Message-ID: <20100807002543.GP2432@linux.vnet.ibm.com>
Date:	Fri, 6 Aug 2010 17:25:43 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	david@...g.hm
Cc:	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	arve@...roid.com, mjg59@...f.ucam.org, pavel@....cz,
	florian@...kler.org, rjw@...k.pl, stern@...land.harvard.edu,
	swetland@...gle.com, peterz@...radead.org, tglx@...utronix.de,
	alan@...rguk.ukuu.org.uk, menage@...gle.com, david-b@...bell.net,
	James.Bottomley@...e.de, tytso@....edu, arjan@...radead.org,
	swmike@....pp.se, galibert@...ox.com, dipankar@...ibm.com
Subject: Re: Attempted summary of suspend-blockers LKML thread, take three

On Fri, Aug 06, 2010 at 04:59:54PM -0700, david@...g.hm wrote:
> one other nice-to-have (or conflicting requirement, depending on
> your point of view), and I think one of the big things causing
> people to dislike wavelocks, is the desire to not have to modify
> applications to have them work with the infrastructure.
> 
> you sort of touch on this when you say that power oblivious
> applications need to be able to be intergrated, but it goes beyond
> what that statement implies.
> 
> with wavelocka, even power optimized applications need to be
> modified, or the system may halt them at any time.

Yes, I believe that Android would require most power-optimized application
be modified to use wakelocks.  But power-optimized applications require
so much tweaking that the addition of suspend blockers (or whatever
other power-control mechanism) is pretty much a non-issue by comparison.

And the number of power-optimized applications should be small, which,
as you noted earlier, greatly eases the overall pain of modifying them.

> one thing that has been very clear over the years is that if an API
> only exists on Linux, no matter how good it is, most application
> developers won't use it.

Many application developers do indeed value portability.  But this in turn
means that most application developers will not be writing power-optimized
applications, because the process of power-optimization significantly
degrades portability.  Just as does the process of performance tuning,
beyond a certain point.

Of course, the reason that application developers value portability is
that this is one way to gain large unit volumes.  Another way to gain
large unit volumes is to code for a very popular platform, which explains
the large number of apps that run only on iPhone, Android, and Windows.

And in my experience, developers who have decided to commit to a single
platform are usually not at all shy about exploiting special facilities
of that platform to the fullest.

> In this case we are in an even worse situation, it's not only
> specific to Linux, it's specific to a subset of Linux systems, and
> not using it will cause no problems most of the time.

It seems to me that PM-driving and power-optimized applications are going
to be highly platform specific, whether that platform be Linux or some
other OS.

> now, android is betting that the apps are all developed specifically
> for the android from scratch, so having a different API is
> acceptable, even if it cuts them off from the rest of the *nix
> applications. For a phone this is not neccessarily an unreasonable
> stance, but as Android moves into the spaces where normal
> applications are in use (netbooks and tablets), this becomes a much
> shakier stance to take.

There certainly does seem to be a large and growing number of Android
apps, so I might be reluctant to bet against them.  And the Android guys
appear to be making another bet as well -- that almost all applications
will be power-oblivious.  Their design handles this rather well, given
that such applications need not worry about special power-control
features.

