Developing Power Friendly Mobile Applications

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Developing Power Friendly Mobile Applications
San Francisco 2005 February 8-9

• Dale Taylor
• Application Engineer
• Intel Corporation
• SSG Software Solutions Group

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Agenda

• Battery Awareness
• Optimizing for performance and battery life
• Being a good Power Citizen
• On Now power management
• System power status
• System power management events
• System sleep criteria
• PeekMessage and Windows
• Windows concerns
• Alternatives code structure
• Alternatives - GetMessage

• Power Evaluation Tool
• Understanding the tool
• Test plan and usage considerations
• How you can get the tool
• Case study PeekMessage vs. GetMessage
• Summary

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Battery Awareness

• Mobile users expect performance and demand longer
  battery life.
• This is not just a hardware issue.
• We as application developers must be good power
  citizens.
• Application decisions effect power consumption on
  a platform level.
• Next Some examples

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Optimizing for Battery Life

• CD drive playing music
• When reading a song, read it all, caching it
  then the drive can spin down
• Battery savings, and other benefits!
• If theres a read error, time to correct without
  skipping
• Enhanced Intel(R) SpeedStepTM Technology cant
  lower processor frequency if your app is keeping
  the system unnecessarily busy
• Avoid spin wait loops (PeekMessage)
• Optimizing for performance and battery life are
  mutually beneficial
• Tune for performance first

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Being a Good Power Citizen

• Understand you are a part of the whole.
• Insight into your applications power consumption
  and performance is your first step.
• Know how your application affects the system,
  devices used, and hardware requirements.
• Pay attention to System messages.
• Standby/Hibernation the lid is going down
• No time to ask the user, make the best choice
• Provide user options if necessary, not in
  real-time
• Use available APIs.
• Lets look at some APIs and examples.

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On Now Power Management

• Supported by most current hardware
• Always on (sleeping), ready for use
• Instantly available
• Controlled by user power policy
• Benefits
• Reduced power consumption
• Fast startup performance - no boot up
• Enables automated tasks, even when off
• Manages power on a per-device basis

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Power Settings IBM T41
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Windows Power Management
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New Power Scheme IBM T41
Medium is Adaptive
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System Power Status

• Indicates source of Power (AC vs. DC), battery
  life, and other information.
• GetSystemPowerStatus() function
• Returns SYSTEM_POWER_STATUS BYTE
  ACLineStatusBYTE BatteryFlagBYTE
  BatteryLifePercentBYTE Reserved1DWORD
  BatteryLifeTimeDWORD BatteryFullLifeTime
• The system broadcasts
• PBT_APMPOWERSTATUSCHANGE
• to all applications when power status changes.
• Applications should also use GetDevicePowerState()
  

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System Power Management Events

• WM_POWERBROADCAST Message
• Used to broadcast to an application notification
  of power-management events
• Change in power supply or battery status
• System is suspending
• Power events can help you prepare for changes in
  mode and not lose data by saving and closing
  files.
• Similar notification happens at restore

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System Sleep Criteria

• When the system determines there is no activity,
  it will go to sleep.
• Certain activities cannot be detected
  Presentations but you do not want the screen to
  go blank.
• SetThreadExecutionState() notifies the system
  that your application is busy.
• You must remember to clear the flag when done!

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How Do We Use This Knowledge?

• Weve seen the APIs, lets apply them to a real
  application.
• As program developers, we all need to check for
  messages.
• Message loops are primary methods of
  communicating with Windows.
• PeekMessage has been around since Windows 3.1.

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PeekMessage and Windows Concerns

• MSDN has the following discussion
• An application should use a PeekMessage() loop
  for as little time as possible. To be compatible
  with battery-powered computers and to optimize
  system performance, every Windows-based
  application should inform Windows that it is idle
  as soon and as often as possible. An application
  is idle when the GetMessage() or WaitMessage()
  function is called and no messages are waiting in
  the application's message queue.
• Common message pump method
• A power-friendly PeekMessage() loop exits when
  background processing is complete because, while
  an application is in a PeekMessage() loop,
  Windows cannot go idle.

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Wise PeekMessage Usage

• Another method for performing idle processing in
  an application involves embedding a message loop
  in one of your functions. This message loop is
  very similar to MFC's main message loop, found in
  CWinThreadRun, thus with MFC it must perform
  many of the same functions as the main message
  loop.
• while ( bDoingBackgroundProcessing )
• MSG msg
• while ( PeekMessage( msg, NULL, 0, 0,
  PM_NOREMOVE ) ) if ( !PumpMessage( ) )
• bDoingBackgroundProcessing FALSE
  PostQuitMessage( )
• break
• // Let MFC do its idle processing
• LONG lIdle 0
• while ( AfxGetApp()-gtOnIdle(lIdle ) )
• // Perform some background processing here,
  call OnIdle

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Controlling Sleep Time Directly

• The WaitMessage() function is one of the simplest
  Windows functions. It simply puts the current
  task to sleep. It is normally used by
  applications that perform background processing
  to control precisely when their application goes
  to sleep. Most applications that do background
  processing implement a PeekMessage() loop in the
  following manner
• if (PeekMessage(...) ! NULL)
• // Translate dispatch the message
• else if (there is background processing to
  perform)
• // Perform background processing
• else
• // No background processing, no messages - go
  to sleep
• WaitMessage(...)

