Re: [PATCH 2/2] mm: forcely swapout when we are out of page cache

[Minchan Kim <minchan@kernel.org>]

On Tue, Jan 15, 2013 at 05:21:15PM -0800, Sonny Rao wrote:
> On Tue, Jan 15, 2013 at 4:50 PM, Andrew Morton
> <akpm@linux-foundation.org> wrote:
> > On Tue, 15 Jan 2013 16:32:38 -0800
> > Sonny Rao <sonnyrao@google.com> wrote:
> >
> >> >> It's for saving the power to increase batter life.
> >> >
> >> > It might well have that effect, dunno.  That wasn't my intent.  Testing
> >> > needed!
> >> >
> >>
> >> Power saving is certainly why we had it on originally for ChromeOS,
> >> but we turned it off due to misbehavior.
> >>
> >> Specifically, we saw a pathological behavior where we'd end up writing
> >> to the disk every few seconds when laptop mode was turned on.  This
> >> turned out to be because laptop-mode sets a timer which is used to
> >> check for new dirty data after the initial flush and writes that out
> >> before spinning the disk down, and on ChromeOS various chatty daemons
> >> on the system were logging and dirtying data more or less constantly
> >> so there was almost always something there to be written out.  So what
> >> ended up happening was that we'd need to do a read, then wake up the
> >> disk, and then keep writing every few seconds for a long period of
> >> time, which had the opposite effect from what we wanted.
> >
> > So after the read, the disk would chatter away doing a dribble of
> > writes?  That sounds like plain brokenness (and why did the chrome guys
> > not tell anyone about it?!?!?).
> 
> Yes, either read or fsync.  I ranted about it a little (here:
> http://marc.info/?l=linux-mm&m=135422986220016&w=4), but mostly
> assumed it was working as expected, and that ChromeOS was just
> dirtying data at an absurd pace.  Might have been a bad assumption and
> I could have been more explicit about reporting it, sorry about that.
> 
> > The idea is that when the physical
> > read occurs, we should opportunistically flush out all pending writes,
> > while the disk is running.  Then go back into
> > buffer-writes-for-a-long-time mode.
> >
> 
> See the comment in page-writeback.c above laptop_io_completion():
> 
> /*
>  * We've spun up the disk and we're in laptop mode: schedule writeback
>  * of all dirty data a few seconds from now.  If the flush is already
> scheduled
>  * then push it back - the user is still using the disk.
>  */
> void laptop_io_completion(struct backing_dev_info *info)
> 
> What ends up happening fairly often is that there's always something
> dirty with that few seconds (or even one second) on our system.
> 
> > I forget what we did with fsync() and friends.  Quite a lot of
> > pestiferous applications like to do fsync quite frequently.  I had a
> > special kernel in which fsync() consisted of "return 0;", but ISTR
> > there being some resistance to productizing that idea.
> >
> 
> Yeah, we have this problem and we try to fix up users of fsync() as we
> find them but it's a bit of a never-ending battle.  Such a feature
> would be useful.
> 
> >>  The issues
> >> with zram swap just confirmed that we didn't want laptop mode.
> >>
> >> Most of our devices have had SSDs rather than spinning disks, so noise
> >> wasn't an issue, although when we finally did support an official
> >> device with a spinning disk people certainly complained when the disk
> >> started clicking all the time
> >
> > hm, it's interesting that the general idea still has vailidity.  It
> > would be a fun project for someone to sniff out all the requirements,
> > fixup/enhance/rewrite the current implementation and generally make it
> > all spiffy and nice.
> >
> >> (due to the underflow in the writeback code).
> >
> > To what underflow do you refer?
> >
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commit;h=c8b74c2f6604923de91f8aa6539f8bb934736754
> 
> That particular bug caused writes to happen almost instantly after the
> underflow ocurred, and consequently slowed write throughput to a crawl
> because there was no chance for contiguous writes to gather.
> 
> >> We do know that current SSDs save a significant amount of
> >> power when they go into standby, so minimizing disk writes is still
> >> useful on these devices.
> >>
> >> A very simple laptop mode which only does a single sync when we spin
> >> up the disk, and didn't bother with the timer behavior or muck with
> >> swap behavior might be something that is more useful for us, and I
> >> suspect it might simplify the writeback code somewhat as well.
> >
> > I don't think I understand the problem with the timer.  My original RFC
> > said
> >
> > : laptop_writeback_centisecs
> > : --------------------------
> > :
> > : This tunable determines the maximum age of dirty data when the machine
> > : is operating in Laptop mode.  The default value is 30000 - five
> > : minutes.  This means that if applications are generating a small amount
> > : of write traffic, the disk will spin up once per five minutes.
> > :
> > : If the disk is spun up for any other reason (such as for a read) then
> > : all dirty data will be flushed anyway, and this timer is reset to zero.
> >
> > which all sounds very sensible and shouldn't exhibit the behavior you
> > observed.
> >
> 
> The laptop-mode timer get re-armed after each writeback (see above
> laptop_io_completion function), even if it was caused by laptop-mode
> itself.  So, if something is continually dirtying a little bit of
> data, we end up getting a chain of small writes which keeps the disk
> awake for long periods of time.

Out of curiosity, for saving the power, why don' you increase the value for
laptop_mode?

> 
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-- 
Kind regards,
Minchan Kim

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