							Thanx, Paul

> David Lang
> 
> On Fri, 6 Aug 2010, Paul E. McKenney wrote:
> 
> >Date: Fri, 6 Aug 2010 15:54:53 -0700
> >From: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> >To: linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
> >Cc: arve@...roid.com, mjg59@...f.ucam.org, pavel@....cz, florian@...kler.org,
> >    rjw@...k.pl, stern@...land.harvard.edu, swetland@...gle.com,
> >    peterz@...radead.org, tglx@...utronix.de, alan@...rguk.ukuu.org.uk,
> >    david@...g.hm, menage@...gle.com, david-b@...bell.net,
> >    James.Bottomley@...e.de, tytso@....edu, arjan@...radead.org,
> >    swmike@....pp.se, galibert@...ox.com, dipankar@...ibm.com
> >Subject: Attempted summary of suspend-blockers LKML thread, take three
> >
> >Final report from this particular angel-free zone for the time being...
> >
> >This is the third and final version of my Android requirements list
> >(last version available at http://lkml.org/lkml/2010/8/4/409).  Again,
> >this email is an attempt to present the Android guys' requirements, based
> >on my interpretation of LKML discussions.  This past week's discussion
> >was quite productive, and I thank everyone who took part.
> >
> >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 our collective understanding of what the Android
> >folks' requirements are.  That said, I do discuss example mechanisms
> >where needed to clarify the meaning of the requirements.  This should
> >not be interpreted as a preference for any given example mechanism.
> >
> >							Thanx, Paul
> >
> >------------------------------------------------------------------------
> >
> >CONTENTS
> >
> >o	DEFINITIONS
> >o	CATEGORIES OF APPLICATION BEHAVIOR
> >o	REQUIREMENTS
> >o	NICE-TO-HAVES
> >o	APPARENT NON-REQUIREMENTS
> >o	SUGGESTED USAGE
> >o	POWER-OPTIMIZED APPLICATIONS
> >o	OTHER EXAMPLE APPLICATIONS
> >o	ACKNOWLEDGMENTS
> >
> >
> >DEFINITIONS
> >
> >These have been updated based on LKML and linux-pm discussions.  The names
> >are probably still sub-optimal, but incremental progress is nevertheless
> >a very good thing.
> >
> >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
> >	externally powered PCs.  Many other people seemed to instead be
> >	thinking in terms of an ill-conceived or useless application,
> >	perhaps exemplified by "bouncing cows".
> >
> >	This document uses "power-oblivious applications" to mean
> >	applications that are well-designed and well-implemented in in
> >	general, but which do not take power consumption or battery life
> >	into account.
> >
> >o	"PM-driving 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 processes are classified as power aware.	Android uses a
> >	user-level daemon to classify app-store apps as PM-driving or not.
> >	More generally, PM-driving applications are those that have
> >	permission to exert some control over the system's sleep state.
> >
> >	Note that an application might be power-oblivious on one
> >	Android device and PM-driving on another, depending on whether
> >	the user allows that application to acquire suspend blockers.
> >	The classification might even change over time.  For example,
> >	a user might give an application PM-driving status initially,
> >	but change his or her mind after some experience with that
> >	application.
> >
> >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
> >	PM-driving applications that have been aggressively tuned to
> >	reduce power consumption.
> >
> >o	Individual devices in an embedded system can enter "device
> >	low-power states" when not in use.
> >
> >o	The system as a whole can enter a "system sleep state" when
> >	the system as a whole is not in use.  Suspend blockers are about
> >	system sleep states rather than device low-power states.
> >
> >o	There was much discussion of "idle" (AKA "deep idle") and
> >	"suspend" (as in current Linux-kernel suspend operations).
> >	The following characteristics distinguish "idle" from "suspend":
> >
> >	1.	Idle states are entered by a given CPU only there are no
> >		runnable tasks for that CPU.  In contrast, opportunistic
> >		suspend can halt the entire system even when there
> >		are tasks that are ready, willing, and able to run.
> >		(But please note that this might not apply to real-time
> >		tasks.)
> >
> >		Freezing of subsets of applications is somewhat related
> >		to the idle/suspend discussion, but is covered in a
> >		later section of this document.
> >
> >	2.	There can be a set of input events that do not bring
> >		the system out of suspend, but which would bring the
> >		system out of idle.  Exactly which events are in this
> >		set depends both on hardware capabilities and on the
