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    SubjectAttempted summary of suspend-blockers LKML thread
    Rushing in where angels fear to tread...

    I had been quite happily ignoring the suspend-blockers controversy.
    However, now that I have signed up for the Linaro project that involves
    embedded battery-powered devices, I find that ignorance is no longer
    bliss. I have therefore reviewed much of the suspend-blocker/wakelock
    material, but have not yet seen a clear exposition of the requirements
    that suspend blockers are supposed to meet. This email is a attempt
    to present the requirements, based on my interpretation of the LKML

    Please note that I am not proposing a solution that meets these
    requirements, nor am I attempting to judge the various proposed solutions.
    In fact, I am not even trying to judge whether the requirements are
    optimal, or even whether or not they make sense at all. My only goal
    at the moment is to improve my understanding of what the Android folks'
    requirements are. That said, I do include example mechanisms as needed to
    clarify the meaning of the requirements. This should not be interpreted
    as a preference for any given example mechanism.

    But first I am going to look at nomenclature, as it appears to me that
    at least some of the flamage was due to conflicting definitions. Following
    that, the requirements, nice-to-haves, apparent non-requirements,
    an example power-optimized applications, and finally a brief look
    at other applications.

    Donning the asbestos suit, the one with the tungsten pinstripes...

    Thanx, Paul



    o "Ill-behaved application" AKA "untrusted application" AKA
    "crappy application". The Android guys seem to be thinking in
    terms of applications that are well-designed and well-implemented
    in general, but which do not take power consumption or battery
    life into account. Examples include applications designed for
    AC-powered PCs. Many other people seemed to instead be thinking
    in terms of an ill-conceived or useless application, perhaps
    exemplified by "bouncing cows".

    Assuming I have correctly guessed what the Android guys were
    thinking of, perhaps "power-naive applications" would be a
    better description, which I will use until someone convinces
    me otherwise.

    o "Power-aware application" are applications that are permitted
    to acquire suspend blockers on Android. Verion 8 of the
    suspend-blocker patch seems to use group permissions to determine
    which applications are classified as power aware.

    More generally, power-aware applications seem to be those that
    have permission to exert some control over the system's
    power state.

    o Oddly enough, "power-optimized applications" were not discussed.
    See "POWER-OPTIMIZED APPLICATIONS" below for a brief introduction.
    The short version is that power-optimized applications are those
    power-aware applications that have been aggressively tuned to
    reduce power consumption.


    o Reduce the system's power consumption in order to (1) extend
    battery life and (2) preserve state until AC power can be obtained.

    o It is necessary to be able to use power-naive applications.
    Many of these applications were designed for use in PC platforms
    where power consumption has historically not been of great
    concern, due to either (1) the availability of AC power or (2)
    relatively undemanding laptop battery-lifetime expectations. The
    system must be capable of running these power-naive applications
    without requiring that these applications be modified, and must
    be capable of reasonable power efficiency even when power-naive
    applications are available.

    o If the display is powered off, there is no need to run any
    application whose only effect is to update the display.

    Although one could simply block such an application when it next
    tries to access the display, it appears that it is highly
    desirable that the application also be prevented from
    consuming power computing anything that will not be displayed.
    Furthermore, whatever mechanism is used must operate on
    power-naive applications that do not use blocking system calls.

    o In order to avoid overrunning hardware and/or kernel buffers,
    input events must be delivered to the corresponding application
    in a timely fashion. The application might or might not be
    required to actually process the events in a timely fashion,
    depending on the specific application.

