Re: syncing the disks when entering sleep

Leisner, Martin
2010-02-10T20:58:54Z

> -----Original Message-----
> From: Rafael J. Wysocki [mailto:rjw@sisk.pl]
> Sent: Wednesday, February 10, 2010 5:34 AM
> To: linux-pm@lists.linux-foundation.org
> Cc: Pavel Machek; Nigel Cunningham; Leisner, Martin
> Subject: Re: [linux-pm] syncing the disks when entering sleep
>
> On Wednesday 10 February 2010, Pavel Machek wrote:
> > > Hi.
> > >
> > > Pavel Machek wrote:
> > > >> So you're asking to give this knob "one shot behavior" (i.e.
> "then
> > > >> next sleep won't sync")?
> > > >
> > > > Yes.
> > > >
> > > >> But I'm primarily interested in the behavior on embedded systems
> > > >> (where you control all the processes running -- there's no "user"
> > > >> involved.
> > > >>
> > > >
> > > > Well, then "one shot behaviour" does not hurt you, right?
> > > >
> > > >> If a user starts messing with default settings, any unwanted
> > > >> behavior is the users problem (besides, this should only be
> > > >> writable as root).
> > > >>
> > > >
> > > > I'd rather not add traps for the user unless absolutely
> neccessary.
> > > > Not even for root user. Pavel
> > >
> > > But that's precisely what you're doing. You're advocating making the
> > > behaviour inconsistent. If what you're suggesting is done, you won't
> be
> > > able to simply cat the sysfs entry to know whether sys_syncing is
> going
> > > to be done on the next cycle. You'll also have to have knowledge of
> > > whether a cycle has been done since the last time the value is set.
> The
> > > end result will be someone getting trapped and caught out because
> they
> > > think '1' in /sys/power/dont_sync (or whatever it's called) means
> what
> > > it says.
> >
> > I'm simply advocating that setting from one suspend should not change
> > other suspends ... because you have multiple different programs
> > wanting to suspend the system, all independend.
>
> Which is wrong. There should be one power manager everybody else calls
> to
> suspend the system. Yes, even if the battery is running critical.
>
> And I guess on the embedded systems in question the situation is exactly
> like
> that.
>
> > See the example about system running low on battery earlier in the
> thread.
>
> Which is irrelevant here.
>
> Actually, doing the sync() in the kernel is a hack that we decided not
> to
> remove just because the users space didin't do the right thing on some
> systems.
> If the user space always synced disks before suspending, we wouldn't
> have to
> do that in the kernel.
>
> Same goes for the kernel VT switch, BTW.
>
> So really, I don't see anything wrong with a knob that will turn the
> kernel
> sync off entirely, because that basically means "my user space is not
> broken".
>
> Rafael

This is interesting. When I saw the sync's I said to myself "WTF?"
It definitely should be a knob because it implements policy behavior --
which is often detrimental to power saving efforts.

marty