Re: [PATCH] writeback: permit through good bdi even when globaldirty exceeded
From: Wu Fengguang
Date: Fri Dec 02 2011 - 05:31:55 EST
On Fri, Dec 02, 2011 at 06:16:06PM +0800, Wu Fengguang wrote:
> On Fri, Dec 02, 2011 at 04:29:50PM +0800, Wu Fengguang wrote:
> > On Fri, Dec 02, 2011 at 03:03:59PM +0800, Andrew Morton wrote:
> > > On Fri, 2 Dec 2011 14:36:03 +0800 Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> > >
> > > > --- linux-next.orig/mm/page-writeback.c 2011-12-02 10:16:21.000000000 +0800
> > > > +++ linux-next/mm/page-writeback.c 2011-12-02 14:28:44.000000000 +0800
> > > > @@ -1182,6 +1182,14 @@ pause:
> > > > if (task_ratelimit)
> > > > break;
> > > >
> > > > + /*
> > > > + * In the case of an unresponding NFS server and the NFS dirty
> > > > + * pages exceeds dirty_thresh, give the other good bdi's a pipe
> > > > + * to go through, so that tasks on them still remain responsive.
> > > > + */
> > > > + if (bdi_dirty < 8)
> > > > + break;
> > >
> > > What happens if the local disk has nine dirty pages?
> > The 9 dirty pages will be cleaned by the flusher (likely in one shot),
> > so after a while the dirtier task can dirty 8 pages more. This
> > consumer-producer work flow can keep going on as long as the magic
> > number chosen is >= 1.
> > > Also: please, no more magic numbers. We have too many in there already.
> > Good point. Let's add some comment on the number chosen?
> I did a dd test to the local disk (when w/ a stalled NFS mount) and
> find that it always idle for several seconds before making a little
> progress. It can be confirmed from the trace that the bdi_dirty
> remains 8 even when the flusher has done its work.
> So the number is lifted to bdi_stat_error to cover the errors in
> bdi_dirty. Here goes the updated patch.
The new trace now shows bdi_dirty=0 in the _majority_ lines. But in
fact it's some small value. In this case the max pause time should
really be set to the smallest non-zero value to avoid IO queue underrun
and improve throughput.
So here comes one more fix.
--- linux-next.orig/mm/page-writeback.c 2011-12-02 18:20:14.000000000 +0800
+++ linux-next/mm/page-writeback.c 2011-12-02 18:20:27.000000000 +0800
@@ -989,18 +989,17 @@ static unsigned long bdi_max_pause(struc
* Limit pause time for small memory systems. If sleeping for too long
* time, a small pool of dirty/writeback pages may go empty and disk go
* 8 serves as the safety ratio.
- if (bdi_dirty)
- t = min(t, bdi_dirty * HZ / (8 * bw + 1));
+ t = min(t, bdi_dirty * HZ / (8 * bw + 1));
* The pause time will be settled within range (max_pause/4, max_pause).
* Apply a minimal value of 4 to get a non-zero max_pause/4.
return clamp_val(t, 4, MAX_PAUSE);
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