Re: [linux-pm] [PATCH 0/8] Suspend block api (version 6)

[Mike Chan]

On Mon, May 17, 2010 at 12:27 PM, Vitaly Wool <vitalywool@xxxxxxxxx> wrote:
> On Mon, May 17, 2010 at 6:12 PM, Kevin Hilman
> <khilman@xxxxxxxxxxxxxxxxxxx> wrote:
>
>>> and #2, the battery lifetime on the N770 and N800 (both of which I have)
>>> is **appalling** **bad**.
>>
>> Appalling bad compared to what?
>>
>> What's probably more interesting in terms of rough comparisons is
>> comparing similar devices with and without opportunistic suspend.  The
>> Nokia n900 (maemo) and the Moto Droid (android) use the same SoC (TI
>> OMAP3) and roughly the same kernel (2.6.2[89], although both are
>> heavily patched from mainline.)
>>
>> The n900 *never* suspends.  It only uses dynamic PM + CPUidle.
>> The droid uses opportunistic suspend (as well as dynamic PM + CPUidle)
>>
>> I don't know of any more objective comparison of the two, but as a
>> user of both devices I can say that the active usage is basically the
>> same (around a day) and the idle use is similar as well, even though
>> the Droid has a slightly bigger battery (1400 mAh vs. 1320 mAh.)  My
>> own usage suggests the n900 is a bit better in idle time, but I have
>> not done any measuring or objective tests.  I'm guessing the
>> difference is probably because the Droid does not use the deepest
>> off-mode power states either in idle or suspend (IIRC) where the n900
>> does.  I suspect that if both were using off-mode and had the same
>> battery, these differences would go away.
>>

Although both are OMAP3430 and run 2.6.29 you cannot compare the N900
and Droid's perceived user battery life to one another to evaluate
opportunistic suspend. There are many factors uncounted for such as:
network reception, screen brightness (and size), button back-light,
keyboard back-light, modem stack (CDMA vs UMTS). Also the difference
in uerspace.

>> While this is not really a scientific comparison, it at least gives a
>> rough idea.  If using opportunistic suspend was adding noticably
>> better battery life, I think this would be a different discussion.
>
> Exactly. The point is, opportunistic suspend doesn't in fact add any
> value compared to dynamic PM + CPUIdle. It only produces some false
> impression that one can handle power management right without using
> dynamic PM. And this false impression is the cause for many really
> ugly designs (like, for instance, 15 minutes touchscreen inactivity
> delay before forcibly shutting down the wireless, as it's done in
> stock Android framework).
>

Opportunistic suspend is an extension to the current suspend model,
not a replacement dynamic / run-time PM. If you can replace good old
suspend then this would be a different story.

As you mention, Droid uses opportunistic suspend + dynamic pm +
cpuidle + freq. So I decided to do some measurements on a Droid using
our 2.9.32 kernel
(http://android.git.kernel.org/?p=kernel/omap.git;a=summary). For a
little better apples to apples comparison.

Droid (idle system, airplane mode, screen off, 3 min interval):
measured average current
- with opportunity suspend: 3.19mA
- without opportunistic suspend: 3.5mA

Stock userspace build, all I did was replace the kernel. We are
hitting retention on idle as well as suspend for omap (instead of full
off-mode).

Also, your point about wifi, the 15 min timeout is in the framework
and is configurable in the code and via UI, nothing to do with kernel,
opportunistic suspend or run time suspend.


-- Mike

> Thanks,
>   Vitaly
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