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Case Study PeekMessage vs. GetMessage

• A simple application that can be compiled to run
  using PeekMessage or GetMessage.
• PeeksCalled TRUE for peeks, FALSE for gets
• The next slide has the source code for both the
  PeekMessage and GetMessage loops.
• Other than the actual Message call, the loops are
  identical.
• I left adding the ability to vary the percentage
  of time spent using each call as an exercise for
  the curious.
• Next To evaluate the real differences, well
  need some tools.

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PeekMessage vs. GetMessage (C-code)
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Valuable Tools and Looking at PeekMessage

• Useful Tools when evaluating an application
• Intel Vtune Performance Analyzer
• Microsoft Performance Monitor (Perfmon)
• Intel Mobile Computing Technology Kit
• Power Evaluation Tool
• Battery Power Information Tool
• Lets take a look at PeekMessage and dive into
  optimizing an application that uses it.

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Power Evaluation Tool

• Command line tool that automates the process of
  determining the total power consumption and net
  power consumption of an application running on
  battery-operated systems.
• Can be used to compare
• Two versions of an application on the same
  system.
• The average power consumption of the same
  workload on two different platforms.
• Two different power management settings (same
  workload and platform).

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Test Plan Considerations

• Perform a baseline test to identify the update
  granularity of the battery power information
  check power management options.
• Extensive testing has shown large variations
  between models and makes (many within 1 to 30
  sec, others longer than 100sec)
• The execution time of the workload needs to be
  long enough for the power evaluation tool to
  generate results.
• Each test should be performed at least three
  times.
• Consider running tests of different length by
  concatenating multiple copies of the original
  test.
• Increased accuracy of average power consumption
  for longer tests

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Evaluating PeekMessage

• Power Evaluation Tool
• Runs in CMD window
• Produces a log file
• Several options
• Look at Results

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PeekMessage vs- GetMessage Results 1
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PeekMessage vs- GetMessage Results 2
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PeekMessage vs- GetMessage Results 3
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PeekMessage vs- GetMessage Results 4
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Summary

• Be battery-aware.
• Be a good software citizen.
• Use the available APIs and tools.
• Optimize your application for performance and
  battery usage.
• Use GetMessage and WaitMessage
• Plan for and handle system messages.
• Low power situations
• Hibernation
• Power Evaluation Tools are available to help.

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Questions?
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References

• KB93915 - MFC Application Idle Processing and
  Power Consumption
• KB40669 - Howto Post Frequent Messages Within an
  Application
• http//intelmktg.com/sdd/ for access to the
  Mobilized Software Resource Kit and the Power
  Evaluation Tool
• Not Off the Hook Why App developers are an
  important link in the power-management chain.
  http//www.mobilizedsoftware.com/showArticle.jhtml
  ?articleId46200579
• DVD/CD Rendering Optimizing for Power on Mobile
  Platforms http//www.devx.com/Intel/Article/21546/
  2217?pftrue

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Intel Software Developer DispatchMobilized
Software Resource Kit

• MSI R2 Kit Content Highlight
• Best-practices content and code samples to plan
  and design mobile applications (39 collateral)
• Offline Capability
• Application Connectivity
• Power performance
• Multiple Platform support
• Mobile Security Manageability
• Intel Mobile Computing Technology Kit (MCTK) 6
  tools, utilities and APIs for Intel Centrino
  mobile technology-based systems
• Performance optimization techniques for Intel
  XScale technology-based mobile devices

• Intel Software Development Products for Windows
  Evaluation licenses
• How to Subscribe and Order
• Developers visit ISDD web page to subscribe
• ASMO http//intelmktg.com/sdd/
• Russia http//apps.apnet.ru/sites/ids_subscriptio
  n/msi2/web_1D.htm
• APAC http//www.softwaredispatch.net/site/cn/reg.
  jsp
• EMEA Using ASMO site
• While At this conference, visit the Intel booth
  and sign up

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Intels Software Solutions Focus

• A worldwide consulting and services RD
  organization
• Specializing in distributed solutions and
  enterprise infrastructure
• Focus Accelerating technology adoption
• Development of practices
• Proving thru end user deployments
• Sharing best known methods with industry