Re: [PATCH] writeback: remove the internal 5% low bound on dirty_ratio

[Wu Fengguang <fengguang.wu@intel.com>]

On Mon, Aug 23, 2010 at 02:30:40PM +0800, Con Kolivas wrote:
> On Mon, 23 Aug 2010 04:23:59 pm Wu Fengguang wrote:
> > On Mon, Aug 23, 2010 at 12:42:48PM +0800, Neil Brown wrote:
> > > On Fri, 20 Aug 2010 15:50:54 +1000
> > >
> > > Con Kolivas <kernel@kolivas.org> wrote:
> > > > On Fri, 20 Aug 2010 02:13:25 pm KOSAKI Motohiro wrote:
> > > > > > The dirty_ratio was silently limited to >= 5%. This is not a user
> > > > > > expected behavior. Let's rip it.
> > > > > >
> > > > > > It's not likely the user space will depend on the old behavior.
> > > > > > So the risk of breaking user space is very low.
> > > > > >
> > > > > > CC: Jan Kara <jack@suse.cz>
> > > > > > CC: Neil Brown <neilb@suse.de>
> > > > > > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > > > >
> > > > > Thank you.
> > > > > 	Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> > > >
> > > > I have tried to do this in the past, and setting this value to 0 on
> > > > some machines caused the machine to come to a complete standstill with
> > > > small writes to disk. It seemed there was some kind of "minimum" amount
> > > > of data required by the VM before anything would make it to the disk
> > > > and I never quite found out where that blockade occurred. This was some
> > > > time ago (3 years ago) so I'm not sure if the problem has since been
> > > > fixed in the VM since then. I suggest you do some testing with this
> > > > value set to zero before approving this change.
> >
> > You are right, vm.dirty_ratio=0 will block applications for ever..
> 
> Indeed. And while you shouldn't set the lower limit to zero to avoid this 
> problem, it doesn't answer _why_ this happens. What is this "minimum write" 
> that blocks everything, will 1% be enough, and is it hiding another real bug 
> somewhere in the VM?

Good question.
This simple change will unblock the application even with vm_dirty_ratio=0.

# echo 0 > /proc/sys/vm/dirty_ratio
# echo 0 > /proc/sys/vm/dirty_background_ratio                                               
# vmmon nr_dirty nr_writeback nr_unstable

        nr_dirty     nr_writeback      nr_unstable
               0              444             1369
              37               37              326
               0                0               37
              74              772              694
               0                0               19
               0                0             1406
               0                0               23
               0                0                0
               0              370              186
              74             1073             1221
               0               12               26
               0              703             1147
              37                0              999
              37               37             1517
               0              888               63
               0                0                0
               0                0               20
              37                0                0
              37               74             1776
               0                0                8
              37              629              333
               0               12               19

Even with it, the 1% explicit bound still looks reasonable for me.
Who will want to set it to 0%? That would destroy IO inefficient.

Thanks,
Fengguang
---
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -542,8 +536,8 @@ static void balance_dirty_pages(struct address_space *mapping,
 		 * the last resort safeguard.
 		 */
 		dirty_exceeded =
-			(bdi_nr_reclaimable + bdi_nr_writeback >= bdi_thresh)
-			|| (nr_reclaimable + nr_writeback >= dirty_thresh);
+			(bdi_nr_reclaimable + bdi_nr_writeback > bdi_thresh)
+			|| (nr_reclaimable + nr_writeback > dirty_thresh);
 
 		if (!dirty_exceeded)
 			break;