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Re: [PATCH 5/5] mm: writeback: Prioritise dirty inodes encountered by di

To: Jan Kara <jack@xxxxxxx>
Subject: Re: [PATCH 5/5] mm: writeback: Prioritise dirty inodes encountered by direct reclaim for background flushing
From: Mel Gorman <mgorman@xxxxxxx>
Date: Thu, 14 Jul 2011 08:03:10 +0100
Cc: Linux-MM <linux-mm@xxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, XFS <xfs@xxxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>, Johannes Weiner <jweiner@xxxxxxxxxx>, Wu Fengguang <fengguang.wu@xxxxxxxxx>, Rik van Riel <riel@xxxxxxxxxx>, Minchan Kim <minchan.kim@xxxxxxxxx>
In-reply-to: <20110713213947.GC21787@xxxxxxxxxxxxx>
References: <1310567487-15367-1-git-send-email-mgorman@xxxxxxx> <1310567487-15367-6-git-send-email-mgorman@xxxxxxx> <20110713213947.GC21787@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
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

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