> >		platform/application policy.  For example, on one of
> >		the Android-based smartphones, touchscreen input is
> >		ignored when the system is suspended, but is handled
> >		when idle.
> >
> >	3.	The system comes out of idle when a timer expires.  In
> >		contrast, timers might or might not bring the system
> >		out of suspend, depending on both hardware capabilities
> >		and platform/application policy.
> >
> >
> >CATEGORIES OF APPLICATION BEHAVIOR
> >
> >There are a number of categories of application behavior with respect
> >to power management and energy efficiency.  These can be classified via
> >the following questions:  (1) What degree of control is an application
> >permitted over its own behavior?  (2) What degree of control is an
> >application permitted over the power state of individual devices within
> >the system?  (3) What degree of control is an application permitted
> >over the system sleep state?  (4) To what degree has the application
> >been tuned to reduce its power consumption, either in isolation or in
> >conjunction with other applications that might be running concurrently?
> >
> >These categories are discussed below.
> >
> >o	What degree of control is an application permitted over its
> >	own behavior?
> >
> >	The Linux kernel already has many controls over application
> >	behavior:
> >
> >	o	the CAP_ capabilities from include/linux/capability.h.
> >
> >	o	Processes can be assigned to multiple groups, allowing
> >		them privileged access to portions of the filesystem.
> >
> >	o	The chroot() system call limits a process's access to the
> >		specified subtree of the filesystem.
> >
> >	o	The ulimit facility can limit CPU consumption, number
> >		of processes, memory, etc. on a per-user basis.  The
> >		rlimit facility has similar effects on a per-process
> >		basis.
> >
> >	o	The mlockall() system call provides privileged access
> >		to memory, avoiding page-fault overhead.
> >
> >	But more relevant to this discussion, real-time processes are
> >	permitted a much higher degree of control over the timing of their
> >	execution than are non-real-time processes.  However, suspending
> >	the system destroys any pretense of offering real-time guarantees,
> >	which might explain much of the annoyance towards suspend blockers
> >	from the real-time and scheduler folks.  For but one example,
> >	Peter Zijlstra suggested that he would merge a patch that acquired
> >	a suspend blocker any time that the runqueues were non-empty.
> >	My first reaction was amusement at this vintage Peter Zijlstra
> >	response, and my second reaction was that it was a futile gesture,
> >	as the Android guys would simply back out any such change.
> >
> >	After more thought, however, a variation of Peter's approach
> >	might well be the key to resolving this tension between
> >	real-time response on the one hand and Android's desire to
> >	conserve power at any cost on the other.  Given that suspending
> >	destroys real-time response, why not acquire a suspend blocker
> >	any time there is a user-created real-time task in the system,
> >	whether runnable or not?  Of course, a simpler approach would
> >	be to make Android's OPPORTUNISTIC_SUSPEND depend on !PREEMPT_RT.
> >
> >o	What degree of control is an application permitted over the power
> >	state of individual devices within the system?
> >
> >	Is the application in question permitted to power down the
> >	CPU or peripheral devices?  As more of the power control is
> >	automated based on usage, it is possible that this question will
> >	become less relevant.  The longer the latency and the greater
> >	the energy consumption of a power-up/power-down sequence for
> >	a given device, the less suitable that device is for automatic
> >	power-up/power-down decisions.  Cache SRAMs and main-memory
> >	DRAM tend to be less suitable for automation for this reason.
> >
> >o	What degree of control is an application permitted over the
> >	system sleep state?
> >
> >	Is the application permitted to suspend the device?  Or in the
> >	case of Android, is the application permitted to acquire a
> >	suspend blocker, which prevents the device from being suspended?
> >
> >o	To what degree has the application been tuned to reduce its
> >	power consumption, either in isolation or in conjunction with
> >	other applications that might be running concurrently?
> >
> >	See the "POWER-OPTIMIZED APPLICATIONS" section below for more
> >	detail on the lengths that embedded developers go to in order
> >	to conserve power -- or, more accurately, to extend battery life.
> >
> >
> >REQUIREMENTS
> >
