Re: Attempted summary of suspend-blockers LKML thread

Paul E. McKenney
2010-08-02T03:03:04Z

On Sun, Aug 01, 2010 at 04:40:26PM -0400, Ted Ts'o wrote:
> On Sun, Aug 01, 2010 at 12:12:28PM -0700, Paul E. McKenney wrote:
> >
> > I understand that you would prefer that we group applications into
> > "good" and "bad" categories, but then again, I suspect that most of us
> > understood that before you posted this message. Given your significant
> > and well-appreciated contributions to power efficiency over the past
> > several years, I must confess to be quite disappointed that you failed
> > to do more than to simply restate your preference.
>
> Paul, I very much agree with what you stated later, with respect to
> doubting whether the whack-a-mole approach to application fixups is
> workable. Given how many applications screw up using fsync()
> correctly, to the extent that XFS, ext4, and btrfs all had to agree on
> a common hueristics to deal with the fact that application programmers
> are aggressively ignorant, and outnumber the file system developers, I
> too doubt the general strategy of relying only on application
> programmers to do the right thing. That's not to say that we
> shouldn't give up on trying to influence application programmers ---
> but relying on that as the only strategy seems to depart from the path
> of wisdom.

Unless someone can reliably automate whacking the moles, history is not
on the side of the mole-whackers. I won't say that such automation
is impossible, but only because of all the "impossible" things in my
lifetime that have turned out to be possible.

And I agree that multiple approaches are likely to be needed here.

> There is however a much bigger point, which is that it's unfortunately
> black and white to talk about applications being "good" and "bad". In
> fact, it's a continuing point of concern I have with the whole qos
> approach to power management. In fact, power is often something that
> needs to trade off against performance. For example, an application
> could aggressively prefetch e-mail messages or web pages that a user
> _might_ view --- or it could aggressively pre-resolve DNS queries,
> etc, which might make perfect sense when the device is hooked up to AC
> mains, but which I might not want to do on when I only have 800mWh
> worth of battery --- however, if I'm using a laptop with 94,000mWh,
> maybe I'd be happy if the application was a bit more profligate.
>
> So for Arjan to claim that all applications will be held to the same
> standard, whether they are hooked up to the AC mains, or are limited
> to 800mWh of battery, or 94,000 mWh worth of power, is vastly
> oversimplifying the problem. Of *course* if I'm writing an
> application that will be running in a cloud data center, I'm going to
> care about power. But I may have different tradeoffs about what might
> considered acceptible when considering the qualify of user experience
> I'm delivering to the end-user when I'm connected to AC mains, versus
> a cell phone battery, versus a 6-cell laptop battery.

And depending on how badly you need the application to run. "Find me
the nearest source of AC power!" "Sorry, can't do that because that app
is too poorly behaved to run when the battery is nearly exhausted." ;-)

Arjan did suggest taking user preferences into account, but we will see.

> This brings me back to a major problem I have with the pm_qos approach
> to power management. It assumes that applications know best, and that
> they should be free to tell pm_qos subsystem whether they need 0ms
> latency for wireless. Right now, I can't even query the pm_qos
> subsystem to see which application is responsible for keeping the
> wireless on 100% of the time! And even if I could find out, maybe
> some power management framework should be allowed to give a override
> to the application's wishes. OK, maybe the Opera web browser is
> requesting the very best wireless QOS because it wants to beat Chrome
> on some silly potato benchmark --- well, it's ***stupid*** to say that
> my power management should be a one-size-fits all because applications
> should be always as power efficient as possible whether they are
> connected to AC mains or I have a 800mWh cell phone battery. Worse
> yet, it's stupid to say that the application should have the last
> word. Darn it, *I* own the mobile device, and I (or my proxy, which
> might be the Android OS, or some power manage daemon) should be able
> to say, "I don't care what the application claimed it wanted for power
> QOS --- it's not getting more than 100ms wireless latency, and that's
> final."
>
> And note that this is something that might even change over time, or
> depending on circumstances. Maybe normally I might be willing to let
> the application be profilgate with power, so that web pages render a
> bit faster than they might otherwise --- but if I'm on an American
> Airlines flight which has retrofitted its power jacks to use an AC
> plug, and I only have a DC adaptor, and my laptop batteries are worn
> out and only have half their endurance as they used to, I might want
> to use a more stringent pm_qos than I might otherwise normally allow.

Agreed!

Thanx, Paul