Re: Wakelocks Rebooted - Power management for embedded devices

[Rafael J. Wysocki]

On Tuesday, August 17, 2010, Igor Stoppa wrote:
> Hello,
> as discussed during the power management mini-summit in Boston 1ast week
> at linuxcon [1] I'd like to reboot the discussion about power management
> on embedded linux devices - mostly focusing on handhelds.
> 
> The previous discussion was focused on getting a certain implementation
> integrated and has certainly been useful to rise the overall level of
> awareness about battery-constrained systems.
> 
> Many people have commented about how it should have started and evolved,
> so I'll try to follow those words of wisdom :-)
> 
> 
> 
> High level goals:
> -----------------
> 
> 1) Preserve power and performance behavior over time
> Iow the system should not age, even after installing random apps with
> different level of power friendliness.
> 
> 2) Easy development of pm-friendly apps (different from pm-aware)
> Most simple apps are anyway active only when visible on the screen.
> (I am assuming that, given the specific device implementation, the vast
> majority of apps will be written from scratch, rather than ported, to
> better integrate with the rest of the system.)
> 
> 3) Easy identification of problematic apps (those that do not conform to
> the desired behavior on a certain platform/configuration)

Well, I guess the ultimate goal is to save as much energy as reasonably
possible without sacrificing the usability of the system.  That requires
both appropriate device power management mechanisms and user space
participation to some extent, but your goals above only seem to address the
latter.

> Implementation requirements:
> ----------------------------
> 
> a) Keep separate policies from mechanisms to ensure cross platform
> portability of high level policies
>       eg: screen on
> 		high level constraint: max latency in UI
> 		low level constraint:  min frequency 
> 
>         the former is portable, the latter is not
> 
> b) Avoid introducing platform-specific knowledge / dependency in generic
> applications, eg. knowledge about "suspend".
> This doesn't exclude explicit management of context save/restore, but
> the application should not concern itself with the way a certain
> platform implements power saving.
> 
> 
> 
> Problems/Gaps identified so far:
> --------------------------------
> 
> I will not replicate Paul McKenney's effort to summarize all the
> discussion on suspend blockers (btw thanks for the great work!), however
> something received less attention than it deserved imo: runtime tracing
> of system hogs.
> 
> How to make the system to idle and how to make sure it stays there can
> be implemented in different ways, but in any case it is necessary to
> track the usage of system resources.
> 
> I)  Present solutions seem to focus mostly on the CPU/SoC, while it
> would be better if the whole system was accountable for power
> consumption - meaning that CPU wakeups are accounted, but also other
> power sinks (eg. 3G radio) are accounted ad well.

Agreed.

> II) Even when analyzing the SoC, rather than other discrete components,
> there is the problem that the same IP block might have different power
> impact on the system, depending on the way it is wired in a specific SoC
> (clock distribution, power rails, etc) and its activity should be
> weighted in a system specific way, without introducing such notion in
> the driver itself.
> 
> This seems to converge in the discussion initiated by Paul Walmsley and
> Kevin Hilman [1] wrt "omap bus" approach where the bus structures could,
> amongst other information, also store the correlation between device
> state and power consumption.
> 
> III) A common way to address device states and states transitions vs
> time seems to be missing.
> This would allow to expose runtime statistics (through relay-fs, for
> example) of residence in a certain state - similarly to what powertop
> does for x86 processors.
> 
> IV) Also performance modes might benefit from being exposed in a
> consistent way which doesn't stop at active/inactive, but can express
> also the device specific states.
> examples: 
> * touch screen sampling rate affects both accuracy and power consumption
> * HW audio codec where different ifle states have different latencies
> 
> 
> Some Thoughts:
> --------------
> 
> * The suspend-based approach doesn't seem to fit our design/approach,
> but otoh if we make it easy enough for developers to improve/fix their
> code when dealing with runtime idle, by exposing causes for not hitting
> low power states, the number of power unfriendly apps will just dwindle
> down.
> Typically such fixes have also significant impact on performance, so
> they are not desirable just from a use-time pov.
> 
> * Introducing a mechanism to prevent power unfriendly apps from ruining
> the overall system performance/use-time seems tp be the way to go (this
> might have both an automatic mode and a user-interactive mode), so long
> as the user can express what he wants and gets it (which might depend on
> what is considered to be paramount in a specific situation)

That seems to be a major problem from the implementation point of view, because
power-unfriendly applications may be related to the power-friendly ones in
various ways.

> * In practice it seems unlikely that the applications to be made
> available by 3rd parties will be ported existing legacy PC code, but
> rather new apps that will be written and targeted to mobile devices.
> 
> Yes, there are plenty of examples that contradict my statement, however
> observing the population of a typical app-store, most of the
> applications are relatively simple and specifically designed to look
> nice on a mobile screen.
> And even for ported apps, if there are no platform specific APIs (like
> suspend blockers), bugfixes can be contributed back to the upstream
> project.

That seems to express the concern about platform-specific hooks added to
applications with power management in mind.  I'm not sure if it's generally
possible to avoid them, especially in applications located in the given
platform's "plumbing layer" (ie. between the kernel and the other apps), but
also I'm not sure if these applications will be portable anyway.

> * Most of the requirements described in the Android discussion can
> actually be handled in userspace because they involve some slowly
> evolving phenomenon (typically mode transitions, such as screen on/off)
> with no strict time constraint.
> We roughly share the same needs/constraints.
> 
> * I see lots of potential overlapping wrt tracking/controlling the state
> of the system, iow defining the kernel interfaces, while the userspace
> implementation could significantly depend on the philosophy followed by
> the system integrator.
> 
> * the same interfaces could be shared also with other tools currently
> used for more static analysis, like powertop, timechart, etc [2]
> 
> * MeeGo/Maemo already have a user space policy manager (currently used
> for audio routing and cgroup policies) that could be leveraged also for
> this sort of power policies.
> Raphael was also thinking about a userspace policy manager and albeit
> I'm not familiar with Android, I believe it must have something similar
> for dealing with bytecode-based apps and their constraints (feedback
> from people with better knowledge would be appreciated).
> 
> * I wasn't able to locate a complete list of Android goals/requirements;
> having such information - coming straight from the source - would
> improve the chances of mutual understanding.
> 
> * I would welcome people from silicon companies to participate in the
> discussion, since they have the best understanding of the real impact of
> a certain power saving implementation on the SoC performance.

Yes, it would be nice to have some input from the hardware folks.

Thanks,
Rafael
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