> >o	Reduce the system's power consumption in order to (1) extend
> >	battery life and (2) preserve state until external power can
> >	be obtained.
> >
> >o	It is necessary to be able to use power-oblivious 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 external power or
> >	(2) relatively undemanding laptop battery-lifetime expectations.
> >	The system must be capable of running these power-oblivious
> >	applications without requiring that these applications be
> >	modified, and must be capable of reasonable power efficiency
> >	even when power-oblivious applications are in use.
> >
> >	In other words, it must be possible to automate the incorporation
> >	of a power-oblivious application into the Android environment,
> >	but without significantly degrading battery lifetime.
> >
> >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 it is highly desirable that the
> >	application also be prevented from consuming power computing
> >	something that will not be displayed.  Furthermore, whatever
> >	mechanism is used must operate on power-oblivious applications
> >	that do not use blocking system calls.
> >
> >	There might well be similar requirements for other output-only
> >	devices, as noted by Alan Stern.
> >
> >o	In order to avoid overrunning hardware and/or kernel buffers,
> >	and to minimize response latencies, designated 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 sleep state is received while the system is
> >	transitioning into a sleep state, the system must transition
> >	back out of the sleep state so that it can hand the user
> >	input off to the corresponding application.
> >
> >	Other input events do not force a wakeup, and such input events
> >	-can- be lost due to buffer overflow in hardware or the kernel.
> >	The response latency to such input events can of course be
> >	unbounded.
> >
> >o	Because Android acquires a suspend blocker as soon as an
> >	input event is noticed and holds it until some application
> >	reads that input event, there must be a way to cause the
> >	suspend blocker to timeout.  If there was no such timeout
> >	facility, a power-oblivious application could block suspend by
> >	opening an input device and then refusing to ever read from it.
> >	(Yes, this can be considered to be a energy-efficiency bug in
> >	the power-oblivious application.  Please see the statistics
> >	requirement below.)
> >
> >o	The API must provide a way for PM-driving applications that
> >	receive events to keep themselves running until they have been
> >	able to process those events.
> >
> >o	Statistics of the power-control actions taken by PM-driving
> >	applications must be provided.	Statistics are aggregated by name,
> >	which is passed by the application in through the suspend-blocker
> >	interface.  The following specific statistics are collected in
> >	the kernel, in roughly decreasing order of importance:
> >
> >	o	total_time, which accumulates the total amount of time
> >		that the corresponding suspend blocker has been held.
> >
> >	o	active_since, which tracks how long a suspend blocker has
> >		been held since it was last acquired, or (presumably) zero
> >		if it is not currently held.
> >
> >	o	count, which is the number of times that the suspend
> >		blocker has been acquired.  This is useful in combination
> >		with total_time, as it allows you to calculate the
> >		average hold time for the suspend blocker.
> >
> >	o	expire_count, which is the number of times that the
> >		suspend blocker has timed out.  This indicates that
> >		some application has an input device open, but is
> >		not reading from it, which is a bug, as noted earlier.
> >
> >	o	max_time, which is the longest hold time for the suspend
> >		blocker.  This allows finding cases where suspend blockers
> >		are held for too long, but are eventually released.
> >		(In contrast, active_since is more useful in the
> >		held-forever case.)
> >
> >	o	sleep_time, which is the total time that the suspend
> >		blocker was held while the display was powered off.
> >		(This might have interesting implications should E-ink
> >		displays every become capable of full-motion color video,
> >		but it is easy to imagine that the definition of "powered
> >		off" would then include only those times during which
> >		the display wasn't actively being updated.)
> >
> >	o	wake_count, which is the number of times that the
> >		suspend blocker was the first to be acquired in the
> >		resume path.  This is less than useful on some
> >		Android platforms; Arve is dissatisfied with it
> >		on Nexus One.
> >
> >	Presumably, the userspace code collects similar statistics on