    In particular, if user input that would prevent the system
    from entering a low-power state is received while the system is
    transitioning into a low-power state, the system must transition
    back out of the low-power state so that it can hand the user
    input off to the corresponding application.

    o If a power-aware application receives user input, then that
    application must be given the opportunity to process that

    o A power-aware application must be able to efficiently communicate
    its needs to the system, so that such communication can be
    performed on hot code paths. Communication via open() and
    close() is considered too slow, but communication via ioctl()
    is acceptable.

    o Power-naive applications must be prohibited from controlling
    the system power state. One acceptable approach is through
    use of group permissions on a special power-control device.

    o Statistics of the power-control actions taken by power-aware
    applications must be provided, and must be keyed off of program

    o Power-aware applications can make use of power-naive infrastructure.
    This means that a power-aware application must have some way,
    whether explicit or implicit, to ensure that any power-naive
    infrastructure is permitted to run when a power-aware application
    needs it to run.

    o When a power-aware application is preventing the system from
    shutting down, and is also waiting on a power-naive application,
    the power-aware application must set a timeout to handle
    the possibility that the power-naive application might halt
    or otherwise fail. (Such timeouts are also used to limit the
    number of kernel modifications required.)

    o If no power-aware or power-optimized application are indicating
    a need for the system to remain operating, the system is permitted
    (even encouraged!) to suspend all execution, even if power-naive
    applications are runnable. (This requirement did appear to be
    somewhat controversial.)

    o Transition to low-power state must be efficient. In particular,
    methods based on repeated attempts to suspend are considered to
    be too inefficient to be useful.

    o Individual peripherals and CPUs must still use standard
    power-conservation measures, for example, transitioning CPUs into
    low-power states on idle and powering down peripheral devices
    and hardware accelerators that have not been recently used.

    o The API that controls the system power state must be
    accessible both from Android's Java replacement, from
    userland C code, and from kernel C code (both process
    level and irq code, but not NMI handlers).

    o Any initialization of the API that controls the system power
    state must be unconditional, so as to be free from failure.
    (I don't currently understand how this relates, probably due to
    my current insufficient understanding of the proposed patch set.)

    o The API that controls the system power state must operate
    correctly on SMP systems of modest size. (My guess is that
    "modest" means up to four CPUs, maybe up to eight CPUs.)

    o Any QoS-based solution must take display and user-input
    state into account. In other words, the QoS must be
    expressed as a function of the display and the user-input

    o Transitioning to extremely low power states requires saving
    and restoring DRAM and/or cache SRAM state, which in itself
    consumes significant energy. The power savings must therefore
    be balanced against the energy consumed in the state

    o The current Android userspace API must be supported in order
    to support existing device software.


    o It would be nice to be able to identify power-naive applications
    that never were depended on by power-aware applications. This
    particular class of power-naive applications could be shut down
    when the screen blanks even if some power-aware application
    was preventing the system from powering down. (I am guessing
    at this one based on the momentary excitement that cgroup
    freezing raised in the Android folks. Of course, this approach
    requires a reliable way to identify such applications.)


    o Transitioning to low-power states need not be highly scalable,
    as evidenced by the global locks. (If you believe that this
    will in fact be required, please provide a use case. But please
    understand that I do know something about scalability trends,
    but also about uses for transistors beyond more cores.)


    A typical power-optimized application manually controls the power state
    of many separately controlled hardware subsystems to minimize power
    consumption. Such optimization normally requires an understanding
    of the hardware and of the full system's workload: strangely enough,
    concurrently running two separately power-optimized applications often
    does -not- result in a power-optimized system. Such optimization also
    requires knowledge of what the application will be doing in the future,
    so that needed hardware subsystems can be proactively powered up just
    when the application will need them. This is especially important when
    powering down cache SRAMS or banks of main memory, because such components
    take significant time (and consume significant energy) when preparing them
    to be powered off and when restoring their state after powering them on.

    Consider an MP3 player as an example. Such a player will periodically
    read MP3-encoded data from flash memory, decode it (possibly using
    hardware acceleration), and place the resulting audio data into main
    memory. Different systems have different ways of getting the data from
    main memory to the audio output device, but let's assume that the audio
    output device consumes data at a predictable rate such that the software
    can use timers to schedule refilling of the device's output buffer.
    The timer duration will of course need to allow for the time required to
    power up the CPU and L2 cache. The timer can be allowed to happen too
    soon, albeit with a battery-lifetime penalty, but cannot be permitted
    to happen too late, as this will cause "skips" in the playback.

