field-emission displays uses an array of cathodes
  for each pixel instead of one gun as in CRT
  displays. Selective activation possible.

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How to Reduce Energy Consumption?

• Energy S Poweri x Timei
• To reduce energy used for task
• Reduce power cost of power state ithrough better
  technology.
• Reduce time spent in the higher cost power
  states.

i e powerstates
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Power Modes of HW Devices
High power cost
transition
Busy
?
Idle
Low power cost
transition
?
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How to Reduce Energy Consumption?

• Energy S Poweri x Timei
• To reduce energy used for task
• Reduce power cost of power state ithrough better
  technology.
• Reduce time spent in the higher cost power
  states.
• Amortize transition states, if significant.

i e powerstates
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StrongARM Processor Power Modes

• 160MHz microprocessor, 2 16kB caches on chip
• Normal active mode 450mW
• Idle mode 20mW, return to normal, no delay.
• Internal clocking stopped
• Sleep mode 150mW, return to normal 140ms
• Internal power to chip off, I/O circuitry remains
  powered, no state saved

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Rambus RDRAM Power Modes
Read/write transaction
Active 1.0x mW
100x ns
1.0x ns
PwrDown .01x mW
Nap 0.1x mW
0.1x ns
Standby 0.6x mW
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spinup
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Wireless LAN Power Modes
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Bluetooth
Listen every 1.28 sec.

• Freq Hop Radio
• nominal range 10 meters
• augmentable to 100 meters with power amplifier
• 721 kbits/sec
• Adaptive range-RSSI (received signal strength
  indicator)

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Rethinking OS Design
Productivity applications Process control
Personal (PDAs),
Embedded
Workload
Services API
Internal Structure
Metrics
Policies / Mechanisms
Energy efficiency
You are here
Hardware Resources
Processor, Memory, Disks (?), Wireless
IR, Keyboard(?), Display(?), Mic
Speaker, Motors Sensors, GPS, Camera, Batteries