> >	application suspend-blocker activity, but that is out of the scope
> >	of this document, which focuses instead on kernel requirements.
> >	Given that the overhead of maintaining these statistics is
> >	quite low, it seems that it would be worthwhile to have them
> >	enabled in production systems, for example, in order to flag
> >	power-buggy applications that the user has naively downloaded.
> >
> >o	Some PM-driving applications use power-oblivious infrastructure
> >	code.  This means that a PM-driving application must have
> >	some way, whether explicit or implicit, to ensure that any
> >	power-oblivious infrastructure code is permitted to run when a
> >	PM-driving application needs it to run.
> >
> >o	If no PM-driving or power-optimized application are indicating
> >	a need for the system to remain operating, the system is permitted
> >	(even encouraged!) to suspend all execution, regardless of the
> >	state of power-oblivious applications.	(This requirement did
> >	appear to be somewhat controversial, both in terms of what is
> >	meant by "runnable" and in terms of what constitutes "execution".)
> >
> >	In Android, this is implemented by suspending even while
> >	PM-driving or power-optimized applications are active, -unless-
> >	a suspend blocker is held.
> >
> >o	Transition to system sleep state must be power-efficient.
> >	In particular, methods based on repeated attempts to suspend
> >	are considered to be too inefficient to be useful.
> >
> >o	Transition to system sleep state must occur very soon after
> >	all PM-driving and power-optimized applications have indicated
> >	that they have no need for the system to remain operating.
> >	Quick transition is expecially important in cases where the wakeup
> >	was momentary, for example, when processing sporadic network
> >	input or processing widely spaced batches of audio output.
> >	For an example of the latter, MP3 playback allows 1-4 minute
> >	spacing between bursts of CPU activity).
> >
> >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 sleep 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	The API that controls the system sleep 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 states.
> >
> >o	Transitioning to extremely low-power sleep 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 transitions.
> >
> >o	The current Android userspace API must be supported in order
> >	to support existing device software.  According to Brian
> >	Swetland:
> >
> >		For Java/Dalvik apps, the wakelock API is pertty
> >		high level -- it talks to a service via RPC (Binder)
> >		that actually interacts with the kernel.  Changing the
> >		basic kernel<->userspace interface (within reason) is
> >		not unthinkable.  For example, Arve's suspend_blocker
> >		patch provides a device interface rather than the proc
> >		interface the older wakelock patches use.  We'd have to
> >		make some userspace changes to support that but they're
> >		pretty low level and minor.
> >
> >		In the current model, only a few processes need to
> >		specifically interact with the kernel (the power
> >		management service in the system_server, possibly the
> >		media_server and the radio interface glue).  A model where
> >		every process needs to have a bunch of instrumentation is
> >		not very desirable from our point of view.  We definitely
> >		do need reasonable statistics in order to enable debugging
> >		and to enable reporting to endusers (through the Battery
> >		Usage UI) what's keeping the device awake.
> >
> >o	Any mechanism that freezes some subset of the applications must
> >	ensure that none of the frozen applications hold any user-level
> >	resources, such as pthread mutexes.  The reason for this is that
> >	freezing an application that holds a shared pthread mutex will
> >	result in an application-level hang should some unfrozen process
> >	attempt to acquire that same pthread mutex.  Note that although
> >	the current cgroup freezer ensures that frozen applications do not
> >	hold any kernel-level mutexes (at least assuming these mutexes
> >	are not wrongly held when returning to user-level execution),
> >	it currently does nothing to prevent freezing processes holding
> >	pthread mutexes.  (There are some proposals to address this issue.)
> >
> >
> >NICE-TO-HAVES
> >
> >o	It would be nice to be able to identify power-oblivious
> >	applications that never were depended on by PM-driving
> >	applications.  This particular class of power-oblivious
> >	applications could be shut down when the screen blanks even
> >	if some PM-driving application was preventing the system from
> >	powering down.
> >