    If MP3 playback is the only application running in the system, things
    are quite easy. We calculate when the audio output device will empty
    its buffer, allow a few milliseconds to power up the needed hardware,
    and set a timer accordingly. Because modern audio output devices have
    buffers that can handle roughly a second's worth of output, it is well
    worthwhile to spend the few milliseconds required to flush the cache
    SRAMS in order to put the system into an extremely low power state over
    the several hundred milliseconds of playback.

    Now suppose that this device is also recording audio -- perhaps the
    device is being used to monitor an area for noise pollution, and the
    user is also using the device to play music via earphones. The audio
    input process will be the inverse of the audio output process: the
    microphone data will fill a data buffer, which must be collected into
    DRAM, then encoded (perhaps again via MP3) and stored into flash.
    It would be easy to create an optimal application for audio input,
    but running this optimal audio input program concurrently with the
    optimal audio playback program would not necessarily result in
    a power-optimized combination. This lack of optimality is due to
    the fact that the input and output programs would each burn power
    separately powering down and up. In contrast, an optimal solution
    would align the input and output programs' timers so that a single
    power-down/power-up event would cover both programs' processing.
    This would trade off optimal processing of each (for example,
    by draining the input buffer before it was full) in order to attain
    global optimality (by sharing power-down/power-up overhead).

    There are a number of ways to acheive this:

    1. Making the kernel group timers that occur at roughly the same
    time, as has been discussed on this list many times. This can
    work in many cases, but can be problematic in the audio example,
    due to the presence of hard deadlines.

    2. Write the programs to be aware of each other, so that each
    adjusts its behavior when the other is present. This seems
    to be current practice in the battery-powered embedded arena,
    but is quite complex, sensitive to both hardware configuration
    and software behavior, and requires that all combinations of
    programs be anticipated by the designer -- which can be a serious
    disadvantage given today's app stores.

    3. Use new features such as range timers, so that each program
    can indicate both its preference and the degree of flexibility
    that it can tolerate. This also works in some cases, but as
    far as I know, current proposals do not allow the kernel to take
    power-consumption penalties into account.

    4. Use of hardware facilities that allow DMA to be scheduled across
    time. This would allow the CPU to be turned on only for
    decode/encode operations. I am under the impression that this
    sort of time-based DMA hardware does exist in the embedded space
    and that it is actually used for this purpose.

    5. Your favorite solution here.

    Whatever solution is chosen, the key point to keep in mind is that
    running power-optimized applications in combination does -not- result
    in optimal system behavior.


    GPS application that silently displays position.

    There is no point in this application consuming CPU cycles
    or in powering up the GPS hardware unless the display is
    active. Such an application could be handled by the Android
    suspend-blocker proposal. Of course, such an application could
    also periodically poll the display, shutting itself down if the
    display is inactive. In this case, it would also need to have
    some way to be reactivated when the display comes back on.

    GPS application that alerts the user when a given location is reached.

    This application should presumably run even when the display
    is powered down due to input timeout. The question of whether
    or not it should continue running when the device is powered
    off is an interesting one that would be likely to spark much
    spirited discussion. Regardless of the answer to this question,
    the GPS application would hopefully run very intermittently,
    adjusting the delay interval based on the device's velocity and
    distance from the location in question.

    I don't know enough about GPS hardware to say under what
    circumstances the GPS hardware itself should be powered off.
    However, my experience indicates that it takes significant
    time for the GPS hardware to get a position fix after being
    powered on, so presumably this decision would also be based
    on device velocity and distance from the location in question.

    Assuming that the application can run only intermittently,
    suspend blockers would work reasonably well for this use case.
    If the application needed to run continuously, battery life
    would be quite short regardless of the approach used.

    MP3 playback.

    This requires a power-aware (and preferably a power-optimized)
    application. Because the CPU need only run intermittently,
    suspend blockers can handle this use case. Presumably switching
    the device off would halt playback.

    Bouncing cows.

    This can work with a power-naive application that is shut down
    whenever the display is powered off or the device is switched off,
    similar to the GPS application that silently displays position.

     \ /
      Last update: 2010-07-31 20:01    [W:0.047 / U:59.192 seconds]
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