Re: Attempted summary of suspend-blockers LKML thread, take three

From: david
Date: Sat Aug 07 2010 - 05:40:41 EST

On Sat, 7 Aug 2010, Ted Ts'o wrote:

On Fri, Aug 06, 2010 at 08:14:09PM -0700, david@xxxxxxx wrote:

that description sounds far more like normal sleep power management
that suspending. especially since they want to set timers to wake
the system up and the defining characteristic of suspend (according
to this thread) is that timers don't fire while suspended.

as I am seeing it, there are two reasons why this don't "just work"

1. sleeping can't currently save as much power as suspending

No, I don't think that's the case at all. The key thing here is that
*most* applications don't need to be modified to use suspend locks,
because even though they might be in an event loop, when the user user
turns off the display, the user generally doesn't want it doing things
on their behalf.

Again, take for example the Mac Book, since Apple has gotten this
right for most users' use cases. When you close the lid, you even if
the application is under the misguided belief that it should be
checking every five seconds to see whether or not the web page has
reloaded --- actually, that's not what you want. You probably want
the application to be forcibly put to sleep. So the whole point of
the suspend blocker design is that you don't have to modify most
applications; they just simply get put to sleep when you close the
MacBook lid, or, in the case of the Android device, you push the
button that turns off the screen.

this doesn't require wakelocks or anything else new. all it takes is setting the policy that you don't want anything to run when the lid is closed and a switch to detect the lid being closed. Laptops have been doing this for years.

So the reason why this doesn't work is that power management for small
mobile devices *is* different from power management for laptops and
data center servers, and if you want a rich application ecosystem,
it's best if you don't require them to be specially tuned to use the
absolute minimum power. (And that means waking up every 30 seconds
might be too much; as Brian and Arve have pointed out, with the G1 in
airplane mode, the CPU might be waking up once every half hour or more
--- and at that rate, powertop will be waking up the CPU more than
Android system would be doing so.)

note that nothing that I have proposed would wake up a sleeping system. the 'every several seconds' thing that I proposed was that on a system that's fully awake, busy and doesn't want to sleep, there would be a context switch periodically by a privilaged process so that the system would not end up deciding it was idle and halt everything. Now that I think about this more, it's not needed if you want to override this to keep everything running for a significant amount of time, just change the power saving mode from "sleep if a privilaged task isn't running" to "disable suspend". This can be done today by changing the right sysfs value. normal privilaged processes would never need to do this, only gatekeeper daemons that want to let unprivilaged processes run even if no privilaged processes want to run would need to do this (i.e. in the current system, whatever process controls the screen would probably be right)

So the real key here is to take most applications, which may be
written using techniques that might be considered well written from a
laptop point of view, but not for a cell phone, and not require
modifications. Even though the application writer might think it's
doing well by waking up every 15 seconds, if the laptop lid is down,
or if the screen is off, for **most** applications, it should be
forcibly put to sleep.

It's only the rare applications that should really be allowed to run
while screen is off. And it's only those applications that need
modifications to use suspend blocker. From your earlier comments, it
seems that this is the key point which you are missing. (No doubt,
some of these applications that do need to know about suspend blockers
are important ones; ones that make sure the battery isn't about to
blow up, or ones which silently wake up every 10-15 minutes to pull
down fresh mail for you from your mail server. But those applications
are the exception, not the rule.)

the question is what it takes to make an application privilaged like this.

what I proposed was to make it possible for the user/admin to select what applications are allowed to keep the system awake.

wakelocks require that the application developer decide that they want to keep the system awake as well as the user/admin

take your example of a mail client waking up every 15 min.

with Android it needs to be privilaged to grab the wakelock while fetching the mail, it also needs to use a privilaged API to set the wakeups to wake it up at those times.

with what I proposed all you need to do is to tag the application as power privilaged and then if the application sleeps for 15 min between doing thing the system will wake up every 15 min, work for a short time, then go back to sleep.

if you want to put everything to sleep when the screen blanks, that's trivial to do.

the fun starts when you want to say that there are some things you don't want to put to sleep.

do you let some processes run while halting others?
in which case how do you prevent deadlocks?
on the other hand, sleeping again is simple, you sleep when there is nothing more to run
or do you let wasteful processes run while you are awake?
this avoids deadlocks, but how do you decide when to sleep again?

Android approaches this by requiring that any program that a user may want to keep running must be modified to use wakelocks.

My suggestion was that the system ignore other processes when deciding if the system is idle enough to put to sleep.

Yes the Android approach works reasonably well on the phones where everything must currently be custom developed anyway (due to screen and UI constraints), but as Android starts getting used on laptops and other larger devices is that really still the right approach? or would it be better to have something that could use standard software in the privilaged mode?

to use your example of a mail client, why should someone not be able to use fetchmail to get their mail instead of requiring that fetchmail be modified to use wavelocks (or a substatute written)?

the theoretical best approach would probably look nothing like either of these. Instead it would probably either

forbid privilaged processes from having blocking dependancies on unprivilaged processes (you can then just halt the unprivilaged processes when you blank the screen and let idle sleep deal with the privilged processes)


use something like priority inheritance through userspace so that if a privilaged process blocks on something where it's waiting for an unprivilaged process, that unprivilaged process gets woken up and allowed to run until it unblocks the privilaged process.

both of these have been deemed too hard

David Lang
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