Re: [RFC][PATCH] mm: stop balance_dirty_pages doing too much work

[Jens Axboe]

On Wed, Jun 24 2009, Andrew Morton wrote:
> On Wed, 24 Jun 2009 11:38:24 +0100
> Richard Kennedy <richard@xxxxxxxxxxxxxxx> wrote:
> 
> > When writing to 2 (or more) devices at the same time, stop
> > balance_dirty_pages moving dirty pages to writeback when it has reached
> > the bdi threshold. This prevents balance_dirty_pages overshooting its
> > limits and moving all dirty pages to writeback.     
> > 
> >     
> > Signed-off-by: Richard Kennedy <richard@xxxxxxxxxxxxxxx>
> > ---
> > balance_dirty_pages can overreact and move all of the dirty pages to
> > writeback unnecessarily.
> > 
> > balance_dirty_pages makes its decision to throttle based on the number
> > of dirty plus writeback pages that are over the calculated limit,so it
> > will continue to move pages even when there are plenty of pages in
> > writeback and less than the threshold still dirty.
> > 
> > This allows it to overshoot its limits and move all the dirty pages to
> > writeback while waiting for the drives to catch up and empty the
> > writeback list. 
> > 
> > A simple fio test easily demonstrates this problem.  
> > 
> > fio --name=f1 --directory=/disk1 --size=2G -rw=write
> > 	--name=f2 --directory=/disk2 --size=1G --rw=write 		--startdelay=10
> > 
> > The attached graph before.png shows how all pages are moved to writeback
> > as the second write starts and the throttling kicks in.
> > 
> > after.png is the same test with the patch applied, which clearly shows
> > that it keeps dirty_background_ratio dirty pages in the buffer.
> > The values and timings of the graphs are only approximate but are good
> > enough to show the behaviour.  
> > 
> > This is the simplest fix I could find, but I'm not entirely sure that it
> > alone will be enough for all cases. But it certainly is an improvement
> > on my desktop machine writing to 2 disks.
> > 
> > Do we need something more for machines with large arrays where
> > bdi_threshold * number_of_drives is greater than the dirty_ratio ?
> > 
> 
> um.  Interesting find.  Jens, was any of your performance testing using
> multiple devices?  If so, it looks like the results just got invalidated :)

"invalidated" is a bit too much I think, skewed is more like it. But
most of my testing has been on a single spindle, only the very first
patches used 10 disks as a test base.

> > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > index 7b0dcea..7687879 100644
> > --- a/mm/page-writeback.c
> > +++ b/mm/page-writeback.c
> > @@ -541,8 +541,11 @@ static void balance_dirty_pages(struct address_space *mapping)
> >  		 * filesystems (i.e. NFS) in which data may have been
> >  		 * written to the server's write cache, but has not yet
> >  		 * been flushed to permanent storage.
> > +		 * Only move pages to writeback if this bdi is over its
> > +		 * threshold otherwise wait until the disk writes catch
> > +		 * up.
> >  		 */
> > -		if (bdi_nr_reclaimable) {
> > +		if (bdi_nr_reclaimable > bdi_thresh) {
> >  			writeback_inodes(&wbc);
> >  			pages_written += write_chunk - wbc.nr_to_write;
> >  			get_dirty_limits(&background_thresh, &dirty_thresh,
> 
> yup, we need to think about the effect with zillions of disks.  Peter,
> could you please take a look?
> 
> Also...  get_dirty_limits() is rather hard to grok.  The callers of
> get_dirty_limits() treat its three return values as "thresholds", but
> they're not named as thresholds within get_dirty_limits() itself, which
> is a bit confusing.  And the meaning of each of those return values is
> pretty obscure from the code - could we document them please?

This is indeed a pretty interesting find!

-- 
Jens Axboe

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