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@xxxxxxxxxxx> 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@xxxxxxx>
> > > > > > CC: Neil Brown <neilb@xxxxxxx>
> > > > > > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> > > > >
> > > > > Thank you.
> > > > > Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> > > >
> > > > 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;
--
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