Ghosts in the Machine: Interfaces for Better Power Management

1
Ghosts in the MachineInterfaces for Better
Power Management

• Manish Anand
• Edmund B. Nightingale
• Jason Flinn

Electrical Engineering and Computer Science
Dept. University of Michigan
2
Need for improved power management

• Capabilities of mobile handheld devices improving
  rapidly
• Wireless connectivity
• Storage capacity
• Battery capacity improving slowly
• I/O devices decrease handheld battery lifetime by
  60
• OEMs provide techniques to reduce battery
  consumption by severely compromising the
  performance
• Careful power management needed

3
Tradeoff between energy and performance

• Goals of power management involve
• Lower energy usage
• Lower interactive response time (performance)

Good performance
Extended battery life
Inherent tradeoff exists between performance and
energy conservation
4
Current device power management
more
less
better performance and energy conservation
MODULARITY
simplicity

• Problem Device-centric strategies do not
  consider the operating environment contexts, such
  as
• Base power of mobile computer
• Activity of other devices
• Application intent
• Our Approach Provide an infrastructure to
  expose additional information about operating
  environment to both applications and devices

5
Goals and contributions

• Goals
• Improve both performance and energy
• Enable cross-device optimization
• Allow end users to specify their relative
  priorities for performance and energy
  conservation
• Contributions Simple interfaces to expose
  additional contexts
• Power manager for querying device state
• Self-tuning power management (STPM) modules for
  disk, n/w
• Ghost hints for cross-device power management

6
Outline

• Motivation and background
• Overview of our solution
• Interfaces for better power management
• Adaptive caching
• Evaluation
• Related work and conclusions

7
Review power management

• Network Device 802.11b wireless
• CAM Continuously Active Mode
• PSM Power Saving Mode
• power usage reduced by 70 to 80
• - delay proportional to polling period (100 ms)
• Disk Device IBM/Hitachi microdrive
• Active
• Low Power Idle / Standby
• power usage reduced by 55 / 90
• - transition time of 300ms / 800ms

8
Impact of power management

• Assumption Fetching data from local storage is
  less costly

• Local data access could be more expensive for
  small files
• Break-even point is dependent on the state of the
  device
• Simple adaptation through cache hierarchy

9
Limitation of adaptation

• Example Browser fetching many small images
• For single image fetching from network is correct
• For many images fetching from disk is correct
• Amortized transition cost over many reads
• Problem Reactive adaptive caching does not help
• Disk continues to be in standby state
• Solution Apps disclose accesses that might have
  been
• We use ghost hints to implement this

10
Outline

• Motivation and background
• Overview of our solution
• Interfaces for better power management
• Adaptive caching
• Evaluation
• Related work and conclusions

11
Energy-aware architecture
Applications
Cache Manager
Data Access Decision
Device Characteristics Current State
Ghost Hints
Ghost Hints
Network Hints
STPM Network
STPM Disk
Power Manager
MobiCom 03
Mode Transitions
Mode Transitions
Notify Network State Change
Notify Disk State Change
Network Device Driver
Disk Device Driver
Operating System
12
Power manager

• Central repository that maintains information
  about
• State of the device
• Performance characteristics of I/O devices
• Energy characteristics of I/O devices
• Power manager interface
• 3 calls for devices register, deregister and
  notify
• 5 calls for applications 4 to query device
  state and characteristics, 1 for registering
  callbacks

13
Self-tuning power management

• STPM modules consider
• Application access patterns
• Base power of mobile computer
• Energy and performance characteristics of the
  device
• Relative priority of performance and energy
  conservation
• STPM modules decide
• Time to transition to high power state
• Time to transition to low power state

Energy conservation
Performance
0
100
knob
14
Disk power management with STPM

• Transition to power saving mode employs
• Break-even heuristic
• Disk is likely to remain idle if it has been idle
  for a period
• Incorporate the STPM principles
• Transition to active mode employs both
• Reactive strategy
• Transition on a request
• Proactive strategy
• Use ghost hints to transition even with no device
  access

15
Ghost hints
Ghost Hint
Ghost Hint
Break-even Threshold
state change on overflow
Expended Energy between hints

• Ghost hints contain opportunity cost in terms of
  time and energy
• Issued by the application, when a poor I/O path
  is chosen due to inappropriate device state
• Decrements energy cost of being active between
  hints
• Transitions to high performance state on an
  overflow

16
Cache manager
PowerManager
device state
Applications
CacheStatus?
Cache Manager
ghost hint
network better
n/w fetch
STPMModule
PutData
data
flush
Disk

• Keeps the meta-data information in-memory hash
  table
• Determines the weighted cost of going to network
  or disk
• Maintains a write queue and delays writes to the
  disk

17
Outline

• Motivation and background
• Overview of our solution
• Interfaces for better power management
• Adaptive caching
• Evaluation
• Related work and conclusions

18
Evaluation

• Goals
• Effect of cache managers adaptive, cross-device
  strategy
• Influence of global knob
• Benefits of ghost hints
• Client iPAQ handheld with Cisco 350 wireless
  card and IBM/Hitachi microdrive
• Test applications
• Email-sync - recorded e-mail activity
• Dillo web browser with wwwoffle - Berkeley web
  traces

19
Effectiveness of energy-aware caching

• Equal priority for performance and energy
• Average response time is reduced by 27 to 42
• Total energy usage is reduced by 5 to 9

20
E-mail and web with no ghost hints
E-mail
Web

• STPM modules base their decision solely on device
  accesses
• No change for knob value less than 95

21
Importance of ghost hints
E-mail
Web

• Email Substantial benefit when performance is
  high priority
• Web Less likely to see run of accesses clustered
  together

22
Importance of ghost hints
Web with full cache

• Ghost hints show a positive effect on the system
• Ghost hints yield substantial benefit for some
  workloads, and do no harm in the situations where
  they seem ineffective

23
Related work

• We are the first system to efficiently access
  data on multiple I/O
• Burstiness for energy efficiency (Weissel 02,
  Papathanasiou 03, Heath 02)
• ACPI similar to power manager but too complex,
  does not disclose device characteristics
• Wake on Wireless (Shih 02)
• OS level power management (ECOSystem, Odyssey)

24
Conclusions

• Contributions Improved power management
  interfaces
• Power manager for querying device state
• STPM modules for disk, network
• Ghost hints for cross-device power management
• Current work
• Modification of cached data
• Multiple network and storage devices
• Feedback loop to dynamically set the global knob

25
Ghosts in the MachineInterfaces for Better
Power Management

• Manish Anand
• Edmund B. Nightingale
• Jason Flinn

Electrical Engineering and Computer Science
Dept. University of Michigan
26
Dynamic Power Management

• The energy used by the I/O devices can be
  prohibitive, without power management

• Battery lifetime is decreased by 60 without
  power management
• Drawback of using power management
• Network device
• Delay proportional to the polling period
• Disk device
• Transition cost to switch to active mode (300ms
  to 800ms)

27
Network access
Disk access
Applications
STPM disk hints
ghost hints
Disk
ghostrealized
disk spin-up
STPM network hints
Network
PSMswitch
CAMswitch
28
Network Power Management with STPM

• STPM network switches from PSM to CAM when
• Application specifies max delay
• Disclosed transfer size break-even size
• Many forthcoming transfers are likely
• To predict forthcoming transfers STPM network
  generates an empirical distribution of run lengths

Transfers
150 ms
150 ms
150 ms
Run
Run
Run
Run