Re: [PATCH 5/5] mm: writeback: Prioritise dirty inodes encounteredby direct reclaim for background flushing

[Mel Gorman]

On Wed, Jul 13, 2011 at 11:39:47PM +0200, Jan Kara wrote:
> On Wed 13-07-11 15:31:27, Mel Gorman wrote:
> > It is preferable that no dirty pages are dispatched from the page
> > reclaim path. If reclaim is encountering dirty pages, it implies that
> > either reclaim is getting ahead of writeback or use-once logic has
> > prioritise pages for reclaiming that are young relative to when the
> > inode was dirtied.
> > 
> > When dirty pages are encounted on the LRU, this patch marks the inodes
> > I_DIRTY_RECLAIM and wakes the background flusher. When the background
> > flusher runs, it moves such inodes immediately to the dispatch queue
> > regardless of inode age. There is no guarantee that pages reclaim
> > cares about will be cleaned first but the expectation is that the
> > flusher threads will clean the page quicker than if reclaim tried to
> > clean a single page.
>   Hmm, I was looking through your numbers but I didn't see any significant
> difference this patch would make. Do you?
> 

Marginal and well within noise. I'm very skeptical about the patch
but the VM needs some way of prioritising what pages are getting
written back to that pages in a particular zone can be cleaned.

> I was thinking about the problem and actually doing IO from kswapd would be
> a small problem if we submitted more than just a single page. Just to give
> you idea - time to write a single page on plain SATA drive might be like 4
> ms. Time to write sequential 4 MB of data is like 80 ms (I just made up
> these numbers but the orders should be right).

It's as good as number as any for arguements sake. It's not the
first time such a patch has done the rounds. The last one I did along
similar lines was http://lkml.org/lkml/2010/6/8/85 although I mucked
it up with respect to racing with iput.

Wu posted a patch that deferred the writing of ranges to a
flusher thread http://www.spinics.net/lists/xfs/msg05659.html
which Dave has already commented on at
http://www.spinics.net/lists/xfs/msg05665.html. The clustering size
could be easily fixed but the scalability problem he pointed out is
a far greater problem.

> So to write 1000 times more
> data you just need like 20 times longer. That's a factor of 50 in IO
> efficiency. So when reclaim/kswapd submits a single page IO once every
> couple of miliseconds, your IO throughput just went close to zero...
> BTW: I just checked your numbers in fsmark test with vanilla kernel.  You
> wrote like 14500 pages from reclaim in 567 seconds. That is about one page
> per 39 ms. That is going to have noticeable impact on IO throughput (not
> with XFS because it plays tricks with writing more than asked but with ext2
> or ext3 you would see it I guess).
> 
> So when kswapd sees high percentage of dirty pages at the end of LRU, it
> could call something like fdatawrite_range() for the range of 4 MB
> (provided the file is large enough) containing that page and IO thoughput
> would not be hit that much and you will get reasonably bounded time when
> the page gets cleaned... If you wanted to be clever, you could possibly be
> more sophisticated in picking the file and range to write so that you get
> rid of the most pages at the end of LRU but I'm not sure it's worth the CPU
> cycles. Does this sound reasonable to you?
> 

Semi-reasonable and it's along the same lines as what
http://lkml.org/lkml/2010/6/8/85 tried to achieve but maybe the effort
of fixing it up with respect to racing with iput() just isn't worth it.

I think I'll leave it as kswapd will call writepage if the priority is
high enough until a good solution for how the VM can tell the flusher to
prioritise a particular page is devised.

-- 
Mel Gorman
SUSE Labs
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