> >	There are two obstacles to meeting this requirement:
> >
> >	1.	There must be a reliable way to identify such
> >		applications.  This should be doable, for example, the
> >		application might be tagged by its developer.
> >
> >	2.	There must be a reliable way to freeze them such
> >		that no frozen application holds a resource that
> >		might be contended by a non-frozen application.
> >
> >		Although the cgroup freezer does ensure that frozen
> >		tasks hold no kernel-level resources, it currently does
> >		nothing to ensure that no user-level resources are held.
> >		There are some alternative proposals, which might or
> >		might not be more successful:
> >
> >		a.	Unfreeze this group periodically to ensure
> >			that any such resource is eventually released,
> >			while keeping power consumption down to a dull
> >			roar.
> >
> >		b.	Perform the freeze at application level, where
> >			it is possible to determine whether an
> >			application-level resource is held.
> >
> >o	Any initialization of the API that controls the system power
> >	state should be unconditional, so as to be free from failure.
> >	Such unconditional initialization reduces the intrusiveness of
> >	the Android patchset.
> >
> >
> >APPARENT NON-REQUIREMENTS
> >
> >o	Transitioning to system sleep states need not be highly scalable,
> >	as evidenced by the global locks.  (If you believe that high
> >	scalability 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.)
> >
> >	That said, it should not be hard to provide a highly scalable
> >	implementation of suspend blockers, especially if large systems
> >	are allowed to take their time suspending themselves.
> >
> >o	Conserving power in the WiFi and cellular telephony networks.
> >	At the moment, the focus is on increased battery life in the
> >	handheld device, perhaps even at the expense of additional
> >	power consumed by the externally powered WiFi and cell-telephony
> >	equipment.
> >
> >o	Synchronizing wakeups of unrelated applications.  This is of
> >	course an important requirement for power savings overall, but
> >	seems to be left to other mechanisms (e.g., timer aggregation)
> >	by the Android folks.  Although one could implement suspend
> >	blockers so as to aggregate timers after a sufficiently long
> >	suspension, there are problems with this approach:
> >
> >	o	There would be a "thundering herd" problem just after
> >		resume completed as almost every timer in the system
> >		would expire simultaneously.
> >
> >	o	The applications would not necessarily stay aggregated
> >		without some other mechanism helping out.
> >
> >
> >SUGGESTED USAGE
> >
> >These are constraints that the developer is expected to abide by,
> >"for best results" and all that.
> >
> >o	When a PM-driving application is preventing the system from
> >	shutting down, and is also waiting on a power-oblivious
> >	application, the PM-driving application should set a timeout
> >	to handle the possibility that the power-oblivious application
> >	might halt or otherwise fail.
> >
> >
> >POWER-OPTIMIZED APPLICATIONS
> >
> >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 sleep 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 achieve 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.	Provide "heartbeat" services that allow applications to
> >	synchronize with each other.  This seems most applicable for
> >	applications that run infrequently, such as email-checking and
> >	location-service applications.
> >
> >5.	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.
> >
> >6.	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.
> >
> >
> >OTHER EXAMPLE APPLICATIONS
> >
> >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 PM-driving (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-oblivious 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.
> >
> >
> >ACKNOWLEDGMENTS
> >
> >	Of course, just because I acknowledge their contributions does
> >	not necessarily mean that I think they agree with my assessment
> >	of the requirements behind suspend blockers.  ;-)
> >
> >	Nevertheless, I am grateful for any and all feedback, whatever
> >	the form of that feedback might be.  I am new to this area, and
> >	have much to learn.
> >
> >	Alan Stern
> >	Anca Emanuel
> >	Arjan van de Ven
> >	Arve Hj?nnev?g
> >	Brian Swetland
> >	David Brownell
> >	David Lang
> >	Florian Mickler
> >	James Bottomley
> >	Kevin Granade
> >	Mark Brown
> >	Matt Helsley
> >	Matthew Garrett
> >	Mikael Abrahamsson
> >	Olivier Galibert
> >	Paul Menage
> >	Pavel Machek
> >	Rafael J. Wysocki
> >	Richard Woodruff
> >	Ted Ts'o
> >
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