Re: Attempted summary of suspend-blockers LKML thread

From: david
Date: Thu Aug 05 2010 - 16:27:44 EST

On Thu, 5 Aug 2010, kevin granade wrote:

On Thu, Aug 5, 2010 at 10:46 AM, <david@xxxxxxx> wrote:
On Thu, 5 Aug 2010, Paul E. McKenney wrote:

On Wed, Aug 04, 2010 at 10:18:40PM -0700, david@xxxxxxx wrote:

On Wed, 4 Aug 2010, Paul E. McKenney wrote:

On Wed, Aug 04, 2010 at 05:25:53PM -0700, david@xxxxxxx wrote:

On Wed, 4 Aug 2010, Paul E. McKenney wrote:

[ . . . ]

however, in the case of Android I think the timeouts have to end up being
_much_ longer. Otherwise you have the problem of loading an untrusted book
reader app on the device and the device suspends while you are reading the
page.

currently Android works around this by having a wakelock held whenever the
display is on. This seems backwards to me, the display should be on because
the system is not suspended, not the system is prevented from suspending
because the display is on.

Rather than having the display be on causing a wavelock to be held (with the
code that is controls the display having a timeout for how long it leaves
the display on), I would invert this and have the timeout be based on system
activity, and when it decides the system is not active, turn off the display
(along with other things as it suspends)

IIRC, this was a major point of their (Android's) power management
policy. User input of any kind would reset the "display active"
timeout, which is the primary thing keeping random untrusted
user-facing programs from being suspended while in use. They seemed
to consider this to be a special case in their policy, but from the
kernel's point of view it is just another suspend blocker being held.

I'm not sure this is the best use case to look at though, because
since it is user-facing, the timeout durations are on a different
scale than the ones they are really worried about. I think another
category of use case that they are worried about is:

(in suspend) -> wakeup due to network -> process network activity -> suspend

or an example that has been mentioned previously:

(in suspend) -> wakeup due to alarm for audio processing -> process
batch of audio -> suspend

when you suspend the audio will shut off, so it's sleep ->wake -> sleep, not suspend

In both of these cases, the display may never power on (phone might
beep to indicate txt message or email, audio just keeps playing), so
the magnitude of the "timeout" for suspending again should be very
small. Specifically, they don't want there to be a timeout at all, so
as little time as possible time is spent out of suspend in addition to
the time required to handle the event that caused wakeup.

it really depnds on the frequency of the wakeups.

if you get a network packet once every 5 min and need to wake to process it, staying awake for 20 seconds after finishing procesing is FAR more significant than if you get a network packet once every hour. It's not just the factor of 20 that simple math would indicate because the time in suspend eats power as well.

I don't know real numbers, so these are made up for this example

if suspend (with the cell live to receive packets) is 10ma average current and full power is 500ma average current

packets every 5 min with .1 sec wake time will eat ~13maH per hour

packets every 5 min with 10 second wake time will eat ~37maH per hour

packets every hour with .1 sec wake time will eat ~10maH per hour

packets every hour with 10 sec wake time will eat ~11maH per hour

so if you have frequent wakeups, staying awake 100 times as long will cut your battery life to 1/3 what it was before.

if your wakeups are rare, it's about a 10% hit to stay awake 100 times as long.

there is a lot of room for tuning the timeouts here.

David Lang
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