Message-ID: <20100811022131.GA2587@linux.vnet.ibm.com>
Date:	Tue, 10 Aug 2010 19:21:31 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	david@...g.hm
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, "Ted Ts'o" <tytso@....edu>,
	Felipe Contreras <felipe.contreras@...il.com>,
	Brian Swetland <swetland@...gle.com>,
	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,
	peterz@...radead.org, tglx@...utronix.de, menage@...gle.com,
	david-b@...bell.net, James.Bottomley@...e.de, arjan@...radead.org,
	swmike@....pp.se, galibert@...ox.com, dipankar@...ibm.com
Subject: Re: Attempted summary of suspend-blockers LKML thread, take three

On Tue, Aug 10, 2010 at 06:28:39PM -0700, david@...g.hm wrote:
> On Tue, 10 Aug 2010, Paul E. McKenney wrote:
> 
> >On Tue, Aug 10, 2010 at 09:38:49AM +0100, Alan Cox wrote:
> >>>situation you call out can occur with manual suspend-to-RAM already:
> >>>the fact is that this is a design choice.  You could indeed make a
> >>
> >>Losing data is a design choice ? The application set a timer, the OS
> >>shouldn't be ignoring it in that situation. It might want to delay it, it
> >>might want to warn the user its hogging things it shouldnt (powertop,
> >>battery usage monitors in Android etc)
> >
> >Hmmm...  Let's put the two approaches side by side so that we can compare
> >them more easily:
> >
> >	Opportunistic Suspend		Idle + Timer Jitter
> >
> >	Set timer			Set timer
> >	Suspend				OS delays timer
> >	Resume				OS continues delaying timer
> >	Timer fires			Timer fires
> >
> >These two cases look quite similar to me.
> >
> >But as you say, the battery can run out.  So let's add that to the
> >comparison:
> >
> >	Opportunistic Suspend		Idle + Timer Jitter
> >
> >	Set timer			Set timer
> >	Suspend				OS delays timer
> >	Battery runs out		Battery runs out
> >	Data loss			Data loss
> >
> >The two cases still look quite similar.  You might note, quite correctly,
> >that the time between suspend and resume might be quite a bit longer than
> >the typical time that the OS delays a timer.  But the power consumption
> >on some platforms is quite a bit lower in the suspend case than it is
> >in the delayed-timer case.
> 
> it has been stated that the android can hit the exact same power
> state either with sleep or suspend, and that the same clock can wake
> it up (it appears as a timer expiring for sleep, or an alarm for
> suspend, but it's the same clock firing the signal)
> 
> so in at least some cases the hardware supports doing both with
> equal efficiency.

It indeed has been so stated.  But in this section we were discussing
data loss, not hardware power-state capabilities.

> >>>But that doesn't guarantee that solutions developed for PCs and laptops
> >>>will be optimal (or even usable) on cellphones.  Sufficient difference
> >>
> >>Your cellphone is to all intents a laptop from ten years ago, it even has
> >>a similar display resolution and internet availability. The underlying
> >>difference between the two is solely form factor - the laptop had a
> >>better keyboard.
> >
> >There are similarities and differences.  You have called out some of
> >the similarities.  Differences include the more-aggressive hardware
> >power management on cellphones, the greater number and variety of
> >hardware accelerators on cellphones, battery capacity, and, as you say,
> >physical size.  People currently use cellphones differently than they
> >do laptops or desktops.  The usage might converge, but we will see.
> >There is as much reason to expect increasing specialization as there
> >is to expect increasing convergence.
> 
> You are talking about Android as if it was a cell phone only thing,
> it's not. there are shipping tablets (and I believe netbooks, i.e.
> laptops) running andoid.

I was talking about cellphones.  But yes, Android (and thus suspend
blockers) are used for tablets as well as cellphones, thank you for
reminding me!

> >>>As to busting all apps, lthough there have been situations where busting
> >>>all the apps turned out to be the right thing to do, I agree that these
> >>>situations are extremely rare.  Such situations are usually associated
> >>>with the emergence of a new high-volume platform.
> >>
> >>Like Microsoft Windows 16bit co-operative multi-tasking ? It's rarely
> >>right. It's merely that in certain cases the value in the market is large
> >>enough that it can be used as a big stick to beat people into doing lots
> >>of usually wasted work.
> >
> >Interesting choice of example.  I do well remember the Sequent hardware
> >guys' frustration when the old printer driver would monopolize the desktop
> >while printing their large documents.  The fact was that Microsoft's
> >co-operative multi-tasking required all applications to be well behaved,
> >just as does your approach to power efficiency.
> 
> and wakelocks require all applications that can take a wakelock be
> well behaved. and applications that do nt take a wakelock directly
> cannot expect to run unless something else takes a wakelock on their
> behalf

Almost.  Suspend blockers require that only those portions of a PM-driving
application that hold a suspend blocker be carefully written to avoid
wasting power.

							Thanx, Paul
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